: Better mileage on regular?!?



RLLOVETT
05-06-04, 01:10 AM
Yup, you read it here first--I'm getting about 18.3 mpg on Shell regular and about 17.3 on Shell premium...so I tried Shell plus, just for the hell of it...only a quarter tank gone so far...
My real question is: does running the lower octane fuel create a problem over the long run? I don't hear any knocking (for the most part) but there is a noticeable performance gap on the lower octane that I don't especially mind. With fuel at 2.05 (prem) today and most likely going up, the 5 percent increase in fuel economy woould be helpful while I save up for my head gaskets! Thanks in advance!

Ralph
05-06-04, 01:34 AM
I think with higher octane, it's supposed to burn slower, therefore save fuel. But according to this article, not always, which is what you claim.

(oops, I just saw the "permission" clause on that one)

RLLOVETT
05-06-04, 09:50 AM
Thanks for all that detail! Sounds like my leaky head gaskets could be affecting the piston cooling rate and changing the mileage characteristics? Although as long as I keep topping off the coolant, the typical engine temp is the same as before I was using antifreeze like chewing gum. Interesting article.

Randy_W
05-06-04, 11:39 AM
Regular fuel simply has more BTU's than premium and should provide better mpg as long as it doesn't create pre-ignition and cause your pcm to retard timing too much. It won't control flame travel as well as higher octane, so be careful of spark knock.;)

Geno Castellano
05-06-04, 12:42 PM
There's no difference in burn rate or flame speed between premium and regular. Regular "burns" at the same rate as premium- not faster nor slower. Premium fuel has a higher octane rating than regular which means that it resists detonation better- so the engine doesn't spark knock or ping on premium and the knock control will not have to retard the spark advance to prevent detonation. If we are comparing regular gasoline to premium gasoline (not an alcohol blend or anything like that) there is no difference in the BTU content of regular fuel and premium. Typically, all things being even, you would get worse fuel economy on regular than premium if the engine is designed for premium - which the 93-99 Northstars are. If the engine is running on regular it's likely that the knock sensor and knock control software will detect some detonation and retard the spark to prevent it and thus the engine would be less efficient and get less fuel economy.

As far as I know, the 93-99 Northstar combustion chamber is pretty good and very efficient so it's very detonation resistent and tolerates regular fuel well. Most all people can run regular under normal day-to-day conditions and not notice much if any difference between regular and premium.

It is, however, hard to justify or explain getting better fuel economy with regular fuel compared to premium. I would believe as good a fuel economy with regular but not better.

Not doubting your experience here, but, there is most likley another explaination for the improved fuel economy that you observed with regular. It's not because the regular burns different or has more BTU's.

Unless...the premium fuel you were buying had a high alcohol content. Methanol and Ethanol are often added to fuel to achieve a higher octane rating. Alcohol is good for octane rating. But, it takes twice as much alcohol (by volume) to achieve a chemically correct A/F ratio. So if the premium fuel you bought was 10 percent alcohol for octane purposes then the engine would immediately be 5 percent lean and use more fuel (by volume) to achieve the same A/F ratio. This could explain the difference in fuel economy between "regular" and "premium". It would really be the difference between "gasohol" and "gasoline" but the pumps were labeled 'premium" and "regular" as alcohol is not found in regular as often.

Otherwise, you may be experiencing a difference in the driving condition, climate/temperature changes, driving schedule, wind, etcetera. There are a myriad of things that can affect mileage significantly making real world fuel economy comparisons almost impossible. Especially over only one or two tanks of fuel. Just the fill difference between two different pump nozzles could account for the difference. And slight wind or driving differences would easly account for that kind of difference on the electronic fuel economy average calculation.

Just throwing out some ideas, here. Not to dissuade you from using regular if you want. As long as you don't hear any spark knock or pinging then it is fine. Just don't expect better fuel economy on regular. There's really no engineering justification for it.

By the way... "Pre-ignition", in the classical sense, wont cause the knock sensors to react and the spark to retard. The knock sensors react to spark knock or detonation - which is the spontaneous combustion of the end gases in the combustion chamber after the spark plug has ignited the mixture. Pre-ignition is the ignition of the mixture before the spark plug has ignited the mixture. Detonation and preignition are two different things. Both combustion related and often confused but distinctly different phenomenon. Pre-ignition leaves holes in your pistons!

There are actually entire books written on the each of the above topics so this is a very short version.

Randy_W
05-06-04, 11:45 PM
Geno, if the burn rate is the same, then exactly why would higher octane matter? It happens that I worked for Exxon in Benicia, CA in the early '70's. We were taught just what I said, I didn't dream it! If you have a much better understanding of fuels than me, and you may very well, explain exactly why octane, which is basically the measure of a fuels ability to resist knock, would matter if the burn rate were the same. I'd like a better understanding of this, since apparently Exxon misled us way back when. Yes you are right, detonation is the uncontrolled explosion of fuel mixture initiated at some point other than the timed firing of the spark plug. Could be pre or post ignition for that matter, or even simultaneously! Pre ignition, obviously happens before the timed firing of the spark plug, usually due to heat build up. Really hard to explain the difference to the average guy so most people use the terms interchangeably to describe what they know as 'spark knock'.

This from a DOE document;


The octane number assigned to a motor fuel has very little to do with the actual chemical "octanes" in the fuel and everything to do with how well the fuel resists detonation (which is directly related to the amount of energy (heat) required to get the fuel burning in the first place).

Therefore, it is possible to assign octane VALUES to fuel which contain no octanes whatsoever.

WHY DO WE CARE ABOUT OCTANE? WHAT IS PREIGNITION? WHAT IS DETONATION?



The octane value of a fuel is an empirical measure of its ability to resist detonation and, to a limited extent, preignition. Technically, octane ratings measure a fuel's ability to resist the spontaneous ignition of unburnt end-gases under controlled test conditions.

What is preignition? Preignition occurs when the fuel/air mixture in a cylinder ignites before the spark plug fires. It can be caused by burning contaminates (such as carbon, or a spark plug of the wrong heat range) in the cylinder or by extreme overheating.



What is detonation? Detonation occurs when the flame-front in a cylinder does not proceed smoothly from the point of ignition (the spark plug) to the opposite side of the cylinder. It refers to the spontaneous ignition of the entire charge in the cylinder. This ignition is often caused by the extreme pressure rise in the cylinder that occurs when the charge is first ignited (by the spark plug).



WHAT ABOUT FUELS?



There are six things to consider when comparing hydrocarbon fuels: 1. Volatility. In short, what's the fuel's propensity to vaporize. This effects the ability to easily mix the fuel with air and the fuel's tendency to vapor-lock. It also determines the pollution characteristics of the fuel where evaporative pollution is a concern.
2. Pre-ignition & knock resistance. Referred to as "Octane value." How much energy does it take to get the fuel burning - how much does it resist auto-ignition from compressive heat? {Also, what is the rate of burn of the fuel (which affects the rate of pressure rise)? }
3. Energy content. How much energy can be extracted from the fuel as a percentage of its volume or mass.
4. Heat of evaporation.
5. Chemical stability, neutrality, and cleanliness. What additives does the fuel contain to retard gum formation? Prevent icing? Prevent corrosion? Reduce deposits?
6. Safety

The first three factors are often confused and interrelated when, in fact, they measure three completely separate things. There is no natural collelation between them.

General rules: Heavy fuels (diesel, jet): Low volatility, low knock resistance, high energy per volume Light fuels (gasoline): High volatility, high knock resistance, low energy per volume



Note that gasoline, partially, makes up for its (relatively) low energy-per gallon by the fact that a gallon of gasoline weighs less (by about 15%) than a gallon of jet fuel.

Octane rating is in no way correlated with engine power or efficiency. There is more potential energy in a gallon of diesel fuel than a gallon of gasoline, yet the diesel fuel has a much lower octane value (more on that below).

HOW DO YOU DETERMINE OCTANE?

Additional refining steps are used to increase the octane; however, these additional steps do not make the gasoline any cleaner or better. "They just yield a different blend of hydrocarbons that burn more slowly." The additional steps also increase the price.

This seems to say that they 'DO' burn at different rates, in fact all the info I found everywhere says they burn at different rates!!!;)

RLLOVETT
05-07-04, 12:59 AM
Geno, I like the gasohol explanation for this phenomenon mainly cuz it fits the facts...it is puzzling...and I don't think it's external temp/driving conditions/etc. I drive about 6 miles each way to work on the interstate @ 50-65 mph plus another 3-7 every other day ferrying kids around @ 20-40 on the side roads. The car tells me my average speed is about 23-5 mph and my pattern is very consistent week to week. Anybody know how Shell processes their products?

Lawrence
05-07-04, 12:10 PM
Interesting discussion. I have always understood premium to burn slower too. I remember back when I had my 74 Z28 and Sunoco sold 260. When I ran their 260 at about 7000 RPM I would get flames about a foot or so out the exhaust tips. Now that was a full exhaust to the rear bumper. But when I ran their 220 this didn't happen. I know I could feel a difference in the power the car made with the two fuels. It was snappier down low with the 220 but on a WOT run always pulled harder up top on 260. I always set the timing back and ran 260 for the weekend. Always was curious about that, and later "learned" that premium burned slower. So theorized that the burn speed difference was just enough to expell some still in burn process fuel deeper into the exhaust, in this case just enough more that it breached the exhaust tips. Which also explained why I had to set the timing back to run the 260. BTW the car was tuned right and was a screamer, going undefeated on the street here for about one and a half years until I retired it.

It was also my understanding that these premium fuels made more power from the slower and hence longer burn, giving a longer push on the piston, being a greater advantage in a long stroke motor. Much like Nitro-Methane in a top fuel motor.
Geno.....I respect your opinion, but damn you make my head hurt. Hows an old fart supposed to re-learn everything he ever "knew"? Just too damn old to go back to the books. :bonkers:

32V_DOHC
05-07-04, 02:54 PM
Detonation is the spontaneous combustion of the remaining burnable gases in the combustion chamber. Basicly as the charge is burning the temp and pressure rises toward a peak value. If the temp and pressure of the unburned gases rise above the autoignition pressure and or temp then they detonate (explode) If they don't reach this pressure or temp then they burn normally. The octane rating is a measure of where that pressure/temp curve is. The higher the octane rating the more pressure/temp it takes to autoignite the mixture.

Octane rating actally doesn't tell you anything about burn speed. Burn speed is mainly a function of chamber design and turbulence. Slowing the burn down will not create more power. More power is created by higher mean effective pressures. One possible way to get a higher MEP is to raise the peak pressure. This requires a higher octane to resist detonation. Sometimes you can get higher MEP by advancing to timing. A slower burning mixture would require more timing to acheive the same peak pressure. The net result would be a greater pressure exerted during last part of the compression stroke. Which translates to less power at the crank. Furthermore after peak pressure the remaining gases would continue to burn slower while the piston expands the cylinder volume. This would result in less pressure and less power. In fact the newest design fast burning chambers create more power with less advance by burning the mixture faster. The only way to get more power from more octane (in the same engine) is to create a greater peak pressure and move the whole pressure curve up.

John

OlManRivah
05-07-04, 03:12 PM
Another interesting thread.......

We always retarded the ignition on lower octane, not higher octane.

Geno Castellano
05-07-04, 03:24 PM
The idea that premium fuel burns "slower" is a common misconception and people that know better often fall into the trap of using the "slower burning" explaination in trying to explain octane ratings and such.

If the fuel burned at different rates then the spark timing would need to be changed when the fuel is changed from premium to regular - and it doesn't. I have done this many many times on a dyno (usually in trying to prove to some kid that premium doesn't burn slower) run the engine on premium and then regular and it makes the best power at the same spark timing at any given point. End of story.

The DOE article is pretty good and accurate. Right up to the last sentence where it falls into the same trap. Interesting really, since the first part of the article is correct and tends to disagree with the last part of the article.

The issue is very confusing in the real world due to the additives used to raise octane levels. Alcohol is frequently added to raise the octane level and alcohol does burn slower. Much slower. Switch an engine to pure alcohol and it needs 20 degrees more spark to achieve the same power point! So alcohol in fuel will tend to slow the burn has nothing to do with the octane rating but it does confuse the issue due to the chemical properties of alcohol. Notice I said specifically "gasoline" in my original answer - not gasohol or alcohol mixtures - when I said that they burn at the same rate.

Practically speaking it's possible that pump premium "gasoline" that's likely a mixture of alcohol and gasoline might burn a little slower than the regular "gasoline" that likely has no alcohol. But the difference is still tiny and doesn't explain or account for the difference in octane rating.

Randy. I read your comment, here, again. "Yes you are right, detonation is the uncontrolled explosion of fuel mixture initiated at some point other than the timed firing of the spark plug. Could be pre or post ignition for that matter, or even simultaneously!"

This is not true the way you wrote it. Detonation is the spontaneous combustion of the end gas in the chamber after the spark plug fires. Not "other than" or "pre-ignition". Detonation is "post ignition" by the spark plug.

Pre-ignition is as you stated - ignition of the fuel mixture before the spark plug fires. Octane has little or nothing to do with preignition per se. Pre-ignition occurs so soon in the compression event that octane has nothing to do with it. There is always some sort of ignition source other than the arcing of the plug that actually ignites the mixture in pre-ignition. Something like the spark plug tip running too hot and glowing or the ground elctrode glowing and igniting the mixture. This is why you do to cold plugs and cut the ground electrode most of the way off when you run NOS in heavy doses. To prevent pre-ignition.

There is a situation called detonation induced pre-ignition. If the engine detonates due to low octane rating fuel it will cause a lot of heat to be transferred into the chamber walls, the piston and the spark plug. If this heat transfer is great enough it can cause the spark plug tip to overheat and induce pre-ignition. So, in this case, octane rating is somewhat related to pre-ignition and low octane fuel could lead to preignition in this example but it's not the direct cause of the pre-ignition. With a colder spark plug the plug would not overheat even with the detonation and pre-ignition would not result even with the poor octane fuel.

As far as what Exxon or anyone taught anyone - who knows? Maybe considering the circumstances it was the easiest and simplest explaination. It's wrong, though, if that's what "they" taught you.

The fuel's octane rating is just that - an octane rating. This is a measure of the fuels ability to resist the temperature and pressure in the combustion chamber as the charge is burning and resist spontaneous combustion. It's a measure of the fuel's "stability", if you will. Alcohol has an "octane rating" but obviously has no octane in it.

Fuel is rated for octane rating on a running engine which is a special test engine that allows the compression ratio to be adjusted while the engine is running. The questionable fuel is put into the fuel system and the engine is run and the compression is increased until spark knock or detonation occurs. This is where the spark is still igniting the charge but the end gas is spontaneously combusting due to heat and pressure building in the chamber. Then a series of test fuels are run in the engine at that same compression ratio. The fuels are gasoline with lesser and lesser percentages of "octane" in them until the fuel is found that knocks the same as the test fuel. It gets that "octane" rating then. This is the only way to test for octane - that's why it's so hard to get fuel measured for octane rating. Try it some time. It cannot be chemically checked for octane rating only by running it in the special lab test engine for comparison to known fuels can it be determined.

There are many other factors (as mentioned in the DOE article) such as the volitility that affect the power of the engine and the driveability. Most high octane, race fuel, such as the Sunoco 260 has high octane ratings and low volitility. The volitility is kept purposefully low to minimize the likely hood of vapor lock - since vapor lock is a problem with high performance engines trying to pump a lot of fuel with an old mechanical fuel pump. Likely the Sunoco 260 had low volitility and a lot of it didn't atomize and burn at low speeds so you had raw gas and flames coming out the tailpipe. Not due to the octane rating nor the burn rate but because some of the fuel was so "dead" it never atomized at low engine speeds and never burned until it was down the hot exhaust pipe. Fuel is atomized by heat and turbulence. At high engine speeds even "dead", low volitility fuels, will atomize and burn cleanly due to the severe turbulence and high flow so the Sunoco 260 felt good on the top end when it was getting completely atomized and burned. At low speeds there was insufficient turbulence to atomize the Sunoco 260 and the engine felt flat because it was actually lean due to the fuel only partially atomizing. The Sunoco 220 was more a street fuel with higher volitility so it worked well at low speeds but probably started to detonate and loose power at higher speeds.

This is why aviation fuel often does not run well in high performance engines even though it has good octane ratings. It's very dead in terms of volitility since it is designed to be used in an airplane that is running at 5000 or 10000 feet of altitude. At those altitides the volitility increases (water ultimately boils in a vacuum at room temperature) so the fuel atomizes fine. That is why you pilots always put the carb heat on for takeoff - to get good atomization of the low volitility av gas until you are at altitude. Nothing to do with the octane rating or detonation.

There are so many books written on these subjects so I can't even hope to cover all the aspects of the great octane debate - but I can assure you, that within practical reason - regular gasoline and premium gasoline burn at the same rates. Premium does not burn slower.

Geno Castellano
05-07-04, 03:42 PM
Another interesting thread.......

We always retarded the ignition on lower octane, not higher octane.

You are confusing the thread's message with compression ratio, burn rate due to chamber design and octane rating.

If all other design parameters stayed the same you would retard the timing on lower octane fuel to prevent detonation. The lower octane fuel has less resistence to spontaneous combustion so the spark has to be retarded to initiate the flame later in the combustion event so that less heat and pressure is put into the end gas so it doesn't detonate.

If you kept the octane rating the same and increased the burn rate by more chamber turbulence (a later model fast burn chamber) the engine would likely not detonate even with higher compression due to the faster burn and due to the faster burn you would not have to light the spark nearly as soon - so it would take less advance to accomplish the same power. "Retarding" the spark is what is required with faster burn chambers on more modern engines.

This whole issue is very confusing because the burn rate of the chamber is expressed in how many crank shaft degrees it takes to complete the burn. At a given fixed RPM this equates to a specific period of time that can be measured by watching the pressure in the chamber with special instrumentation.

It is possible to make a chamber "burn faster" in several ways. The simplest way is to move the spark plug to the center of the chamber. The mixture burns in a geometric progression like the spreading ripples in a pond when you toss a stone in it. If the spark plug is at one end of the chamber then it takes a longer time for the burn to reach the other end so the spark would need to be more advanced to complete the burn on time. Simple geometry of the chamber and plug location thus affects the burn rate. If the plug is at one end then the end gases are subjected to heat and pressure longer than if the plug were in the middle - so they would have a greater tendency to detontate and would therefore need higher octane fuel if the plug were at one end. Moving the plug to the center of the chamber means that the flame front reaches the recesses of the chamber sooner (since they are closer) so less spark advance is needed (retarded compared to the other example) and the chamber will have less tendency to detonate and require less octane rating.

The confusion evident in this example comes from the fact that the chamber burn rate and the "burn rate" of the fuel are being interchanged. The burn rate of the fuel is defined as how fast the flame front progresses thru the mixture. This doesn't change with the chamber geometry described. But, the chamber does "burn faster" due to the geometry and the fact that the centrally located plug gets the flame front to the edge of the chamber faster.

If the geometry and plug location of the chamber stays the same and, instead, much greater turbulence is added due to intake charge motion or swirl then this could also lead to a fast burn chamber. In this case, the actual flame speed is going to be faster because the added turbulence increases the flame speed through a faster chemical reaction. So, burn rate, in this example, is referring to the burn rate of the flame front due to the in-cylinder mixture motion.

If you take a modern combustion chamber like on the 2000 and later Northstars they optimize both of the above factors. The geometry of the chamber is such to minimize the chamber burn time due to the centrally located plug and chamber shape and the actual burn rate of the mixture is sped up by creating the correct in-cylinder motion to add turbulence to the charge to expedite the burn. That is how it can make good power, with low spark advance on regular fuel with lower octane ratings without detonation.

Geno Castellano
05-07-04, 03:52 PM
One interesting feature of an internal combustion spark ignited gasoline engine is called LOCATION OF PEAK PRESSURE. If you measure the pressure in the combustion chamber during the combustion and power events and plot the pressure vs. crank shaft angle the location of peak pressure (if the engine is sparked and timed correctly and operating at maximum efficiency) is always at 14 degrees After Top Dead Center.

Depending on the chamber design, geometry, burn rate, etcetera, the engine might need more or less spark advance to achieve the 14 ATDC reading but you can rest assured that when the optimum conditions are reaced the LPP will always end up at 14 ATDC.

If you think about it the engine is a combination of a mechanical device (a piston hooked to a rod that is hooked to a crank that is turning at a certain speed) and a combusting mixture that is governed by the ideal gas laws of physics. The gas is going to expand at a certain rate and the pressure is going to increase along the way but it takes a finite amount of time for this to happen and the piston can only move so fast so the whole event has to be timed so that the best compromise can be reached for the most efficient operation. Light it too soon and the piston is fighting the combusting gases to reach top dead center. Light it too late and the gases are still expanding when the piston reaches bottom dead center. You know if it's "right" if the LPP is at 14 ATDC. This is why there are doctorates of doctorates in combustion. This is a complex subject. There really are no "simple" answers.

dloch
05-07-04, 04:36 PM
This is quite an education here...... especially about the chamber design of a cylinder head. I.E. Vortec heads, fast burn heads just to name a couple of very good factory GM heads that make very good power at a relatively low timing advance, 30 - 32 degees.

Geno do your fingers get tired? :bouncy:

Randy_W
05-07-04, 05:27 PM
Geno, that is indeed what "they", Exxon, taught us!

I looked at seven different sources, one engine builder, one research lab, one from the DOE and four oil companies. All stated at some point that higher octane fuels burn at a slower and more controlled rate. I guess you should write to thier engineering departments and tell them of thier mistake! I'm absolutly sure they would want correct themselves.
They are; Shell, Exxon-Mobile, Sun Oil,, BP-Amoco, Petty Enterprises,Synlab and the United States Department of Energy.


As far as doubting that's what I was taught, whether what I was taught was right or wrong, is accusing me of not telling the truth. You are very smart, but not a mind reader. So before you 'imply' that someone is not telling the truth as they know it to be, you should know that person a lot better.

You obviously are a very learned person, but it is also obvious your bedside manor could use some work. I'm a big boy I don't need to prove me right or you wrong. I don't need a d**k measuring contest to prove myself on every issue. If you do, go for it! I will continue to look here for good info, a lot has come from you, for that, thanks! However, you don't know it all, every time. That's o.k. though, niether do any of the rest of us, some of us are just willing to admit it.

Have a nice day.;)

OlManRivah
05-07-04, 09:36 PM
You are confusing the thread's message with compression ratio, burn rate due to chamber design and octane rating.
Actually, I was just remembering what we did to use cheapo gas, retard the timing a little. This after Mr. Lawrence said he use to set the timing back when going to Sunoco 260.

But, you are somewhat correct. I do stay confused......and you're not helping....lol!

I've always ran 93 octane in my '97, but after reading your threads, I may try dropping back to 89. Or, am I still confused. You did imply that I don't have a modern combustion chamber.......:banghead: !

Ralph
05-07-04, 09:47 PM
Here is an "easier" article to help figure it out hopefully.

http://www.howstuffworks.com/question90.htm

Ralph
05-07-04, 09:52 PM
Here is a more complex article that goes into great detail:

I always heard detonation (engine knock) described as "two flame fronts colliding" therefore creating excessive pressure in the combustion chamber, and possibly causing piston or spark plug damage? That's why when I heard my Dad's Cad knocking, I told him to immediately put in the required Premium fuel, and it stopped! As you will see, Geno is correct in that it is an extremely complicated topic. Lots of overlap.

http://www.prime-mover.org/Engines/GArticles/octane.html

Pimpin_Whity
05-07-04, 10:04 PM
i'm not really sure on how this works but my car gets better gas mileage and more power with a higher octane fuel

Lawrence
05-08-04, 12:27 AM
One interesting feature of an internal combustion spark ignited gasoline engine is called LOCATION OF PEAK PRESSURE. If you measure the pressure in the combustion chamber during the combustion and power events and plot the pressure vs. crank shaft angle the location of peak pressure (if the engine is sparked and timed correctly and operating at maximum efficiency) is always at 14 degrees After Top Dead Center.

Depending on the chamber design, geometry, burn rate, etcetera, the engine might need more or less spark advance to achieve the 14 ATDC reading but you can rest assured that when the optimum conditions are reaced the LPP will always end up at 14 ATDC.

If you think about it the engine is a combination of a mechanical device (a piston hooked to a rod that is hooked to a crank that is turning at a certain speed) and a combusting mixture that is governed by the ideal gas laws of physics. The gas is going to expand at a certain rate and the pressure is going to increase along the way but it takes a finite amount of time for this to happen and the piston can only move so fast so the whole event has to be timed so that the best compromise can be reached for the most efficient operation. Light it too soon and the piston is fighting the combusting gases to reach top dead center. Light it too late and the gases are still expanding when the piston reaches bottom dead center. You know if it's "right" if the LPP is at 14 ATDC. This is why there are doctorates of doctorates in combustion. This is a complex subject. There really are no "simple" answers.

Very good reading. You learn something new everyday. Just too bad in my case it just pushes out something else. :bighead: :)

Here is another possible myth you shed light on. Years ago as leaded fuel was being phased out, we used to mix regular unleaded with premium unleaded. The thought was that it increased octane by more than the sum of the two. We did this in the 2 stroke motorcyles because we still wanted the lead but needed the higher octane fuel. Any truth to this?

Geno Castellano
05-08-04, 01:53 AM
i'm not really sure on how this works but my car gets better gas mileage and more power with a higher octane fuel

Perfectly understandable. A Northstar has a knock sensor that detects spark knock or detonation. If you use a lower octane fuel the engine will likely spark knock slightly, the sensor will pick up the knock and the PCM will automatically retard the spark advance to prevent the spark knock and protect the engine - so, due to the retarded spark advance the engine will make slightly less power and will get slightly poorer fuel economy. Under most conditions most of the time the 93-99 Northstars will run acceptably well with little or no loss in performance on regular fuel. However, in hot, dry climates the engine will likely knock on regular (you won't hear it because the knock sensor will catch it first and retard the spark) enough to cause a perceivable loss in performance and fuel economy. So, yes, it will "run better" on premium fuel because it has a knock sensor and very effective knock control software.

Engines, like the 4.9, that don't have a knock sensor will knock on regular or lower grades of fuel but will not slow down or get poorer fuel economy. Conversely, if a 4.9 or 4.5 is happy on regular due to cool, humid conditions it will not make any more power on premium fuel. It would be a waste of money in that case.

Engines that don't have a knock sensor and/or engines that are tuned for regular fuel (lower compression/less spark advance/faster burn chambers/etc...) wont make any more power on premium either. To make more power on premium the engine would have to be modified with more compression/spark/etcetera to take advantage of the better fuel.

Geno Castellano
05-08-04, 02:07 AM
Geno, that is indeed what "they", Exxon, taught us!

I looked at seven different sources, one engine builder, one research lab, one from the DOE and four oil companies. All stated at some point that higher octane fuels burn at a slower and more controlled rate. I guess you should write to thier engineering departments and tell them of thier mistake! I'm absolutly sure they would want correct themselves.
They are; Shell, Exxon-Mobile, Sun Oil,, BP-Amoco, Petty Enterprises,Synlab and the United States Department of Energy.


As far as doubting that's what I was taught, whether what I was taught was right or wrong, is accusing me of not telling the truth. You are very smart, but not a mind reader. So before you 'imply' that someone is not telling the truth as they know it to be, you should know that person a lot better.

You obviously are a very learned person, but it is also obvious your bedside manor could use some work. I'm a big boy I don't need to prove me right or you wrong. I don't need a d**k measuring contest to prove myself on every issue. If you do, go for it! I will continue to look here for good info, a lot has come from you, for that, thanks! However, you don't know it all, every time. That's o.k. though, niether do any of the rest of us, some of us are just willing to admit it.

Have a nice day.;)

Randy, I really don't want to waste the time arguing with you. If you do sufficient reading on the subject you will come to understand that I'm correct. I really have nothing to prove to you or anyone else so it's not a contest to me. People ask questions and I get some degree of satisfaction trying to answer them. If I don't know the answer, I say so. If I do know the answer, I try to convey it in as succinct and clear a term as possible. If you think my bedside manner needs improving I have news for you. I don't sugar coat it or BS. You get the facts as clearly and succintly as I know how to type it. Get used to it or skip over my posts.

I don't think all those folks misunderstand combustion or are trying to misrepresent the facts. I don't recall calling anyone a liar. I simply think there's either a misunderstanding in their explainations or they are trying to oversimply the explainations for the laymen out there. I'm not sure and don't really care and am not going on a mission to correct them.

The subject of combustion and flame speeds and burn rates is very complex as I mentioned. I could carry the argument quite far that there is a difference in burn rates in gasolines - because from certain viewpoints there can be given specialty blended fuels (such as F1 rocket fuel - hardly "gasoline" but it does meet the F1 requirements for "hydrocarbon fuel") but my comments on here reflect the practical side of the discussion regarding gasoline in passenger cars. I am really not interested in nipicking burn rates and acedemic discussions of theoretical flame speeds of outlandish fuels and such. My comments reflect the real world products that are in the market and I try to explain why some of the myths that are floating around are not true - and why they may have occurred. It will take more than a google search on "octane" to change my mind. Sorry.

By the way, there are a lot of text books on combustion that cover this subject at a very detailed level. After studying several of them I am sure you could take my place in this discussion and explain why regular and premium burn at the same rate.:annoyed:

RLLOVETT
05-08-04, 02:15 AM
The following link is not directly related to this thread but if you have a high speed connection, the video presentation of the new Bugatti W-16 engine is worth the price of admission!
http://marshallbrain.blogspot.com/2004_02_01_marshallbrain_archive.html
Kudos to all for a truly educational keyboard hoedown!! Thanks.

Geno Castellano
05-08-04, 02:24 AM
Here is a more complex article that goes into great detail:

I always heard detonation (engine knock) described as "two flame fronts colliding" therefore creating excessive pressure in the combustion chamber, and possibly causing piston or spark plug damage? That's why when I heard my Dad's Cad knocking, I told him to immediately put in the required Premium fuel, and it stopped! As you will see, Sal Collaziano is correct in that it is an extremely complicated topic. Lots of overlap.

http://www.prime-mover.org/Engines/GArticles/octane.html


Another famous myth. "The noise of two flame fronts colliding"

Not true, either. Sounds like a good explaination and seems to make sense and analysis of the pressure data from the chamber indicates that the detonation "noise" coincides with the "two flame fronts colliding" but that is not the noise.

When the end gas in the chamber spontaneously combusts the remaining mixture burns very quickly. Partly because the heat and pressure is increasing exponentially, partly because the turbulence in the chamber just increased dramatically and mostly because the remaining mixture is now being consumed "from both ends" of the chamber. The result is a dramatic, sudden increase in pressure in the combustion chamber. On a pressure trace of the combustion pressure the pressure rise in the cylinder actually looks fairly smooth until detonation occurs and then the pressure increase is like a straight line upwards by 700-800psi. That sudden, violent increase in pressure "rings" the combustion chamber and the engine structure just like striking it with a hammer. That causes the noise just like hitting the inside of the combustion chamber with a hammer would sound. The detonation frequency is about 6000Hz which is what you hear and what the knock sensors are tuned for. So, you're not hearing the sound of the two (or more) flame fronts meeting or colliding - you're hearing the sound of the engine structure resonating from the detonation frequency of 6000Hz.

The proof of this is listening to a diesel engine run. Diesels sound like "spark knock" when they run due to the rapid pressure rise in the chamber combined with the high compression ratio of diesel engines. Since diesels don't and can't "detonate" in the sense that there is no end gas to spontaneously combust the noise cannot come from two flame fronts colliding because there aren't two flame fronts. Diesels inject the fuel into the combustion chamber at the moment that combustion is desired so the fuel is burning as it is injected - there is nothing but air in the rest of the chamber and thus there is no fuel with it to spontaneously combust. But the diesel sure sounds like it is spark knocking or detonating when you listen to it run. And, if you measure the frequency of the sound that is emanating from the diesel it is located around 6000Hz that same frequency emanating from the gasoline engine when it is detonating. So, the noise comes from the rapid pressure rise in the chamber, not due to the two flame fronts colliding.

The fact that the maximum pressure spike occurs just when the two flame fronts meet causes the noise to emanate from the chamber about the same instant but that is the cause and effect of why the noise happens then - not the cause of the noise per se.

Geno Castellano
05-08-04, 02:32 AM
Actually, I was just remembering what we did to use cheapo gas, retard the timing a little. This after Mr. Lawrence said he use to set the timing back when going to Sunoco 260.

But, you are somewhat correct. I do stay confused......and you're not helping....lol!

I've always ran 93 octane in my '97, but after reading your threads, I may try dropping back to 89. Or, am I still confused. You did imply that I don't have a modern combustion chamber.......:banghead: !

You need to read my posts again. I specifically indicated that the 93-99 Northstars had a very good combustion chamber that tolerated lower octane fuels and provide good driveability even when spark retard for detonation prevention was present. Your owners manaul for your 97 indicates regular fuel is fine I believe. Around 97 the fuel requirement for the Northstar was specifically changed from "Premium required" on the 93-96 engines to "premium recommended" on the later engines meaning that regular was okay. Check your owners manual.

Ralph
05-08-04, 02:43 AM
Another famous myth. "The noise of two flame fronts colliding"

Not true, either. Sounds like a good explaination and seems to make sense and analysis of the pressure data from the chamber indicates that the detonation "noise" coincides with the "two flame fronts colliding" but that is not the noise.

When the end gas in the chamber spontaneously combusts the remaining mixture burns very quickly. Partly because the heat and pressure is increasing exponentially, partly because the turbulence in the chamber just increased dramatically and mostly because the remaining mixture is now being consumed "from both ends" of the chamber. The result is a dramatic, sudden increase in pressure in the combustion chamber. On a pressure trace of the combustion pressure the pressure rise in the cylinder actually looks fairly smooth until detonation occurs and then the pressure increase is like a straight line upwards by 700-800psi. That sudden, violent increase in pressure "rings" the combustion chamber and the engine structure just like striking it with a hammer. That causes the noise just like hitting the inside of the combustion chamber with a hammer would sound. The detonation frequency is about 6000Hz which is what you hear and what the knock sensors are tuned for. So, you're not hearing the sound of the two (or more) flame fronts meeting or colliding - you're hearing the sound of the engine structure resonating from the detonation frequency of 6000Hz.

The proof of this is listening to a diesel engine run. Diesels sound like "spark knock" when they run due to the rapid pressure rise in the chamber combined with the high compression ratio of diesel engines. Since diesels don't and can't "detonate" in the sense that there is no end gas to spontaneously combust the noise cannot come from two flame fronts colliding because there aren't two flame fronts. Diesels inject the fuel into the combustion chamber at the moment that combustion is desired so the fuel is burning as it is injected - there is nothing but air in the rest of the chamber and thus there is no fuel with it to spontaneously combust. But the diesel sure sounds like it is spark knocking or detonating when you listen to it run. And, if you measure the frequency of the sound that is emanating from the diesel it is located around 6000Hz that same frequency emanating from the gasoline engine when it is detonating. So, the noise comes from the rapid pressure rise in the chamber, not due to the two flame fronts colliding.

The fact that the maximum pressure spike occurs just when the two flame fronts meet causes the noise to emanate from the chamber about the same instant but that is the cause and effect of why the noise happens then - not the cause of the noise per se.

I guess I don't fully understand how the noise is created. I've read of physical spark plug damage from detonation. I don't think that the flames themselves could cause the electrode of the plug to become bent, so if I understand this correctly, it is just the "pressure" causing the damage? If pressure is causing the noise, what would cause the physical engine damage? I guess I'm as confused as Natasha looks right now!

Diesels sound terrible, one of the reasoms I don't like my friends TDI Jetta.

Geno Castellano
05-08-04, 02:44 AM
Very good reading. You learn something new everyday. Just too bad in my case it just pushes out something else. :bighead: :)

Here is another possible myth you shed light on. Years ago as leaded fuel was being phased out, we used to mix regular unleaded with premium unleaded. The thought was that it increased octane by more than the sum of the two. We did this in the 2 stroke motorcyles because we still wanted the lead but needed the higher octane fuel. Any truth to this?

Did you mean you mixed unleaded premium with leaded regular? Two strokes don't "need" lead. The lead was in the fuel for octane improvement. Nothing in a two stroke needed lead for lubrication or anything. You might have spiked the octane of the unleaded fuel by adding some lead to it with the leaded fuel but it was probably unnecessary. But, hey, you got away with it so who cares?

Lead is a good example of the "octane rating" of the fuel. The addition of tetraethyle lead to gasoline raises the octane rating considerably. No more "octane" per se in the fuel but the knock resistence improved such that more compression could be run in the test engine equating to a higher equivalent octane rating. The addition of lead does not slow the burn rate of the fuel down any it just makes the fuel more resistent to spontaneous combustion of the end gas in the chamber. The exact same gasoline will have a much higher octane rating with the addition of the tetraethyl lead but it is the same hydrocarbons and the same burn rate.

By the way, the effect of tetraethyl lead on octane was a technical breakthrough that was sought to improve the output of high performance aircraft engines before world war two. The piston fighter and bomber engines were at the limit of power without better fuels so as to be able to increase compression. The "discovery" of tetraethyl lead by the Ethyl Corporation lead to the development of the high output aircraft engines that helped to win the war and then was used for passenger car engine development after the war. The addition of lead also, coincidentally, helped with the lubrication and wear resistence of valve seats but that's not why it was put in the fuel. It was just a nice side benefit that everyone got used to and then freaked out over when lead was removed from the fuel so as to not poison the cat. Engines before 1940 ran fine on "unleaded" fuel because it didn't exist.

Ralph
05-08-04, 02:56 AM
Also, last week on the highway, I thought I heard some "high speed knock" from the engine!! Since I have ran premium for over a year, and I don't have any knock at city speeds, how is that possible?! I would hope that there is nothing wrong with my engine. Perhaps it was due to a warmer day, as you mentioned that it is more likely to knock in hot climate.

Geno Castellano
05-08-04, 03:01 AM
I guess I don't fully understand how the noise is created. I've read of physical spark plug damage from detonation. I don't think that the flames themselves could cause the electrode of the plug to become bent, so if I understand this correctly, it is just the "pressure" causing the damage? If pressure is causing the noise, what would cause the physical engine damage?

Diesels sound terrible, one of the reasoms I don't like my friends TDI Jetta.

You hit the nail right on the head. The intense pressure spike is what causes the damage. Detonation pressures can crack spark plugs, break the ground electrode off, break the upper and second ring lands on the piston, break piston rings, etcetera in addition to causing overheating and heat damage to the piston crown and wrist pin. The combustion chamber and the piston crown are "protected" from the intense heat of combustion by a boundary layer of gases that cling to the cooler surfaces and quench the intense temperatures of combustion. When detonation occurs, the intense shock wave breaks down the boundary layer of gases exposing the piston to the combustion temperatures which are well above the melting point of aluminum. More heat is driven into the combustion chamber surface as well causing more heat to be driven into the coolant. It is sort of a death spiral. When harsh detonation at full throttle occurs the piston and chamber get hot which heats up the coolant which heats up the chamber and end gas more and thus leads to more detonation, etcetera. Not good.

Understand that the mild pinging that you hear at part throttle will usually not hurt anything. Engines are designed to be able to run with mild to heavy pinging at part throttle - there is sufficient design strength in the parts to take the detonation. So part throttle detonation is really okay as long as it is mild. It was common to tune engines to "top dead ping" for fuel economy. You "liked" to hear a little constant pinging at part throttle cruise if you were calibrating the engine for max economy. Detonation at full throttle when there is much greater heat input, however, can be very harmful to the engine. Even so, the engine will usually take some full throttle detonation before it will hurt anything. Usually, the worst case that happens is that the detonation, if it gets severe, will overheat the spark plug and induce preignition which instantly puts a hole in the piston. Detonation induced preignition the damage is due to the preignition but it would have never happened if the engine did not get into detonation. This is true of engines in production passenger cars and that level of specific output - not of racing engines. A racing engine that starts to detonate at full throttle will usually live only a short period of time before something breaks.

This is why diesel engines are so robust. Look at the pistons in a diesel. They have 4 or 5 rings instead of 3. The pistons are often cast iron on the crown with aluminum skirts to handle the intense heat caused by the shock load of the sudden combustion. Piston pins are almost always full floating pins and pistons are routinely cooled with oil squirters. Diesels are effectively built to withstand constant "detonation" because their normal combustion characteristics and pressures are so close to what a detonating gasoline engine is like.

The noise is there because older diesels injected all the fuel for that cylinder event at one time so there was a tremendous spike in the cylinder pressure at the moment the fuel was injected because all the fuel burned at once. That has lead to the greatest advance in diesel engines - the multiple injections of fuel during the cylinder event with electronic injectors and a "common rail" fuel injection system. With the advent of electronic controls on the diesel injection the modern diesels are quite quiet and smooth and make a lot of power. Some of the European performance sedans are actually diesel powered. Diesels can easily be turbocharged since they cannot detonate so you can pack a lot of air into a diesel and make huge power/torque and get excellent fuel economy due to the high compression ratio and the higher BTU content of diesel fuel.

Ralph
05-08-04, 03:18 AM
Can one assume then, that if an engine has detonation damage that it will indicate this by an occasional depletion of coolant from the tank reservoir? I know that some will be lost to evaporation, but occasionally I have to add some coolant. Not often, mind you. Even my Grand Marquis went through some. Never my 1980 Pontiac though.

Geno Castellano
05-08-04, 03:39 AM
Can one assume then, that if an engine has detonation damage that it will indicate this by an occasional depletion of coolant from the tank reservoir? I know that some will be lost to evaporation, but occasionally I have to add some coolant. Not often, mind you. Even my Grand Marquis went through some. Never my 1980 Pontiac though.


I wouldn't jump to that conclusion, no. An engine would have to detonate pretty severely for a long time and there would be definite signs of overheating as a result for coolant loss to occur. If the cooling system is steadily loosing coolant then there is a leak somewhere - either externally that's so slow that it evaporates as fast as it leaks or internally and it is consumed by the engine or oil - detonation, within reason, would not cause any coolant loss - no. Since the cooling system is closed there is very little to none lost to evaporation. There can be tiny seepages of coolant from gaskets and even the water pump seal (all water pump seals seep a little coolant by nature of their design and operation) that will go undetected due to dripping while the car is in motion or just drying up on the hot surface of the engine. That is one of the arguements for using the coolant supplement/sealer material.

Lawrence
05-08-04, 05:17 AM
Did you mean you mixed unleaded premium with leaded regular?
Sorry Geno, Yes I did.

Randy_W
05-08-04, 04:07 PM
WE mixed 'leaded' regular with unleaded premium, and I have no clue if it raised the octane.

Ralph
05-08-04, 05:18 PM
I wouldn't jump to that conclusion, no. An engine would have to detonate pretty severely for a long time and there would be definite signs of overheating as a result for coolant loss to occur. If the cooling system is steadily loosing coolant then there is a leak somewhere - either externally that's so slow that it evaporates as fast as it leaks or internally and it is consumed by the engine or oil - detonation, within reason, would not cause any coolant loss - no. Since the cooling system is closed there is very little to none lost to evaporation. There can be tiny seepages of coolant from gaskets and even the water pump seal (all water pump seals seep a little coolant by nature of their design and operation) that will go undetected due to dripping while the car is in motion or just drying up on the hot surface of the engine. That is one of the arguements for using the coolant supplement/sealer material.

Ok, thanks. I'll pick up those pellets next time I'm at the dealer. I want my coolant to remain orange however. :p

Cadillacattack
06-18-04, 02:04 PM
I use regular on a regular basis. I do notice less milage in the city but not on the parkway. There is No knock or ping at all coming from the engine even on steep grades. So I am not paying 2.39 a gallon when I can drive at 1.98 a gallon to 2.10 a gallon in upstate NY.


Picked up my 1996 Deville for 4,000.00 1/10 of original cost. Ya got to laugh if you walk into a dealer and see the prices of these cars new. 45,000.00 and up lol.

A good well kept used one with some milage, driven to and from the golf course by the former rich doctor owner saved me a fortune cause he said it doesn't have that new car smell any more as much : )