: Which octane level to use?



iwinagn
02-09-07, 11:02 PM
The owners manual states that using 91 and higher octane is suggested but it also states in the same paragragh that you can use 87 octane. What do you guys suggest? I ran 89 in it today no pinging at all.. I read somewhere though that 91 or higher can help with mpg is that true? Not that i really care about gas milage but every little bit helps right! On the expressway today i was running 70-75 and was getting around 20 mpg is that about right you think? It does only have 217 miles on it i assume it will get better after it breaks in some.. thanks for any future help!

ctsvett
02-09-07, 11:06 PM
I would stick with premium (91 or above) an only use something else if you cant get premium....

The fuel maps will change if you use lower octane and you will (I have on other cars) fell a performance difference between 91 and 87... Also if you have a tune, thats a big NO NO

My opinion.

Reed

iwinagn
02-09-07, 11:14 PM
I would stick with premium (91 or above) an only use something else if you cant get premium....

The fuel maps will change if you use lower octane and you will (I have on other cars) fell a performance difference between 91 and 87... Also if you have a tune, thats a big NO NO

My opinion.

Reed

Reed
thank you I will only use 91 or above from now on then.. I wasnt sure so i thought i had better find out from the guys that have been around these cars. I did search but came up with nothing.. How bout the mpg am i right that once it breaks in some i should see some increase on the eway? Thats where most of my driving is i try to keep it right around 70 mph and i was seeing about 19-20mpg

Florian
02-09-07, 11:39 PM
always run the good stuff...your fuel maps will retard timing if you use the garbage and you wont be getting all the ponies from your V.


F

rand49er
02-10-07, 09:14 AM
Using a lower octane fuel will, under certain circumstances such as higher throttle openings, cause your PCM to retard the timing to avoid knock. If the timing is retarded, your fuel economy and/or performance will be affected detrimentally. I just bite the bullet and run 93 all the time.

For the purposes of comparison, as an old man, I've averaged 21 MPG overall since I bought the car over two years ago. (Somebody's gonna get a real creampuff from my estate sale someday. :D) At a steady-state 70-75 MPH, I get about 23-24 MPG. BTW, I used to get 25-26 at that speed before I installed the headers -- apparently, the stock manifolds scavenge better at typical road-load conditions.

ctsvflorida
02-10-07, 10:30 AM
That is very low for 70-75 mph but it is probably because your car is brand new. My mileage barely changed with the headers, if at all! At 75mph I will get 24-25 mpg with the air conditioning on! For the past 3 months my ave. mpg has been 20.4 and driving around town it is horrible to say the least!

The octane thing is really a no brainer sir! Highest you can buy at normal pumps!

HiTechRV
02-10-07, 12:45 PM
I believe the PCM is actually smart enough not only to retard timing with knock, but also to advance it. I run 93 or 94 octane most of the time - whatever the maximum available is.

On a dumb engine with no knock sensors and a dumb controller, you want to run the LOWEST octane that does not ping for power ind mileage. On a modern engine like the LSx, I'd go for the highest.

Luna.
02-10-07, 02:24 PM
Also, I don't believe that using a higher octane fuel will result in any significant difference in gas mileage

rand49er
02-10-07, 02:36 PM
Also, I don't believe that using a higher octane fuel will result in any significant difference in gas mileageCorrect!

Octane is simply a measure of a fuel's resistance to knock; by itself, it has nothing to do with a fuel's energy content.

The opposite of octane is cetane, a measure of a fuel's propensity to autoignite, which characterizes Diesel fuel.

HiTechRV
02-10-07, 03:21 PM
Higher octane allows more timing advance. Timing advance allows for more power per gallon, OR fewer gallons for a fixed power level.

So at WOT, higher octane will not get you better mileage. But at a steady speed, the power required is constant, so higher octane *in concert with advanced timing* will actually burn less fuel, since the advanced timing is getting you more out of the fuel you are burning.

On a car that cannot advance the timing to take advantage of the octane, higher octane actually robs you of a small amount of power and mpg.

50 4Ever
02-10-07, 03:29 PM
On the mpg question, in my Corvette I averaged 30.6 MPG with the cruise set on 85 mph from Rock Springs Wyoming to Salt Lake City using 93 octane. The highest I can get in CA is 91 octane and can only get about 28 mpg at 75. If I could get 93 octane here I would buy it.

:thumbsup:

rand49er
02-10-07, 03:50 PM
Higher octane allows more timing advance. ...However, advancing the timing does not always mean more power.

Ignition for gasoline piston engines is nearly always in terms of BTDC (before top dead center), in other words, before the piston has reached full compression. Advancing the timing too far causes the pressure resulting from the explosion in the cylinder to increase too early thereby reducing power.

Let me repeat, octane by itself has nothing to do with the available energy in a given quantity of gasoline. (I used to work for Ethyl Corporation as a young engineer testing engines on engine dynomometers doing WOT, LBT, MBT fish hooks ... and we did this stuff all day long every day.)

Luna.
02-10-07, 05:37 PM
Rand speaks the truth!

I recall reading several articles on the subject and they all echoed Rand's sentiment. One analogy I recall was, "Octane is like running shoes. If you wear a size 10, wearing a size 12 running shoe isn't going to make you sprint any faster."

Chef
02-10-07, 05:40 PM
race fuel - daily driver:stirpot:

124turbo
02-10-07, 06:54 PM
use the good stuff

50 4Ever
02-10-07, 06:59 PM
race fuel - daily driver:stirpot:

That's Chef for ya, always stirring the pot... :thumbsup:

I can tell the difference between 91 octane and 100 octane on the track. I can also notice the difference when I have a 200# passenger riding with me.

:stirpot:

cts-v ls6
02-10-07, 09:27 PM
I have 95k miles on my V, and I always use the cheapest "Walmart "
gas I can find. It's true that 91 0r 93 octane will net you 1-2 mpg more, but if you're running your V on the highway 52k miles per year like I am, who cares?! The V has 2 or 3 different air/fuel tables in the PCM, and yes, it retards timing depending on the load seen and the octane available. But with a StealthV tune, I can stand on it, and no ping, no detonation. I do have water methanol injection which helps boosts the octane at WOT, but I've never used high octane and never will, unless I know I'm going to race.

Jack

Chef
02-11-07, 10:55 AM
That's Chef for ya, always stirring the pot... :thumbsup:

I can tell the difference between 91 octane and 100 octane on the track. I can also notice the difference when I have a 200# passenger riding (http://www.cadillacforums.com/forums/#) with me.

:stirpot:

copy that! 91 is minimum here.

Silver Dollar
02-11-07, 01:07 PM
The only reason to run race fuel is for the smell.

I love the smell of CAM II in the morning...... :)

v-ape
02-11-07, 06:56 PM
87 all day.

As for retarding timing, I'm not sure why it does it with a lower octane...

Chef
02-11-07, 09:21 PM
I love the smell of CAM II in the morning...... http://www.cadillacforums.com/forums/cadillac-cts-v-series-forum/images/smilies/smile.gif

Smells like VICTORY...
Where's Lance?

50 4Ever
02-12-07, 03:21 PM
87 all day.

As for retarding timing, I'm not sure why it does it with a lower octane...

The lower the octane rating the more volitale the fuel. Sometimes in high compression engines, low octane fuel will ignite by itself without the spark, that is usually where knock comes from. I would imagine that by pulling timing out, or retarding it, you are trying to get the fuel to ignite at or a little after TDC (top dead center) of the piston stroke.

:alchi:

v-ape
02-12-07, 03:42 PM
The lower the octane rating the more volitale the fuel. Sometimes in high compression engines, low octane fuel will ignite by itself without the spark, that is usually where knock comes from. I would imagine that by pulling timing out, or retarding it, you are trying to get the fuel to ignite at or a little after TDC (top dead center) of the piston stroke.

:alchi:

so by retarding the timing, you're doing it AFTER the compression (when it would have pre-ignited)... thats the part that confuses me. TDC would seem to be where the mixture is most compressed and therefore most succeptable to pre-ignition. If you're igniting it there or after, then thats not the reason right?

then again, maybe I'm "retarded" :D

rand49er
02-12-07, 04:26 PM
Unlike the Diesel engine which burns (ideally) at constant volume and coincidentally was invented by Rudolf Diesel, the spark ignition engine invented by Nicolaus Otto burns (ideally) at constant pressure. Now, there is an optimum time or point in the four-stroke engine cycle to initiate spark at which torque output is maximized. MBT, or minimum spark advance for best torque, occurs at some number of degrees before the piston reaches top dead center during its compression stroke. Unlike the Diesel engine where the explosion occurs almost simutaneously in all parts of the cylinder with a sharp "pinging" sound and occurs almost at constant volume (especially in open-chamber configurations), in the gasoline engine a flame front begins at the spark plug and proceeds away from the spark plug toward the far reaches of the shrinking cylinder cavity as the piston is reaching top dead center and proceeds back down during its power stroke. (This combustion process occurs roughly at constant pressure.) Advancing the timing beyond the MBT point gets you no additional power, by definition, and advancing it too far results in reduced power. And, this advancing of spark timing will risk the tendency of the end gases (those at the far reaches of the cylinder which haven't yet been reached by the flame front) to be pressurized to the point where they auto ignite, i.e. "knock."

You don't want knock in a spark ignition engine because it wasn't built to withstand the very high rates of pressure rise associated with auto ignition. Diesel engines are built beefy (and consequently heavier) to withstand these sudden rates of pressure rise. Further, the primary reason Diesel engines offer better fuel economy is twofold: they have higher compression ratios (typically in excess of 13 to 1) and they are unthrottled. They would be even that much better than spark igition engines at achieving higher fuel economy except Diesel fuel does not contain as much energy per unit weight (or volume <-- not absolutely sure about the volume one).

There's more, but I'll save that for another time. :alchi:

v-ape
02-12-07, 04:45 PM
^^^ gotcha

ctsvflorida
02-12-07, 06:15 PM
Higher octane allows more timing advance. Timing advance allows for more power per gallon, OR fewer gallons for a fixed power level.

So at WOT, higher octane will not get you better mileage. But at a steady speed, the power required is constant, so higher octane *in concert with advanced timing* will actually burn less fuel, since the advanced timing is getting you more out of the fuel you are burning.

On a car that cannot advance the timing to take advantage of the octane, higher octane actually robs you of a small amount of power and mpg.

I agree 100% with this and don't really know what all the technical talk is actually saying here (yes Randy, you and the other guys, lol) but I can say that, for all of the non-believers...please put 87 octane in your car and drive the same speed, same way, or whatever test you'd like, and then use 93 octane and let me know if your gas mileage is different!? I will assure you that it will be better.

50 4Ever
02-12-07, 06:16 PM
^^^ gotcha

:yeah:
That makes sense. Good explaination.