: Octane

10-03-12, 08:31 PM
Hey everyone just wondering if anybody knows the proper mixing ratio if I was to mix vp racing fuel 100 or 109 octane with my 91 octane here in Cali what ratio would I need to achieve a 93 octane rating??

larry arizona
10-03-12, 09:36 PM
for every gallon of 110 it will raise you one octane point. so 2 gallons of 110 will turn 91 into 93.

10-03-12, 09:54 PM
Wow that would get expensive to mix enough to make 93 octane. Thanks!

10-03-12, 10:04 PM
Get a catch can, that'll bump your octane up very slightly.

Gary Wells
10-03-12, 11:55 PM
Generally, anything above 100 octane is leaded and that will play havoc with your cats in short order. 93 octane will not give you any more RWHP or RWTQ than 91 will. I ran 100 octane VP unleaded to find this out.

10-04-12, 03:17 AM
Hand-Dandy Octane calculator http://www.bazellracefuels.com/Calcs/OC1.htm

4 gallons of 100+14 gallons of 91=93...:thumbsup:

larry arizona
10-04-12, 06:50 AM
running a couple gallons of leaded per tank will not harm your cats or o2 sensors. i have run at least 50 gallons of 110 leaded with no issues.

10-04-12, 07:45 AM
Gary.... You are saying the extra money for 93 octane over 91 octane is a waste???

Gary Wells
10-04-12, 10:00 AM
Yes, I experimented with this very same subject about 2 years ago and came to the conclusion that there was no "seat of the pants" difference in RWHP and / or RWTQ. Not without a tune anyhow. I have heard from someone that did do before & after dyno runs and found no difference between the 2. With a tune. they picked up,I believe 16 RWHP / RWTQ.
I ran 100 octane straight for about a month but just decided that with dyno variations, even with the same dyno, that a after dyno run was just going to be an additional cost.
Definitely smells great though, and changes the tone of the exhaust slightly.

larry arizona
10-04-12, 11:54 AM
With tunes 45-50 HP on an E85 tune is pretty easy so I am sure the same can be done on 110 leaded. So octane and tune is required. I have a tune with 2 degrees more advance and you can absolutely tell the difference, but I have to run 95+ octane mix to stay out of KR.

Lead wont plug your cats but overtime it could coat the platinum enough to lessen their catalytic effect, but only really matters if you are environmentally concerened. I suppose your 02's could eventually get lazy but they can be replaced and adding a couple gallons per tank won't hurt them for a LONG time.

10-04-12, 12:19 PM
Generally, anything above 100 octane is leaded and that will play havoc with your cats in short order. 93 octane will not give you any more RWHP or RWTQ than 91 will. I ran 100 octane VP unleaded to find this out.

That's not really what higher octane does, or how octane works in general. You can't make blanket statements about what higher octane does for your car because it's different for every car in different parts of the country, as it's dependent on atmosphere, temperature and elevation.

Higher octane raises the compression threshold in your cylinders to make sure the spark is what detonates the gas, and not the compression of the gas itself. When the gas ignites due to compression, that's detonation. Preignition is very, very bad, and higher octane prevents it by raising the compression threshold. When the gas ignites by compression rather than the spark, that's what causes the knocking sound you hear shortly before your engine explodes.

These days our engine electronics have the capability to increase or retard ignition timing electronically, so if the knock sensors detect preignition the ECU can retard the ignition timing which creates a safety buffer and prevents early fuel ignition, or preignition. Retarding the ignition timing also has the effect of reducing horsepower.

This is where the misconception of higher octane raising horsepower comes in: IF - and only if - your car is suffering from preignition due to bad gas or abnormally high cylinder temps, your ECU will retard your ignition timing and you will lose some power. This is the only scenario in which going to higher octane will add more horsepower - or more accurately restore the horsepower that was lost when the ignition timing was retarded. When you add higher octane gas in this scenario, the lack of warning from the knock sensors lets the ECU gradually increase ignition timing again until it's back at full strength. The net result is that you have gained horsepower, but you have not added any additional horsepower on top of what you were supposed to have.

Being in Vegas, the likelihood of preignition is probably higher for me than for most of you. Not only is it really hot here in the summer, but we're also stuck with the CA/NV/AZ terrible oxygenated maximum 91 octane gas, the worst in the country. Our 91 is probably equivalent to, say, Sunoco's 89 on the east coast. So it's the worst of both worlds out here. Despite that, my engine isn't knocking, I'm not getting any preignition or detonation. But I can't be sure that my timing hasn't been retarded, because I have no way to tell if it has been without a timing light.

If my ECU is retarding timing, then I will gain horsepower by going with a mix of 100 and 91. If it's not, then I won't. I'm guessing it's fine though, because stock manufacturer tunes are usually very conservative for this reason. So for anybody on a stock tune it's probably safe to say that as long as you're running 91, you're already running well and adding higher octane won't do anything.

All of this goes out the window when you start tuning. When you do a custom tune, it's based on the quality and octane level of the gas in your tank at the time, in addition to the ambient temps and elevation. A good tune will maximize ignition timing without inducing preignition. But if you fill your tank with 100 and get it tuned, you will get more power out of it because the tune can be a lot more aggressive since the higher octane is raising the flash point of the fuel in the cylinder, allowing for more timing advance. The downside is that the margins are much slimmer, and if you're tuned for 100 and start running 91 again, your ecu will retard timing to make sure your engine doesn't blow up, and you will lose power. And if the ECU can't compensate, you will start detonating.

So the correct answer is that 93 or 100 octane CAN give you more power, but in most cases you will need a custom tune to actually get the power out of it. 93 and above octane can also restore any HP you've lost if you were pinging at any point and your ECU retarded your timing. But if your car is running 100% on 91, then no amount of octane will add horsepower.

I will say that in my experience I have been able to get additional dyno-proven horsepower from using 100 octane in SoCal on my completely stock Subaru WRX (turbocharged), simply because Subaru's stock map was ridiculously conservative AND because the quality of the gas in CA/NV/AZ is unbelievably, almost criminally terrible. Just raising the octane level allowed the ECU to push up the ignition timing and boost pressure, and the car dynoed about 20 AWHP higher than most of the other stock WRXs at that particular shop. I don't think the CTS-V has that problem, this engine is less high strung, less prone to detonation and more accepting of lower quality gas, but it's worth keeping in mind.

So my advice to the OP is that if you want to mix fuel to get a 93 equivalent rating, make sure your car can actually make use of it first. If you're already running fine and your ECU isn't retarding timing, then higher octane won't do anything for you.

10-04-12, 02:42 PM
Well done, Xaqtly. Best explanation of what octane is, and actually does, I've read...

10-04-12, 03:01 PM
Well done, Xaqtly. Best explanation of what octane is, and actually does, I've read...

Agreed. Nailed it to a "T". Good to have accurate info posted here.


V Wagon
10-04-12, 03:35 PM
The original question is ridiculously simple to figure out without some online calculator. You can do it right at the pump, and it works for whatever fuel octanes you happen to find.

Defining parameters
O1 = Octane of fuel 1
G1 = # of gallons of fuel 1
O2 = Octane of fuel 2
G2 = # of gallons of fuel 2
Ot = Total Octane

Ot = [(G1*O1)+(G2*O2)]/(G1+G2)

This can be rearranged to suit what you are looking for. So, you are trying to achieve 93 and know you have 91 and 100 to work with and say you know you need 15 gallons of gas, you can figure out how many gallons of each you need.

93 = [(G1*100)+(G2*91)]/15
93 = {[(15-G2)*100]+(G2*91)}/15
1395 = 1500 - G2*100 + G2*91
G2*9 = 105
G2 = 11.6666666 gallons of 91
G1 = 15-11.6666666 = 3.33333333 gallons of 100

If you happen to pull up to a station and need to work with 87 octane and 100 octane or whatever you may happen to encounter you can do that math to reach the desired octane by switching the numbers.

10-04-12, 03:39 PM
Yeah. That does look ridiculously simple. Much easier to solve than copying the URL to my iPhone browser, and using it at the pump. Now where's I put my slide rule???:bigroll:

V Wagon
10-04-12, 03:49 PM
for every gallon of 110 it will raise you one octane point. so 2 gallons of 110 will turn 91 into 93.

That's a really misleading statement, using the above, 1 gallon of 110 would raise the octane of 8.5 gallons of 91 to 93. Beyond that it really depends on how much fuel you need. In a V with an empty tank 2 gallons of 110 would be pretty close to giving you 93. But if you've got a supercharged Suburban and need 93 you're going to need a lot more 110. And if you are in a V and don't have an empty tank you won't ever need 2 gallons of 110.

larry arizona
10-04-12, 04:22 PM
I was speaking specifically on the V and its 18 gallon tank.

(# gallons x 91 octane) +( # gallons x110 )/ 18 gallon tank = octane rating of mix.

example 16x91 + 2x110/ 18 = 93 octane.
example 15x91 + 3x110/ 18 = 94 octane
example 14x91 + 4x110/18 = 95 octane

So its pretty easy AND convienent that our V has an 18 gallon tank and for every gallon of 110 you add to (any octane gas) bumps the base octane 1 point.

example 16x93 + 2x110/18 =95 octane

Pretty easy......

Yes I took the assumption that you would fill your tank with the base fuel and add race fuel based on the octane you desire and every fill up after that would net you slightly higher octane based on that formula if you mixed race gas every fill up. Its the easiest way I can figure it with my walnut brain.