SundayDriver

I just read yet another thread about fuel octane (this time over on the GT3 board) - filled with misinformation and BS. I am not going to post over there - way too much BS but I thought you guys might like some real info on octane and what it does and does not mean. FYI - Waaay back in the stone age, I did fuel research as an automotive engineer.

Octane is a measure of a fuels resistance to detonation. Nothing more! But in one sense, octane rating is not consistent. The test methodology is to run the fuel in a test engine. (This info is based on my work many years ago. I am assuming the test method has not substantially changed.) The test engine is a single cylinder engine running at VERY low rpm. The head is adjustable to vary the compression ratio. The test method is to run the engine and increase the compression ratio until trace knock is detected. There are two reference fuels defined at 0 and 100 octane. A blend of these fuels defines other octanes. 50/50 would be a 50 octane fuel. Over 100, for example - 103 Octane was defined as the knock resistance of 100 octane plus 3 grams/gallon (as I recall) of TEL (lead). Test conditions are tightly controlled - air temps, humidity, etc.

Now here is where it starts to get really messy. There are two sets of conditions. These were designed for one to more closely simulate the conditions of a manual transmission vehicle while the other was close to an automatic trans. These were RON and MON. Since most cars were autos, that was the number published at the pumps. Enter the EPA who decided, in their wisdom, that we should use and average of the two. So you take a particular fuel and test it for both RON and MON the average to get the number on the pump. Are you getting the idea of one major issue? Two different 93 octane fuels will not necessarily have the same RON and MON - only the same average. So one may be more resistant to knock in your car, but the other does better in my car. Not all 93's are created equal, though they are not going to be terribly far apart.

As far as HP, knock is actually a good thing. More energy is released under knock (detonation) conditions, than normal combustion. Top fuel dragsters run under sever knock, or explosive, conditions. But knock is VERY hard on parts - one reason that these same dragsters change pistons almost every run. You must stay out of knock conditions for the motor to survive.

Remember that I said the tests are at controlled conditions. This is because the octane requirement of an engine changes with load, rpm, inlet air temp and inlet air humidity as well as internal temps. The sensitivity of one engine is not the same as another. That means you could have two different designs that 'require' 93 octane and both do fine. But at 90 degrees ambient, one is knocking and the other is not (same operating conditions). Now at temps go up, octane requirements go up. With less dense air (high altitude) octane requirements drop. This is why high altitude areas have lower octane fuel - that is all you need. Humidity (or water injection) reduces the octane requirement - that is why some high performance cars ran water injection (it also cools the inlet air making it denser but that raises combustion pressures and increases octane requirements).

Too much octane does not hurt an engine. But do not confuse octane with other factors. To get that higher octane, other compromises may be made - BUT is is not the Octane that can cause problems it is the other thing. For example, racing fuel is high octane. It is blended for best full throttle performance. There is little concern with throttle response, so that is sacrificed in race gas blending. We also don't care about cold starting - if you need help in a race car, jump it or spray starting fluid. Street fuel is blended for these and other factors. What that means is that some race fuels will not feel very good driving around town - the car will feel sluggish in its throttle response.

So, that is my treatise on Octane for a Saturday morning. Now to do work on the race car.

38D

Thanks for the info. Question for you. I run 100 octane in my C2 Turbo. I do this only to help eliminate any potential for detonation. Is this a waste?.

SundayDriver

When I had my TT, I blended about 50/50 100 octane and pump premium for the street and only ran straight 100 at the track. The lower price fuel was just not worth the risk of detonation, as I was in CA. If 93/94 had been available, I would have run that on the street, unless it was really hot weather. You are proabably running more octane than you need, but better to have too much than too little. If the 100 is typical 76 or equivalent, it is still a good street blend.

Bill L Seifert

Mark,

Glad you brought up the subject. I heard that 87 octane actually has more energy per gallon that 93 Octane. That always confused me, so because you seem to understand it. Does 87 have more energy than 93, or was whoever said that feeding me a line of bull?

Bill Seifert

1987 944S Race Car

SundayDriver

Bill, it depends though that is often true. If the 93 was obtained in part with more ethanol, then that will be true. Ethenol has about half the btu per gallon vs. gasoline. It also needs to run about 2:1 richer so when you run pure alcohol it balances out (actaully alcohol will win out for other reasons). In our cars, if there is more alcohol, the car can not adjust at full throttle, you run leaner and also with less power.

So the answer is yes (usually).

Joe Weinstein

Hi. To a degree it is true, but this is understandable because the ability to detonate is
related to the amount of potential energy availabe. Tap water has an infinite 'octane
rating' because it will never detonate. Dynamite has more energy per volume than
gasoline... However, if getting all the energy in 87 octane gas (or dynamite) is only
possible by burning it at a lower compression ratio or detonating it, then the energy
is relatively unavailable for non-damaging engine use compared to 93-octane.
Joe

Steve Weiner-Rennsport Systems

Hi:

Great topic and quite complex due to all the variations in pump and racing gasolines.

You'll find some further information at http://www.rennsportsystems.com/2a.html

RJay

Question or two just out of curiosity Mark. You mentioned that the fuel rating is different between autos and manuals. Is this due to engine load and the torque converter? And if theres an impact on octane, does it work in favor of manuals or against them? ie. which is higher the RON or the MON?

SundayDriver

The typical auto trans vehicle runs at much lower rpms due to the early shifting. The specs for RON were intentionally designed (this goes way back) to be closer to those conditions while MON was more aligned with manual trans operating conditions. RON always produced a higher number, as I recall.

So it is not that the rating is really different for auto vs. manual, but the test conditions tended to be biased towards one or the other (RON vs. MON). However, manual trans vehicles tend to run at higher loads at the lower rpms as there is no automatic downshift.

Bill935K3

HI Sunday,
Always enjoy reading about this subject. Here is an interesting side with many calculators.
http://www.turbofast.com.au/javacalc.html
Way back I used it to come up with compression under boost. At the time I think I came up with 7.5 to 1 compression plus 1.2bar of boost came in to a total compression ratio of 14+ to 1. And a need for a minimum of 105 octane fuel to be safe.
As far as race fuel power goes I remember my short track friends always trying get away with the use of oxygenated race fuels. As you have said Octane creates power only by the fact that you can run optimum mixture without detonation.

JW in Texas

Bill,
That sounds about right. It has been a while since I figured this stuff for my turbo carrera but it seems like it was each 1/10 of a bar = .5 compression point (ie; 1.2 bar bumps compression up 6 points over static).

Nordschleife

Where people run their turbo cars at maximum speed for long periods (like Germany), there are real benefits from using the best available fuel (read 100 RON). A bad load of fuel can destroy turbos (and manifolds) in these conditions, if the motronic unit can't cope with what is happening. I have had major replacements under warranty and when I asked what caused the problem I was told 'bad fuel', the engineers felt that they should have programmed the engine to cope with the problem - the joys of German angst!
One of my tuner friends, who is vastely experienced and very careful, uses quite different programs for people who live in areas with great heat or lower octane fuel, a combination of poor fuel and heat soaked intercoolers can prove quite problematic as some German drivers have found when they have driven hard and fast from Germany into southern Italy on hot days.
With turbos, I don't notice the difference between 98 and 100+ RON most of the time, but if I am fast running then I do notice that the engine pulls harder at the top end with the higher octane gasoline.
There is another theory with fuel, the tuner I mentioned above is very close to a new gas station. The gas station is the first stop for the tanker when it leaves the refinery. Tests show that this gas station's 98 produces as good results on the rolling road (which is provided with huge air fans for the intercoolers) as 100 does. However, I still get them to tune usinng 100+ as this is now freely available throught most of central Europe (well its not free, but you know what I mean).

R+C

Honkity Hank

Thanks for posting this Mark. I never new how MON and RON differed, and never considered that diffferent mfr's will blend differently. Be nice to know who's gas is better for track conditions. At Road Atlanta for example we have a choice of Sunoco (at the track) race fuels, or if you run down the street, BP, Racetrak, Standard. I wonder if one of the off track retailers is going to be better?

38D

I'm just not sure I buy that 100+ has any performance advanatge, unless the engine was specifically built for it. I've tested this once. Back in 2003, DrJupeman and I brought our identical Boxster Ss to Watkins Glen. He ran 93 and I put 100 in my car as a test. There was absolutely zero difference in straight line speed. What I took this to mean is that the computer may retard the timing to cope with knock from "bad gas", but it does not advance the timing to handle the better stuff. I do put 100 in my turbo for track use, but only to avoid any possibility of detonation. I have run 93 in it, and there's no difference in pulling power.

DrJupeman

Fwiw, I'm running 97 octane on the track with my turbo (mix of Sunoco 94 and 100).