JamE55

Was just wondering if there's a limit as to how high of an octane rating can you put in your gasoline tank? I'm currently runing 99% 101 octane which is great and all but was just wondering if i can go higher lets say a 110 or 120+?

E552006

The issue is not the octane as much as if it is a true unleaded race fuel. Some of the fuels with octanes that high have lead in them. That is bad on O2 sensors. Other than that I would think that at levels above 104+ octane you will being playing with the law of deminishing returns.
I really doubt that the stock ECU will deal any differently with octanes that high. If you had an aftermarket ECU then it could be programmed with different fuel tables for higher octane.

JamE55

So are you saying that even with higher rating they have certain mixture of lead in them? Yes i know it's bad for the O2's. But is there any adverse effects to running higher? And i'm not running stock ecu. It's a custom one that will handle higher. I was just wondering if there's a limit as to how high i can go without causing any problems.

Fast55

James, you can get pretty high octane unleaded, but the most performance you'll get out of anything is to use the LOWEST octane the car is happy with. There definitely IS a point of diminishing returns. The higher the octane, the LESS the fuel wants to burn. Beyond the point where the ECU can add timing and fuel with good A/F's, you will begin to lose power. I would be surprised if anything above 104-106 would be a help UNLESS Kleemann were to tune your car specifically for that high of an octane rating.

JamE55

Brad,

Thanks for the info and i think i know what you're getting at. I was just experimenting with the idea about running higher to see if there's a little more power it will give out. The ecu is tuned at 91 but it's also good for running at higher rating.

E552006

James
There is a good point made here.
Higher octane fuels burn SLOWER. That is why they resist detonation better. There is less energy in 104 octane than plain old 87 octane fuel. Higher compression engines and the use of forced induction requires higher octane due to the higher cylinder pressures made by these engines.
"octane ratings measure a fuel's ability to resist the spontaneous ignition of unburnt end-gases under controlled test conditions. "

Josh

E552006

The only advantage you could gain in an ECU tuned for higher octane would be either by adding more boost or if there are specific fuel tables for higher octane...I doubt any tuners are writing tables for anything above 104.

JamE55

Thanks for the info Josh! Think i'll just stick to the ole $5.21per gallon 101.

medici78

Cool, that's only $2/gallon more than most are paying for premium!
I wish we had a distributor for that kind of octane in my town. The highest octane available here is 91 octane ****.

AMGE

I agree with 5506, anything over about 100 octane is not helpful and in fact could be harmful by washing down cylinder walls with fuel and or additives that does not burn properly, and or cause emmissions realted problems. In order to capitilize on higher octane the compression would need to come up a couple points and then you could not run on anything less then 100 or more octane.
With an expensive complicated computerized emissions system I would say you are playing with fire...no pun intended.

Grumpy666

Under your worst-case driving conditions (max boost @ high temp, etc), use the lowest octane rating that still prevents the ECU from pulling timing. This will give you the highest output for your current configuration. Any higher octane w/o adding timing will begin to diminsh power. As was mentioned above, octane rating indicates the fuel's resistance to ignition, and it burns slower. You want maximum cylinder pressure to occur at about 10-15 degrees ATDC on the power stroke. If the fuel burns slower, this maximum pressure point occurs when the pistion is farther down in the cylinder, which will reduce the amount of power delivered to the crank.

PTE

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Fast55

Exactly what I was refering to. The required octane also goes down as altitude goes up, so you guys out in Denver shouldn't be wasting your money on 94 if the car has no problem on 92 (it should be fine). This is precisely the reason mfgs. will advise against putting higher octane in yer' grocery getter. All that happens is the car makes LESS power and get WORSE mileage. It can and does cause driveability problems in cars that don't need high octane.