Octane rating is really a measure of how resistant to combustion the fuel is. I guess the theory of it losing power on 100 octane is that the fuel is more resistant to burning, so not all of the fuel burns and you don't get the full power out of the charge. Meanwhile on 91, your getting the same amount of fuel and roughly the same energy content except ALL of it is burning creating power.

The advantage of higher octane is you can run a higher compression ratio without knocking (detonation, which damages your engine) which is thermodynamically more efficient resulting in a power gain. If your engine isn't tuned to run higher octane you're just throwing money away and in some cases, you may be losing power.

I don't know if my first paragraph holds true for our cars, that's something that will have to be tested, but the idea is sound. If your car isn't tuned for higher octane, it won't do much other than lighten your wallet.

If the extra $8 a tank is worth the psychological benefits who am I to stop you? If you're actually trying to gain measurable speed for competition that's what practice sessions are for, although you'd probably get more time out of tire pressure adjustments or playing with your driving technique.

Has anyone mixed E85? I've been putting a bit of 100 octane in each tank to boost to 93 or 94, and was wondering if it would be cheaper to use E85. I have heard that if your car isn't set up for E85, it will cause problems.

Why are you wasting money on mixing in some 100 octane in the first place?

Your car might gain 1/8th of a WHP, but maybe not even that. Extra octane doesn't help you unless your engine is tuned and maximized for it. It certainly isn't "better" for you car. Buying quality fuel without additives (like extra ethanol) is more important than octane. I'd put 87 from Shell/Chevron in my car before 93 from Joe's Fuel Shack.

Octane rating is just the temperature at which the fuel detonates (better explanation a few posts up). It's not a "better" fuel.

Yes, I understand that (see my earlier post). Maybe it's a 'placebo effect', but it seems to me that the engine tends to hesitate slightly with 91 octane when I jump on it, and not so much with the higher mix. Maybe I'm getting crappy gas from Arco(BP), Valero, etc and the 76 100 Octane is better gas. It was your earlier post that actually got me thinking about using the E85. I'm a cheap skate, so I never use a higher octane like some folks that think it's better gas. I put regular in my wife's MDX that calls for premium, haven't heard a knock yet.

Check my post earlier in the thread, I agree, car ecu is programmed to run 93, but "learns" and compensates to run CA 91 gas by sacrificing some response (not necessarily power).

I believe you will need a tune (Ecutek, etc) to run E85 properly without messing up your engine. Had a few guys out here who have tried E85, but I guess it could be tricky finding a gas station (not as common here in the bay compared to the valley)

If you're cheap why do you spend more on pointless fuel for your car? The difference between 91 and 93 is vertically immeasurable from a performance standpoint while driving the car. Literally on a stock tuned car the dyno would read the exact same. Maybe 10ths of a HP.

Poor fuel would cause hesitation, the ECU is smart enough to know its being fed 91 and shouldn't hesitate vs. 93.

The reason you don't hear knocks in the MDX is the ECU can adjusting the timing for 87. BRZ/FR-S will do the same. Other than loosing a HP or so you can safely run 87 in your BRZ.

I know I'm necrobumping this a bit but here goes.

I subjectively feel performance and smoothness gains from 91 up to about 95 octane, above that I think it falls off on a stock tune. I understand that higher octane gas doesn't "add power" but giving the ecu the ability to run more aggressive timings does add power even on a stock tune.

I'm curious to see if anyone has any dyno results from playing with octanes on a stock tune. I think there is more gains here than people realize. I would expect at least 5-7hp gain. Timing on high compression and turbo engines can make quite a difference.

The ecu learning does make timing changes significant enough to pop CEL's if you change fuels. I was running a mix of 100/91 for about 94 octane, my wife put in some 91 and I got a CEL on my first highway pull.

Anyone have any real numbers on this?

A local guy Rick had custom dyno tuning for 91 oct and then 94 oct. His car was not stock however, it had exhaust installed.
He gained 1whp and 1tq between 91 to 94.

http://86drive.com/2014/03/01/na-brz...n-engineering/

I dont know if mental feeling or what.
I tried both, I felt that the E85 has more power than the 100 octane.