What happens when the knock sensor sees detonation? Backs off on timing and increases mixture... My guess is that if you are somewhere around 91, 92, 93 octane, your ecu will compensate. If you run crap 91 cali pump gas, you probibly won't see the entire "300 crank hp" and "300 tq" Suby clames the STI will have from the factory because of the detonation compensation.

I think the technical stuff is great (Im a chemical engineer) but I also don't think that the 2 octane difference will make effect your engine's lifespan.

Seriously, I love seeing these technical discussions above... for the tech geek in all of us its really interesting.

 

There is almost no way to get an accurate mixture (or even have a rough idea) of what the final product of the mix might be. The gas gauge itself is ridiculously inaccurate (for instance, at lowest "E" my tank has between 2.6 and 1.2 gallons left in the tank, and the light comes on within a 2-gallon range before that) so you've got no idea what you are starting with. You don't want to actually drain the tank, as even a year-old car has a good buildup of sediment on the bottom of the tank that you don't want sucked into your fuel line and delivered to your injectors.

Those facts alone are why mixing grades and adding octane boosters (or incredibly toxic, cancer-causing chemicals like xylene or tolulene) is junk science, and everyone who thinks they have the "magic formula" is fooling themselves.

 

Now, using 91 won't harm your engine--it just prevents it from working optimally. So what are the benefits of using a higher octane? Well, the UK tv show Fifth Gear did a test with three cars and one of them was a 2006 STI.

I was from a new episode in November where they compared the bhp and torque of a 2006 STI with different octane fuel. Actually, they tested three cars: a Clio, a GTI, and an STI. Obviously, since the Clio wasn't designed for higher octane, there was no difference.

I don't know how to convert the octane ratings they use in the UK, this is the gas they used: Supermarket (95), BP Ultimate (97), Shell Optimax (98). The Shell must be like the 93 or 94 we get here.

Anyway, here are the results for the STI:

Supermarket: 235 bhp, 235 ft lbs.
BP Ultimate: 248 bhp, 248 ft lbs.
Shell Optimax: 249 bhp, 258 ft lbs.

You can watch the episode yourself by downloading it here:

http://www.finalgear.com/