Octane

[CROSSTBOLT]

Go to "SuperchargersOnline.com" and click on the thread "The Mystery of Octane explained." It seems that we all had certain valid inputs about using higher octane fuel in an 87 octane engine.

[LameRandomName]

The most important thing to keep in mind is that an engine rated for 87 octane is rated under optimal conditions and when new.

 

Honestly, it's a good rule of thumb to go up one octane rating in real world conditions, especially if you do a lot of city driving in traffic.

 

 

Plus, many engines are tuned to run on 87 and made safe that way by the use of a knock sensor. I'm actually unaware of ANY new car that doesn't do knock sensing. There might be, I just don't know about it.

 

I have found, with several different vehicles, that running 92/93 octane gas lets the engine management systems run the timing up enough to make a small but noticible difference in the seat-of-pants feel and a measurable difference in gas mileage.

 

On my 2000 Forester I have actually seen a 3mpg difference going from 87 to 93.

 

 

Finally, you can actually make your own octane booster at home. Well actually, you don't make it, you just by Toluene at the hardware store and add it to your gasoline.

Since the gas companies already add toluene to gas to enhance octane you should stay below a 4:1 mix (20% added toluene.

If you're in a country that has that cheap 80 octane straigh gasoline, you can go 2:1 (33.333% added toluene.

 

Here are some links you may find to be interesting:

 

Rocket Fuel FAQ

 

An Audi Site

 

A Turbo Buick Site

[loudsuby]

I have noticed many of the same changes. I never took the time to measure them, but there was a difference in my 90 leagacy.

[CROSSTBOLT]

The one thing I failed to consider was the timing varying at the command of the computer from the input of the knock sensor. What I said was true with fixed timing (P&W 985) and centrifugal/vaccuum advance/retard timing. Everything changes with knock sensors and ECU.

 

That was very interesting stuff about toluene. It would be more reliable than MAYBE getting 93 out of the ground at the quick-stop.

[LameRandomName]

For my Forester, I'll just continue using the premium gas at the pump, which gives me slightly better mileage.

 

For my hotrod though, the toluene method is for damn sure cheaper than buying 100 octane CamII at the station down the road.

[sprintman]

I use 98 octane and nothing else. It actually works out cheaper with the additional mileage.

[DerFahrer]

My Legacy likes 89 octane, 93 actually makes it run worse! My XT likes 93, but I'm pretty sure its disty is one tooth advanced, so when I fix that, I'll re-evaluate it.

 

Here is the simple rule I follow: the higher the octane, the more energy is required to burn the fuel. Cars that typically require premium fuel have higher compression ratios (or forced induction) and more advanced ignition systems. Cars designed for 87 do not have these features and may not run as well on high test gas...

[CROSSTBOLT]

Ok, Sprintman. How much does each fuel cost, 87 and 98 octane, and what kind of mileage do you get on each?

 

[sprintman]

We dont have 87 octane. I think 92 is our lowest. 9 cents a litre difference between them. Oh and never buy fuel with Ethanol in it. Low mileage and other problems too numerous to list