Octane - Fuel Economy, Hypermiling, EcoModding News and Forum

hyperyaris · 07-28-2009, 01:42 AM

All a higher octane rating does is cause the fuel to burn more slowly. Sure, this can help one to advance or retard the timing, but octane alone offers no benefits whatsoever by itself, except a lighter wallet.

2000mc · 07-28-2009, 01:44 AM

thanks

SVOboy · 07-28-2009, 01:48 AM

Yes...In general one should not put things in their car their car was not designed to burn.

mobythevan · 07-28-2009, 03:30 PM

But higher octane has a useful purpose.

hyperyaris · 07-28-2009, 04:09 PM

Quote:

Originally Posted by mobythevan

But higher octane has a useful purpose.

Its only purpose is to deal with timing changes, and yes, that is useful to some I agree.

mobythevan · 07-28-2009, 04:18 PM

It is not that simple. Higher octane allows higher compression which in turn allows more power per unit of fuel to do work and not get wasted out the exhaust or as heat into the engine cooling system. So an engine built with low compression cannot really make use of high octane fuel like you say. However, an engine with high compression can use high octane fuel more efficiently and produce more useable power per unit of fuel. Higher octane has a purpose. Slow burning fuels allow more power to be useable and not wasted in a piston engine. Quick burning fuels (low octane) lead to more wasteful piston engines. It is more of an engine design question than what fuel to put in your tank.

hyperyaris · 07-28-2009, 04:26 PM

Well, I can agree with that. I guess what bugs me is people who drive cars designed for 87 who think that 93 will "give them more power." No, it won't.

mobythevan · 07-28-2009, 05:27 PM

Yeah, that bugs me too. The only possible way they would get more power is if the computer has been pulling timing because of knock with low octane and then they put in higher octane and the timing is allowed to advance by the computer. I doubt that happens very often.

chuckm · 07-28-2009, 06:16 PM

The volumetric fuel value (in btu/gal) is almost identical from grade to grade (not accounting for differences in ethanol additions). For many years, I had thought that premium had a slightly lower volumetric fuel value, but the difference is maybe 1%.

Big Dave · 07-28-2009, 07:26 PM

Actually, modern cars have knock sensors that automatically retard the ignition if you are running a gasoline with low enough octane to knock. The more retarded the ignition, the less HP and less FE.

The FE effect is not overly dramatic. I'm baselining a 96 Impala SS with a LT-1 Corvette engine. FE drops off about 1.5 MPG on 87 octane vs 94 octane. The power difference is quite noticeable.