octane?

[m5james]

Originally Posted by JCL

Do what M5James did. If it runs crappy on a certain fuel, get a better one. He ran his 740 on 87 and it fell on its face. Good reason not to run 87 in that engine under those conditions. But if it ran fine on 89, then there would be no reason to put 91 in it except to satisfy the original risk management strategy of the BMW marketing department, who spec'd it to account for the widest range of variables that their engines would see in the real world.

This instance happened at a Chevron no less...some say it may not matter, but I try to stay within th Top Tier gasolines, so Chevron, Shell or Texaco. I should note that my 7 isn't stock and has internal performance mods pushing it to around 325hp, but that made it much more reactive to the lower grade fuel than a normal BMW would.

I used to fill my E34 M5 w/ Shell regularily when I was doing construction (we had an account w/ them) and he just happen to sell race gas as well. I only ran the race gas a few times, but in regards to Shell, I could actually feel the difference in using Shell vs any of the Top Tier gasoline brands at the time. I'd spoken to the owner about it, and he went into this lengthly explanation about gasoline qualities, additives, yada yada and while he wasn't trying to say Shell was the best, he said there must be something in it that clearly my engine liked better.

I agree about the whole 87, 89, 91/93 thing...to a point. If it runs like crap on 87, and good on 89, I'd still rather stick w/ 91/93 just for the added cushion and force of habit for when I do random things that'll load the engine more and possibly prevent detonation...WOT, carrying lots of passengers, elevation changes, etc. I can't think of a single manufacturer that suggests actually using 89, it seems mostly it's either 87 or 91/93. The only time I've told people to deviate from using 87 is in a vehicle where they've got clearly audible pinging (poor head design, years of poor maintenance) and going up in fuel grades until the sound disappears, citing that the pinging is not only robbing power, but ruining mileage as well. Once I explained this to my mother who tried cheaping out on gas in her 328ci, telling her she'll pay more now at the pump or pay more later w/ lower mileage, she finally listened. I used the same analogy on dyno vs synthetic oil changes...change more frequently or pay more upfront but go 2-3x as long between changes.