At the extreme, diesel fuel
>(very low octane) contains much more usable energy per unit volume
>than any gasoline. Hence the inherent high mpg figures with 
>those engines. 

I believe that the diesel engine is more efficient because the area under 
the carnot cycle is greater.  This is due to the crazy compression ratios.  
My Mercades has 22 to one, or around 300 psi.  I could be wrong, but I 
recall some of this from physics.  I don't mean to pick a fight here, but 
thought I'd rather just comment.

>So the idea is to use the lowest octane 
>fuel which does not cause trouble (pre-ignition mainly) in 
>a given engine.  And, as another correspondent already pointed
>out, this magic number may vary with engine condition, degree of 
>carbon fouling, type of plugs etc. As far as I know, premium 
>(ie high octane) fuels are seldom required except in antique
>detroit '70's iron and really old carboned up or modified 
>engines.
>
Agreed.

  Chris Bridge
  
  ''71 Westy (Sportsmobile?)
  Favorite quotes:
  "Virginia Tech- yea, the best 5, or maybe even 6 years of your life."
  "They even throw in the stomach ulcers for free." 
  E-mail: cbridge@vt.edu