>Along those lines, I have another question. I have a 1.8 Liter VW water
>cooled engine in my Lola T540. It has 10:1 pistons in it, a hot cam and two
>DCOE 40 Webers. I run 113 octane leaded. Since you run only 104, am I
>wasting money on 113?
>-
Octane is (simplistically) resistance to knock, or detonation. Compression
ratio is one of the things that influences knock, but not the only one.
Ignition timing and mixture also cause knock, and can be a reason for higher
octane gas. In showroom stock racing, race gas is often used (by
cheaters;-)) to allow the ignition to be advanced beyond normal limits, for
a gain in power.
Static compression, which is what the compression ratio measures, is a
mechanical measurement. "Real" compression is affected by the cam timing,
intake and exhaust flow capabilities, carburation and intake flow, exhaust
headers, and so on. Two good examples of how static compression is used to
overcome intake flow restriction, for example, are restrictor motors in
Winston Cup, which can run 18:1 compression or higher, and my old 1070cc
mini motor, which ran 14.5:1. It had a lousy head, so I whacked the CR up
and rev'd the hell out of it. Once, I even ran it on pump gas, with octane
booster. My theory was that if it survived, I saved on gas prices, and if
it broke, I got to build a new motor! The thing lasted three seasons, and I
retired it when a harmonic balancer exploded, and caused the crank to crack.
Your VW may have radical cam or ignition timing, or a miracle head, so that
10:1 static compression is actually fairly high. My street car has
10.5:1,and I run it on regular unleaded, and some Jaguars had 11:1 stock -
so static compression is only a rough guide to octane requirement.
Cheers, Brian Evans
|