boost vs. octane rating

[M3 Special]

whats the theoretical boost number you can run at 8.5:1on 91 octane pump gas? and does anyone know where i can find an octane vs. boost chart?

[TcmaBoy]

There is no such thing. Each motor is a little different. Boost level and octane ratings aren't the only factors involved in detonation. Ignition timing, intake temps, combustion chamber temps, compressor efficiency, inter cooler efficiency, and plug type are some of the things that play a part. The way to find out how much you can get out of pump gas is to strap your car to a dyno and tune it.

[M3 Special]

oh. well... i understand most of that, but how does ignition timing and compressor effeciency affect detonation? and doesnt air/fuel ratio affect detonation too?

[madrox68]

ignition timeing tells when to spark if its too early or too late the motor will not run right and get bad milage. yes air/fuel mixtire makes a diffrence too.

[GadflyII]

Tons of things affect detonation.

compression ratio
A/F ratio
Spark-plug temps
intake charge temp
timing

Pre-detonation is what you are trying to aviod. This is what kills motors, because the fuel explodes as the piston is still traveling towards the head.

So you

reduce compression ratio
richen A/F
retard timing
cool the intake charge
run cooler plugs.

On a MKIV turbo,

93 Octane is good to about 16psi (some will aurgue 18PSI)
104 race unleaded race gas is good to 23psi
50/50 mix 92, and C16 race fuel 26psi
C16 35+ PSI