Build my new engine

[guyryan100]

I ran across that same 330 HP motor and since I'm replacing a 250 I will need the intake and carb, so this is a good setup. One thing I'm not sure of is the factor that determines the fuel octane required - I noticed that as the horsepower increases there comes a point where 92 octane is required - is this based on compression ratio? Is there a rule of thumb here? I would prefer to burn the 87 octane if possible.

[chevytruckfan]

Originally Posted by guyryan100

is this based on compression ratio? Is there a rule of thumb here? I would prefer to burn the 87 octane if possible.

yes it is based on CR. The higher octane helps prevent autoignition or knocking in high compression engines. Stay under 10.5:1 CR if you want to stay on 87 octane. Any higher Compression and you will have to increase your fuel octane. As for a rule of thumb <10.5:1 87 octane, 11:1 89 octane, >12:1 92 octane.