Compression

joe boom

New member
What is a realistic compression ratio for a modded flatty .080 over ,arc billit head ,4.5 rod,eybrows cut ??? 9,10,11 to1 ??
 

Lkerley

Member
Did not realize this.I was thinking at least 10.1...so when running gas on a Flathead with most any cam or stroke there's never any need for anything above 87 octane? Thku.
 

CarlsonMotorsports

Premium User
Did not realize this.I was thinking at least 10.1...so when running gas on a Flathead with most any cam or stroke there's never any need for anything above 87 octane? Thku.
Absolutely correct.
In fact, we used to use stale gas back in the '80s before everyone (WKA & IKF) went to alcohol.

OP, as soon as you cut those eyebrows, and valve relieved the head for a bigger lift cam, you lost compression ratio.
You're likely around 6:1 now even with some pop-up and a flat-milled head. There are billet heads that will help some, but the majority of the loss is over the valves.

Keep this low CR (and coil retard) in mind when setting your ignition timing!


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