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 ??
 

flattop1

Dawg 89
Not much more then stock .
Screenshot_20190909-171245_Chrome.jpg
 

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

Site Supporter
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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OVALTECH1

Site Supporter
Mine is .030 over with a 12:1 pop up piston and 2 lobes off a 292 Duntov cam welded to the Briggs gear! :ROFLMAO::LOL::p
 

alvin l nunley

Site Supporter
in ga. we have this 85 octane at Sunoco...Runs like crap in mine...
Very interesting, why do you suppose that is? Do you change the jetting when you run the E85? I can only imagine, but I would think, just as a guess, you would need about 5 to 10 sizes bigger jet. Was this E85 fresh out of the pump? Alcohol can go bad if not properly stored.
 

CarlsonMotorsports

Site Supporter
Very interesting, why do you suppose that is? Do you change the jetting when you run the E85? I can only imagine, but I would think, just as a guess, you would need about 5 to 10 sizes bigger jet. Was this E85 fresh out of the pump? Alcohol can go bad if not properly stored.

I think he's referring to 85 octane, Al, NOT E85 (=85% ethanol.)
 

Lkerley

Member
Very interesting, why do you suppose that is? Do you change the jetting when you run the E85? I can only imagine, but I would think, just as a guess, you would need about 5 to 10 sizes bigger jet. Was this E85 fresh out of the pump? Alcohol can go bad if not properly stored.
NO ITS NOT E85........ITS 85 OCTANE RECREATIONAL GAS.....
 
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