Quote:
Originally Posted by Tim Ayers
As Drolston mentioned, those numbers are referring to the amount of can lift the heads will take. Meaning, 400 will accept a .400" lift cam, 425 will accept a .425" lift cam.
So if you have the stock cam in there, 375's would also work. If you plan to change to the cam to a different grind, you'll just need to be mindful of that.
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I am running the 425 heads on a 284 inch engine and can still advance the spark by three degrees (one and a half marks) on the crab distributor without getting pinging (spark knock, pre-ignition) or regular 87 octane with 10% ethanol. With my Schneider 348F cam valve lift of 0.350" I could have run any Offy head, but chose the lower compression 425 because it is strictly a street engine for which I did not want to get into advance curves and the like to get the most out of the higher compression
I did not mention the 375's because they will be old and used and hard to find. On a big inch engine you might have to go to premium fuel with the 375's.
I am suspicious of old used aluminum heads based on one bad experience many years ago. But I am sure there are good ones out there.