Some of the big truck engines came relieved from the factory. That said I am with Tubman on this one you are working with an 8:1 (or less) motor to start with and everything you remove will lower that. @tubman you have been around a while tell me if this is something you are familiar with. I used to work around an old guy that was Mr Flathead (still in the '70s). He used to mill the heads then open up the pocket over the valves and cut reliefs into the combustion chamber on the head side instead of the block. he said that it really let 'em breath and did not believe in relieving a block at all.
Beaner, To me, that sounds to me like the best way to go. In all truthfulness, I build my flatheads for the best street performance and don't worry about esoteric tricks to get the last 2% (which amounts to 2 or 3 horsepower) out of them. That being said, after listening to guys like "Ol' Ron" and a few others, I have become a true believer in getting optimum "quench" over the piston. I built a big bucks (for then) 276" flathead for a '36 three window I had in the mid '90's. Not knowing any better, I just bolted on a set of "8.5 to 1" Offenhauser heads and called it good. The 255" Merc in my '51 with carefully prepared Edmunds heads, a Rochester 2G, and a properly set up Mallory dual points is much more satisfying than the 276" was.
Tubman and Porkbeaner: Do you have any photos of heads that show the machining you described earlier? Would be neat to see. "He used to mill the heads then open up the pocket over the valves and cut reliefs into the combustion chamber on the head side instead of the block."
What are peoples thoughts on un-shrouding the valves by removing material from the heads?? Does it work??
The vast majority of the flow is from the one side of the valve that faces the bore. So, most of the flow improvements are related to the valve size, the work in the port runner and the bowl, etc.. There are various opinions on using 'reliefs' -- how much, what RPM they're good for, etc.. On very high-end racing flatheads, some guys will rework the back-sides of the combustion chambers (in the heads) to radius what is normally a straight/flat wall. This has been shown to give some improvements on the initial part of the valve lift - though it is not a lot. I've seen quite a bit of this done on Harley KR race heads, where they're trying to squeeze every last bit of HP out of them. I've done it on blower motors, where I didn't mind losing some compression as a trade-off for flow (with the blower boost making up for the compression loss). On a street engine, I believe that keeping the compression higher is worth more than you'll ever get by working the back side of the valves area in the heads. So there yah have it . . . some more opinions to add to the flathead story . . .
The combustion process is a very complex system to model, with many variables other than CR and flow rates to consider. In fact, even complex computer models are incapable of accurately modelling the real world with precision. Many assumptions must be made. For those curious, have a look at the paper presented in the link below for an eye opener. https://link.springer.com/article/10.1007/s40095-018-0265-9
My two cents, which is probably worth less than that, my 59L motor (factory relieved), built with a 4" crank, (276ci) did 173 hp on dyno. Hard to beat that naturally aspirated. Did factory relief help or hurt? I'd argue it didn't hurt. I'm a Joe Abbin disciple, his calculated hp numbers matched my output almost exactly. Y'all can take it from there...
The Jalopy Journal
