Quote:
Originally Posted by cuzncletus
I'm just getting started on building the last flathead in my garage. It will be a .125 over 239 8ba at 258 c.i., L100 cam, dual intake, aluminum heads; in other words a pretty standard street flathead. Machining hasn't started yet except for having the crank turned/balancing.
My thoughts were using 1.6 intakes, 1.5 on the exhaust for valves. My reasoning is that it would give a little extra margin of metal between the exhaust seat and the bore where flatheads normally crack. I'm not sure the performance difference would be much but I'm also not sure how much insurance I'm buying myself. Either size valve and machining is the same cost. Comments from experienced builders, please?
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Hi Pat, save your money and stay with the "standard" FH valve sizes. You'll gain nothing in the larger picture, been there and done it!
Use the "standard" lengths also, you'll avoid running into installed spring heights and pressure issues.
You'll probably wind up somewhere around 125/130 HP with a bit less Torque than a stroker would yield. With the same bore and a 4.250" arm we generally see 150/160 HP and 250/260 Torque on the dyno!
Thanks, Gary in N.Y.
P.S. I would strongly recommend the bronze-lined guides however, these eliminate any valve "hanging" issues during longer storage periods! We use and sell hundreds during any given year along with the stainless valves in a package.