Well of corse it'd help, manifolds or not, do the angle smoothing and guide shape, it's safe, u cant really screw up if you take you're time at slower grinder speed. look at the 1st page of this thread.
At the very least...
cc them heads, then just have them milled=60 bucks.. and u wont be losing any cylinder pressure and it will be a win by virtue of good sealing valves.
as for the last question..lol...you gotta consider the rings, if they are not sealing 'more blowby from more cyl pressure' and say the the cyl pressure is way down already=110'lbs....dont expect a lil more flow to cure the loss of power once u start burning oil ...and again, measure both chambers of the 2 heads...and u can figure the diff and get a rough idea
if it were me, i would have done a leak down test before the old heads came off.
every 5 cfm is said to give 10 hp increase, that is if the cam works at the lifts flow increase.
what do u think? ;)
