340 xhead spring shimming, height and pocket question(s)
Yeah, these heads were done 30 years ago lol and at that time I was fine with "stock". I didnt realize this cam was that much above stock?
Thanks for your input.
The 340HP spring is good for .500", but it has to be at the stock installed ht., which is 1.685" iirc. When You sink the valves during the valve job, it will increase the height, so shims will restore that. Those HP springs are short & high-rate, which means(just like suspensions) as fast as the load rises on compression, it disappears on extension. So it's critical to get that right & maintain that 70# on the seat. With a stock, or stock lift cam, the max was .475". Adding .025" of shim plus the amount the valves sank will ensure seat load, and not bind the coils, but throw another .020" lift on it & now You're on the razor's edge.
Aggressive profiles require more seat load, so total lift isn't the only consideration. For instance, My plan to use LS beehives from Comp(all roller cams & 1.7 or 1.8 rockers) with a .904" Mopar specific aggressive lobe was torpedoed, because neither the PAC 26915(1st level LS upgrade) nor the 26918 were up to the task. The reality is that an aggressive FT will out-accelerate a roller, the roller can maintain it's lift velo longer tho', I couldn't even use a Ford .875" ramp lobe. So I settled for a very nice pair of Extreme Energy solid lobes, Comp had just come out with the even higher load LS beehive, but I am not confident the stock-type valves can handle that much...so I compromised.