Thought I'd bounce a theory off you guys.... Valve-piston clearance is tightest near TDC when exhaust valve is closing, and intake valve is opening....basically between 15 degrees BTDC and 15 degrees ATDC during valve overlap..... With that said, increasing duration or tightening the lobe spread of a cam would increase overlap and reduce v-p clearance. Isky cams uses a term 'VOTC' which means 'valve opening at top dead center' as a guideline for determining adequate v-p clearance.....see their website....interesting concept.:Smarty: CompCams also lists a 'Tappet lift @ TDC' spec in their 'master Lobe catalog' :TU: The 110 degree spec appears good for most intake profiles. I feel this could be a great guideline to determine what cams MIGHT fit without v-p clearance issues. Some real numbers: - My stock '66 401 cam has VOTC of about 0.030". - The Poston NH400 cam I installed is 0.050"..... - The Isky cam i put in another engine has VOTC of 0.085-0.090". Pistons had to be cut, even with a 0.040" head gasket. Details: The 401 I built with Poston NH400 cam had the following: -Stock 0.045" deck clearance -Steel 0.015" head gasket -0.007" shaved off the heads -Replacement Zollner pistons. Valve-piston clearance was in the 0.090- 0.110" range....fairly safe. Adding Tom's roller rockers reduced v-p clearance about 0.020" - clearly, a bigger cam would have insufficient clearance. The Idea.... With that said, a VOTC around 0.050" should work on a stock block. 'Might' be able to go 0.060-0.065" with a thicker 0.040" head gasket. -There are a lot of variables, but it would be nice to get some ballpark numbers on which cams might fit without problems. We might even be able to put a limit on overlap specs for a stock build....I'm thinking 'any' overlap at 0.050" might be a problem... Opinions on this theory appreciated! Actual measurements appreciated even more!!:TU:
I'm thinking 'any' overlap at 0.050" might be a problem That means 180 @ .050" for both lobes, less than the stock cam.
I'm showing (minus)19 degrees overlap for the stock cam. and (minus) 5 degrees for the Poston NH400 cam. at 0.050" lifter rise..... ....advertised duration specs are a whole different story.
Walt, love the idea. So far, the biggest cam I'm readily aware of fitting, as I've said before, is the TA Performance 413-401. Stealing some data from Comp and calculating a few other things, I get that the VOTC for the 413-401 should be around (.0625" intake/.08125" exhaust), actually it should be slightly less, which is pretty close to your ballpark. I think you've got a good method here Walt :TU: !
Walt here is the cam from Schneider that we are using in the dragster. We are also using Tom's rockers...
Sorry to bring an old thread back up, but I was doing a little searching and ran into this and was wondering if anyone has computed the numbers on the Thumpr cam that seems pretty big but actually fits :Brow: Would it be 0.0695 Int. and 0.0634 Ex.?
