Xtnct00WS6

iTrader: (8)

Is there any basic rule of thumb that says something like for every 2 degrees of duration in a cam, you lose .005 worth of piston to valve clearance?

My setup right now consists of 63 cc 243 heads (.010 mill) and I have .130 clearance on the intake. I actually plan on milling a bit further to a total of .024. So theoretically I'll be at .116 from the extra mill. I ALSO am planning on stepping up the cam to about 230/226 on a 112 +4 with 600ish lift (From a 224/224 .563 on 114).

So......If I'm at .116 PTV clearance and add 6 degrees of duration, do you think that will bring me into the sub .100 land of PTV problems??

redtail2426

iTrader: (10)

good question and i am also curious on this.I will quess and say for every two degrees is about .005.I think if you retard the cam some you can gain some clearance,but need to confirm this also.

Xtnct00WS6

iTrader: (8)

Nobody knows how much PTV clearance you lose for a given amount of duration in a cam? (for every __ degrees, you have __ less ptv clearance?) Any guesses?

Gearhead1

It would depend greatly on how agressive the lobes you've chosen to run are. If you've had your cam "cam doctored" you can get a ballpark figure of how close you are from the "lift at TDC" numbers. My cam doctor sheet says I have .0654 lift at TDC with my cam installed straight up @ 109.2 intake centerline. .0654 lift x 1.7 (rocker ratio) = .11118 lift @ TDC. The free drop on my Darts is .150 (unmilled) so, .150 - the lobe lift of .11118 = .03882. From this you subtract your out of the hole measurement of the piston (I used .010 to be safe) and now you have .02882. This is how far my intake valve is from my piston top @ TDC. Add in your compressed head gasket thickness (mine is .056) .02882 + .056 = .08482, and that's how much clearance I should have at TDC with my setup. I realize that there isn't a 1:1 ratio between the gasket thickness and the free drop because of the angle that the valve is coming down at. Maybe some of you smart guys can figure it out, but I can't!! The minimum is said to be .080 Int and .100 on the Exh. so I should be safe. That being said, the only truely safe way to do it is to check it with a dial indicator. All of you cam guys out there feel free to correct any mistakes you see in my numbers. I'm still on a learning curve on this type of calculating and I welcome any constuctive criticism or attaboys.