Originally Posted by jtz55
just got done with the heads and pistons.the pistons held appox. 22.5cc's (dished) and the heads toped off at 65cc's .the block has zero piston to deck height .this is in a 421sbc 4.155 bore by 3.875 stroke JE pistons with 6" rods .running a solid roller xr286r comp cam.with this info what would be the diff.in compression with a .039 versus a .041 head gasket.the heads are dart iron eagle 215's.a little worried about pump gas .PS did this with a veterinary syringe and plexy glass could be a little off.is this enough info. for the dcr.?
I think we can muddle through it.....
861 cc's in the cylinder
22.5 cc's in the piston crown
65 cc's in the chamber
0 cc's in the piston deck height
8.5 cc's in the head gasket
Mr. Gasket Ultra-Seal Head Gaskets 5801G - SummitRacing.com
Total 957.0 cc's
Subtract cylinder cc's from total and find 96.0 cc's
Divide 957 by 96 and find 9.97:1 SCR with a 0.038" squish.
Intake valve on the XR286R cam closes at 50 degrees ABDC @0.050" tappet lift.
Using these figures in the KB calc.....9.970, 3.875, 6.000, 65.000,
I get a DCR of 7.989:1 (call it 8:1).
I like to see in the vicinity of 8.5 on this calc, so I'd be thinkin' that to show that number the motor would need either more SCR or less cam. An intake closing at 40 makes 8.55:1 DCR. Then again, with a range of 3000 to 7000, this big cam might make up the difference in flow and let the motor make more power than a smaller cam with the "ideal" DCR. At 8:1 DCR and a 0.038" squish, I don't think there's much to worry about in the detonation department on pump gas. I'd run it with the big cam if it was mine.