Using the following numbers:
Chamber CC = 104 ("702" castings are listed as having 96.5 chambers. Mine were cleaned up so I am assuming a net of 104 CC's)
Piston dome = 20 CC's (I was advised that the domes come in at 21 CC's but the sharp edges of mine were smoothed a little)
.020 deck (after being advised that replacement forged pistons don't have the pin raised an additional .020)
Bore = 4.155 (.060 over 396)
Head gasket = .041 thick and bore size of 4.370
Stroke = 3.76

The numbers above give a SCR of 9.479.

Now, using a UD solid cam advertized as 264/264 on a 110 LSA and an ILC of 107 (BTDC=25, ABDC=59, BBDC=65, ATDC=19), I come up with a DCR of 7.909. Interestingly, with the numbers above, the calculator shows total cam advance as 3 degrees, not the 4 that's supposed to be ground into the cam. If I adjust the ILC to 106 the calc then shows total advance at 4 degrees, but then the valve timing numbers all change by one, the intake now closes at 58, causing DCR to go to 7.962. If I change the ILC to 104 to give the recomended 6 degrees of advance, the intake then closes at 56, pushing DCR to 8.064.

A few questions: Am I using the numbers correctly? Are there any special considerations for calculating the numbers of a solid cam given .020 of valve lash? Is a DCR approaching 8.1 pushing it for iron heads on 93 octain - assuming a well set up QJ and good cooling?

Sanity check, please.

Dan

You'll be fine at 7.9 DCR. You should be able to run 8.25 or a bit more without problems.

As you can see, with a 110 LSA and an ICL of 107, that is 3º (LSA-ICL=cam advance).

If the lash is .020" then the valve is starting off the seat at .016" (.01576") tappet lift (lash (.020)/rocker ratio (1.7)+.004"). This means the cam is actually bigger than speced if it is speced at .020" tappet lift. I don't know what the dur would be at .016" lift. If all this is the case, the DCR will be lower than what you calculated.

Depending on what piston you are using, the results can vary dramatically. I have some accurate measurements for my 408 (402 +.030).
Pistons : TRW 2354F030 19.8CC
Heads : 290's cut .010, polished chambers & relieved around valves - will be CC'd to be exact, but estimate 102CC (original published data is 101CC.
Piston/deck ht measured @ .016
Head Gasket : GM Steel @ .022"
Cam : Hyd Roller 280 Adv. Int closes @ 68ABDC.

My calculated Dynamic CR with 101CC head would be 8.05. Even if the heads are 102 - 103 cc, the dynamic CR will still be in the 7.90+.

If you haven't done so, I would verify the actual piston/deck ht. With a .020" deck and a .041 gasket, you are leaving a lot of open space above the piston and eliminating the built in quench design of the head which can lead to excessive detonation despite the lower compression ratio.

Thanks Pat. Because some of the numbers I'm working with are educated guesses, I probably should have asked "what's the max DCR I can get away with". Even if I'm off a little and my SCR is a little more than I think I'll still be in the ballpark. Your 8.25 number is reassuring.

GRN (sorry, name?), the pistons are L2240NF60's and have a dome height of .182, which another TC'er advised works out to be 21 CC's. The decks and heads have never been cut. It's not had any detonation problems yet. I realize a quench of .061 - give or take, may be considered a little wide by some but there isn't much I can do about it unless I pull the whole thing apart. All I am hoping to do before Spring arrives is get a cam installed to wake it up just a little.

Dan