Miles in Florida

The cam is a comp XE-R 238/244 .605/.612 112LSA

Can anyone tell me the dynamic compression ratio, or point me to an online calculator, or site that can explain it?

Thanks,
Miles

PS: If anyone wants to talk about valve event timing, please do, I'd love to hear it. Most of what you say will go over my head, though.

Joel_SS

iTrader: (8)

you will need more info than that to calculate DCR.. post a pic of your cam card

pianoprodigy

iTrader: (1)

Does the cam have any advance ground in? (+2, +4, etc.)

What ccs are your heads? What Gasket?

Your duration at .006 is 287 & 293 respectively.

Miles in Florida

Unfortunately I still haven't gotten the cam card from the engine builder.

I don't know if there is any advance, can you compute SCR assuming zero, just to give me a ballpark figure? The heads are 72cc, with a 0.054" gasket. I don't know how much the pistons come out of the hole, but assuming zero, I compute a 10.9:1 compression ratio. The engine builder said static compression is 11:1, so either he rounded, or the piston # changes it, or I'm off somewhere else. In any case, it's approximately 11:1 static compression.

Thanks

pianoprodigy

iTrader: (1)

No idea how you're getting 11:1 SCR with a 72cc chamber and a .054" gasket.

Assuming no advance and the piston does not come out of the hole I get 9.59 SCR and 6.98 DCR.

DanZ28

iTrader: (5)

There's really no way to ball park that. Are you having an engine built? We need to know piston bore, rod length, piston out or in the hole distance, valve reliefs?, stroke, etc. Also the ICL and LSA of the cam make a big difference in your DCR.

On a stock engine using the stock head combustion chamber, the piston would have to be out of the hole .030" to get 11.0 SCR. I get the same numbers as pianoprodigy does with your specs so far.

Miles in Florida

Sorry guys, my brain has been out to lunch all day, between car problems and problems at work. I didn't mention it's a 402, with 4" bore and 4" stroke.

I must be confused about how dynamic compression ratio is computed, I assumed that once I said static compression is 11:1, you wouldn't need to know displacement, gasket thickness, combustion chamber volume, etc., and that all those just went into computing static compression. So I thought if I said up front what static compression is, dynamic compression is determined just from that, and the cam specs.

Sorry for the confusion,
Miles

Miles in Florida

I looked around for formulas online. I think if static compression is known, you can compute dynamic compression with just that, rod length, stroke, and the intake closing valve event. But most calculators that compute DCR are set up so you enter everything needed to compute SCR, not just have you enter SCR, and then go from there.

I found a calculator here:
http://www.empirenet.com/pkelley2/DynamicCR.html

I entered everything, assuming no advance ground in, and it computed the intake closing at 75.5 degrees, and dynamic compression as 7.97:1.

With 4 degrees of advance, the DCR jumped up to 8.27:1. 6 degrees advance, comes out to 8.42:1.

I really wish FFHP sent me the damn cam card. I'm gonna call them again, and ask specifically for the ICL/advance. And also the deck height, while I'm at it. I'm thinking maybe I should have paid more attention to the cam, instead of just using their recommendation. Actually, I asked them to go smaller, I think originally they recommended a 244/250. Maybe I should have gone even smaller still, for better dynamic compression? Or else had them drop the gasket thickness a bit? Too late to do anything now, except maybe get an adjustable timing chain and advance the cam a bit more, if it's not got any advance ground in....

How much power am I losing, if my DCR is 8.0:1, instead of the more desireable 8.5:1? Or should I have tried for even higher than 8.5:1 DCR? I want this thing to run on pump gas, and I'm currently in Florida, where it's hot as hell, and I'm gonna be going to Colorado, at 5000ft elevation. Perhaps the low DCR is for the best, so I don't have to worry about it knocking or pinging?

Thanks guys, and sorry for not having my **** together before that first post.

DanZ28

iTrader: (5)

I come up with the same as you did on a different calcultor. I'm going to guess you might have 4* advance in the cam, seems typical for most vendors. If you use a .040 gasket you would jump to 8.5 DCR and a .035 gasket puts you at 8.6 DCR. if you find out your deck height, you will know if you can use a thinner gasket. Some thought should have gone to Quench area on the build. Personally, I wouldn't worry so much about the DCR, as long as your combination was assembled to work together, that's more important. I've seen 12.0 SCR/8.9 DCR cars running on 91 octane pump gas, but not in Florida. Run the right temp plugs and get a real good tune on a load dyno and can be done.

Dan

Miles in Florida

I got the following data from FFHP:

4.005 bore
4.00 stroke
-2cc pistons
4.060 gasket bore
.054 gasket thickness
72cc chambers
-.010 deck height

Using that, I get 8.12:1 as the dynamic compression, static is 10.90:1. I think it's good enough that I won't worry about it any more for now. What brought this on was low dyno numbers, but it looks like I'll have to look elsewhere for the cause.

Thanks guys,
Miles

PS: Low dyno number thread:
https://ls1tech.com/forums/showthrea...0&page=1&pp=20