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I put together a 6.0 with aftermarket flat top pistons no valve reliefs. Stock 862 heads running the summit 8728R1 camshaft. I was curious if anyone could help me with dynamic compression ratio. This will be pump gas carbed in a boat. I have 91 close to me and 93 a little farther away. Just trying to figure out what I can run. Tia.!!
Do you just need a number thrown out there? Or do you need help in actually computing the DCR? I'd think you could get away with around 8.5-8.7 DCR if your quench is where it should be ( around .035 ). Maybe even a bit higher, considering you'll have evaporative cooling due to the carb. Just remember, you need to know the IVC at lash in order to compute the number correctly. You can fudge it with the .006 duration values, but your number will be artificially higher because the valve won't fully shut for a couple more degrees. It'll be close enough, however.
Im just trying to make sure i can run on atleat 93. So pretty close i would assume.
I have 4" bore
3.622 stroke
.051 gasket
4.100 gasket hole
Flat tops no reliefs
61 cc chambers (from my reasearch)
my cam card says 57* ivc @.006
im coming up with like a
11:23:1 static
9:26:1 dynamic
would that sound right i can attach card My main problem is i didnt check deck height
That's pretty short cam timing for such high compression, but you could maybe make it work if you installed the cam 4 degrees retarded. It's already ground 3 degrees advanced, so it would end up 1 degree retarded, which isn't a big deal. All it will do is move your powerband up a little bit. Adding in deck height will likely raise your compression calculation a bit, because LS engines are generally around .006 out of the hole. I calculated it out, and I got 9.44:1 dynamic, so it seems like you may just be pushing the envelope a little. If I were you, I'd take the time to unshroud the intake valve by grinding the chamber wall back slightly, which will increase flow AND lower your compression by adding a couple CC's to the chamber. This is, of course, if you're dead set on using this specific cam. If you're not, I'd get something in the low 220's for intake duration and the later IVC will bleed off enough compression to work on pump gas.
That's pretty short cam timing for such high compression, but you could maybe make it work if you installed the cam 4 degrees retarded. It's already ground 3 degrees advanced, so it would end up 1 degree retarded, which isn't a big deal. All it will do is move your powerband up a little bit. Adding in deck height will likely raise your compression calculation a bit, because LS engines are generally around .006 out of the hole. I calculated it out, and I got 9.44:1 dynamic, so it seems like you may just be pushing the envelope a little. If I were you, I'd take the time to unshroud the intake valve by grinding the chamber wall back slightly, which will increase flow AND lower your compression by adding a couple CC's to the chamber. This is, of course, if you're dead set on using this specific cam. If you're not, I'd get something in the low 220's for intake duration and the later IVC will bleed off enough compression to work on pump gas.
I just wanted to say thank you for taking time out of your days to help me. Means a lot I have a set of 317s layin here that would be a better fit and given my particular circumstance I can swap heads in about 30 minutes. I’d have to pull the engine for a cam swap. I think for my needs and this pieced together thing I have I’ll just swap to 317s. You probably saved my son and I from grenading going down the river. Thanks a million!!!
Reverse Cooling allows for the Cylinder Head to be colder than the crankcase where the pistons "live".
The cylinder wall should be at least 190* good for ring life.
With this method, your CR will be fine,
Lance
06-24-2024, 02:00 PM
DCR help? 
