Stroker Crank and compression ratio question?

[spaz1]

I seem to be having some trouble figuring this out. It seems simple enough but.... If i swap out my 3.622 stroke crank and throw in a 4.00 inch stroke crank i'll up the displacement aprox 36 cubic inches. So 4.5 cubic inches per cyl which is aprox 74 cc per cylinder. that makes sense up to this point. So how much dish do i need in the pistons to keep the same compression ratio.
if i go cly volume plus cumbustion chamber volume and devide it bu combustion volume even with stock bore stroke and 65cc heads i'm out to lunch at 12:1. What am i doing wrong. I am trying to build aprox stock comp ratio engine. with 243 heads and then maybe swap them out for some 317 heads and run boost.
Thanks,C

[ForcedTQ]

I'll try to help you here, you are forgetting a step in the calculation. You need to add the volume that the head gasket will give you, then either add/subtract the volume that the piston displaces (coming out of the bore / staying below engine deck). Then you will use those values to divide from your overall cylinder volume to get compression ratio. Total Cyl. Volume / ((Combustion Chamber volume + Volume from Head Gasket +/- piston displacement above/below block deck +/- dish/dome of piston)). That should get you in the ballpark!

[1 FMF]

http://www.csgnetwork.com/compcalc.html

why not use a compression ratio calculator? you're most likely not accounting for head gasket size and it's throwing your numbers off.

for the above calculator,
stock ls1 bore and stroke is 3.9" x 3.62",
head gasket diameter is typically 3.910",
head gasket thickness on average is 0.050",
combustion chamber volume i thought for stock heads is 67cc,
for stock setup use 0 for dish/dome pistons,
and piston deck clearance is typically 0.005".
That results in the stock CR of 10.1.
change only stroke to 4" and CR is now 11.06:1 but this is keeping everything else the same so in order to do that the piston pin location is going to be moved upward by 0.19" to keep the same piston to deck clearance, otherwise if you reuse the stock pistons then the piston will come up out of the bore by that much minus the original deck clearance which is 0.185", which does not work. So what you choose for pistons, and whether you go different rod lengths not sure what's available, will be the biggest determining factor. But if you get all the parts to work correctly and keeping the same 0.005" piston to deck clearance to have that 11:1 CR, a dish piston of 5cc results in 10.45 CR and 10cc dish results in 9.9CR, find a slightly thinner head gasket and/or trim combustion chamber volume and you're back at 10.1 CR with a -10cc piston.
I thought lunati has a 4" stroker kit with pistons and everything, check out and see what their measurements are.