I've calculated compression ratio before, but i have one unknown right now: stock honda gx390 dished piston volume. I wasn't planning on modifying the compression ratio, but changed my mind after i put it in the block and don't want to pull it to CC the dish.
Heres my stats
Bore 88mm
Stroke 64 mm
Piston .005" in the hole at TDC
Head gasket .040-.043"
cylinder head volume stock ~ 41cc
piston dish volume ????
I am trying to figure out how much to mill off of my head to get a 10.2-11.0 MAX compression ratio (i want to keep pump fuel and keep it somewhat reliable).
I cant find anywhere where I can get the dished piston volume.
With that said, i looked at the vegas karts site, and they had a little guide, but i think its for the flat top pistons... (copied and pasted below)
Stock = 41cc (8.75:1 CR)
Milled 0.025" = 37cc (9.5:1 CR)
Milled 0.050" = 33cc (10.2:1 CR) *Max for 87 Octane
Milled 0.075" = 30cc (10.8:1 CR) *Max for 91 Octane
Milled 0.100" = 27cc (11.5:1 CR)
Milled 0.125" = 24cc (12.3:1 CR)
Milled 0.150" = 21cc (13.3:1 CR) *Maximum Recommended
So i'm thinking between .050" and .075" shaved should be good provided that the valves clear the piston. But id rather just do the math with the dish volume to be 100% sure. I'm going to measure the piston to valve clearance tonight before I get the head milled tomorrow.
Heres my stats
Bore 88mm
Stroke 64 mm
Piston .005" in the hole at TDC
Head gasket .040-.043"
cylinder head volume stock ~ 41cc
piston dish volume ????
I am trying to figure out how much to mill off of my head to get a 10.2-11.0 MAX compression ratio (i want to keep pump fuel and keep it somewhat reliable).
I cant find anywhere where I can get the dished piston volume.
With that said, i looked at the vegas karts site, and they had a little guide, but i think its for the flat top pistons... (copied and pasted below)
Stock = 41cc (8.75:1 CR)
Milled 0.025" = 37cc (9.5:1 CR)
Milled 0.050" = 33cc (10.2:1 CR) *Max for 87 Octane
Milled 0.075" = 30cc (10.8:1 CR) *Max for 91 Octane
Milled 0.100" = 27cc (11.5:1 CR)
Milled 0.125" = 24cc (12.3:1 CR)
Milled 0.150" = 21cc (13.3:1 CR) *Maximum Recommended
So i'm thinking between .050" and .075" shaved should be good provided that the valves clear the piston. But id rather just do the math with the dish volume to be 100% sure. I'm going to measure the piston to valve clearance tonight before I get the head milled tomorrow.