View Full Version : How to determine comp ratio?

02-08-2011, 09:07 PM
How do I figure out how much I would need to deck a block to get a certain compression ratio and how do I determin how far I can go before valves start hitting pistons?

02-08-2011, 09:22 PM
There is a long process to do this. It involves putty.

I can't remember the exact formula, but there are several steps to it.

02-08-2011, 09:26 PM
Im reading something on carcraft.com and its a very indepth formula and Ima have to make some measurments. Thats just for comp raito tho.

02-08-2011, 09:28 PM
You should try and get ahold of WarTowels, he is currently doing some similar measurments and might be helpfull.

02-08-2011, 09:40 PM
PM'ed him. If I can just figure out how close my piston gets to the valves when they are open Ill be happy. If I knew I had .050 clearance, I would get it decked .030 and be happy.

02-09-2011, 12:14 AM
Rumer has it I want atleast .100" valve to piston clearence and the only real way to measure is with clay on the piston and measure how far the valve goes into the clay.

It just hit me that valves are adjustable, of course I want them to open as far as possible, but what about a valve that open slightly less, like .010 less, so i can get the c/r higher? Im just rambling stuff as I think of it.

02-09-2011, 01:25 AM
Then you would end up with too much lash, how about cutting the valve reliefs on the pistons deeper?

02-09-2011, 02:14 AM
I donno that seems extreme. Didnt think about lash. Ima just figure out how much piston to valve clearence I have and check with the machine shop to see how far I could go. My ideal goal is 11:1 without buying expensive pistons but since I need to know ALOT of info to figure out what would put me to 11:1 Ima just deck to a resonable amount and call it done.

02-11-2011, 12:47 PM
What engine are you working with here? To figure compression ratio, you need to find out what your compression chamber volume is vs your bore stroke.

Measure the volume of your compression chamber of the head by flipping the head over, place a piece of plexiglass (with a small hole drilled in it) over it using a graduated pipette, fill the chamber till the oil is flush with the top of the hole in the plexiglass (take the measurement of the oil it took to fill the combustion chamber of the head)

for flat/dished type pistons: Measure the volume still left in the piston to deck (while at TDC) using the same method with the pipette and the plexiglass. (for domed pistons, sink the piston a known distance, and remove this volume from your measurement)
Measure the volume of the hole you drilled in the plexi

Volume of the head + volume in piston to deck height + volume of your compressed headgasket bore - 2x volume of the plexi hole.

Your compression ratio is this volume vs your bore X stroke.

To check piston to valve clearance, place the piston at TDC and lock the crank in place so the crank can't move. place machinist's putty in the valve reliefs. Lightly coat the top of the putty with a light oil. Bolt the head to the block (with head gasket) and torque to spec. set your valve clearances to stock. Rotate the camshaft so that the valve is fully extended. Flip the block over, and using some type of pole, rock the pistons on the wrist pins. Flip it back over and rotate the camshaft a few more rotations. Remove the head, carefully peal the putty off of the piston face and place on a sheet of wax paper. Slice the putty into thin strips using a new razor blade and measure the thinnest areas... this is your valve clearance.