The key tool is a measuring burette which allows you to accurately measure volumes down to
Gasket volume is similar:
Bore X Bore X 0.7854 X Gasket thickness = G
If the gasket thickness is 0.020-inch, then the formula looks like this:
4 X 4 X 0.7854 X 0.020 = 0.251328 ci
Now, let's assume that the piston's negative volume is 5 cc's (0.305 ci) and the combustion chambers are 64 cc's (3.906 ci). Now we can figure out the compression ratio:
(43.731 + 0.305 + 0.41469 + 0.251328 + 3.906) / (0.305 + 0.41469 + 0.251328 + 3.906) = 9.97
Seal the edges of the combustion chamber with a light coating of grease or petroleum jelly
So the compression ratio is 9.97:1. A little low for a race motor, but the numbers work for the purpose of this example. If, by the way, you have a zero deck height, you can leave that one out.
Making Your Own Measurements
If there are volumes that you don't know, such as the piston volume, it may be just as simple to make the measurements yourself. All you need is a burette, a cover plate and a little grease. We got our kit from Powerhouse Products, which also includes a stand to hold the burette.
To measure the deck clearance volume, start with at least one piston (with the top ring) and rod installed in the bore of the block and attached to the crank. Move the piston part way down in the bore and wipe a small amount of grease or petroleum jelly around the inside of the bore near the top. Move the piston to TDC and wipe away the excess grease above the piston top. The rest of the grease should seal up the crevice volume, which is the area between the piston and the bore between the top of the piston and the top of the upper ring. This keeps the fluid from leaking past the gaps in the rings. On racing pistons, the crevice volume is minimal, but sealing it up provides you a safety net of sorts. Most tech inspectors allow 1 cc for crevice volume.
Now use the burette to fill the void in the chamber with fluid through the hole in the cov
Now use some grease or petroleum jelly to provide a seal between the deck and the cover plate. You don't need much here, just enough to keep fluid from leaking out. Fill the burette with either mineral spirits or isopropyl alcohol and then fill the void in the chamber you are measuring. Check the volume on your burette. This is your bore volume at TDC (BVTDC). Measuring the volume of the combustion chamber in the cylinder heads is even simpler since you don't have to worry about sealing the piston. The formula simplifies to:
(D + BVTDC + CC) / (BVTDC + CC) = CR
The ability to precisely deter-mine both your displacement and compression ratio is a vital tool for anyone making alterations to a racing engine. Plus, you can also determine how one change might affect other things. Now you have the tools to know when you're bumping up against the rules and when you have shot right past them.
Here, you can see the chamber is completely filled with the fluid. Check the level on the
A sports bottle also comes in handy when it's time to clean up the fluid.
A glass burette like we used can be a bit pricey if you aren't a professional engine shop.