Coat the deck of the head around the edge of the combustion chamber with a thin coat of gr
Each of the volume variables is easy to determine. Displacement, as mentioned above, is for one cylinder only. If you have modified either the combustion chambers in the heads (evidenced by dished valves and any grinding work or decking of the heads) or the piston tops (evidenced by valve reliefs or fly cutting), the specs given by the manufacturer are no longer valid. Jeff Dorton of Automotive Specialists demonstrated the practice of cc'ing the combustion chambers, which is shown in detail in the accompanying photos. If you need to confirm the piston volume, the process is nearly the same, except it also takes into account deck clearance volume and gasket volume, which changes the formula. We will cover this later.
Deck clearance volume is calculated almost exactly like displacement:
Bore x bore x 0.7854 x distance from piston top at TDC to deck of block = DC
For instance, continuing our example of the Chevy 350, if the deck clearance is 0.050 inch, the deck clearance volume is:
4 x 4 x 0.7854 x 0.050 = 0.62832 ci
Gasket volume is similar:
Place the cover plate so that it completely covers the combustion chamber. Dorton saves a
Bore x bore x 0.7854 x gasket thickness = G
If the gasket thickness is 0.020 inch, the formula looks like this:
4 x 4 x 0.7854 x 0.020 = 0.251328 ci
Now, let's assume that neither the piston tops nor the combustion chambers have been modified. Piston volume is 5 cc's (0.305 ci), and the combustion chambers are 70 cc's (4.272 ci). Now we can figure our compression ratio:
(43.731 + 0.305 + 0.62832 + 0.251328 + 4.272) (0.305 + 0.62832 + 0.251328 + 4.272) = 9.01
The compression ratio is 9.01:1. Pretty low for racing, but the numbers are nice round figures for the purposes of the example. If you have a zero deck height engine, you can leave the DC variable out.
If there are volumes that you don't know, such as the piston volume or the compressed gasket thickness, it may be just as simple to make the measurements yourself. Just like cc'ing a combustion chamber, all you need is a burette, a cover plate, and a little grease. You can buy kits from Powerhouse Products or make your own. The burette can be purchased from medical supply houses, and the cover plate is simply a piece of thick, clear plastic. The only thing you need to do to make your own cover plate is drill one hole large enough for the end of the burette and several smaller holes to allow air to escape.
Fill the burette (you can get them from engine builders' supply houses such as Powerhouse
To measure the piston volume, deck clearance volume, and gasket volume, start with at least one piston (with rings) and rod installed in the bore of the block and attached to the crank. Move the piston part way down into the bore, and wipe a small amount of grease or Vasoline around the inside of the top of the bore. 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 ring. On racing pistons, the crevice volume is minimal, but sealing it up provides you with a safety net. Most tech inspectors allow one cc for crevice volume.
Add a used gasket of the type you plan to run on top with the cover plate on top of that. You may need to hold down the plate and gasket with a couple of head bolt studs and nuts. Just tighten it enough to seal-don't try to torque to spec. Now, using a burette filled with mineral spirits or some other type of fluid (water isn't a good idea), fill the void. The grease between the piston and bore should keep the fluid from leaking past the gaps in the rings. Check the volume on your burette-this is your bore volume at TDC (BVTDC). The formula for compression ratio now simplifies to:
(D + BVTDC + CC) (BVTDC + CC) = CR
The ability to precisely determine both your displacement and compression ratio is a vital tool for anyone making alterations to a racing engine. Now you can determine how one change might affect other things. For example, touching up the bores with a hone might only open them up a few thousandths of an inch, but it's enough to increase both the displacement and the compression ratio. Now you have the tools to know when you're bumping up against the rules and when you're busting right through them.
When the chamber is filled, make sure there are no air bubbles trapped behind the plate. If you consistently get bubbles that you cannot get rid of, you need more air holes. Now read the level on the burette to get your volume. You can simply dump the mineral spirits out over your parts washer or use a suction bulb if you want to be a little bit neater.
This is the same method tech inspectors use when you are checked at the track. You'll be required to remove one head for the inspector. Even a single race carbon buildup can affect your chamber volume, so most inspectors will allow you to wipe down a chamber before inspection. If your combustion chambers are right up against the minimum limits (and they should be), be sure to ask if you can do this. A rag with any type of chemical thinner should do the trick.