Several factors must be included...
Several factors must be included if you are going to accurately determine your compression ratio. Calculations like the compressed gasket thickness, deck clearance, and either piston dish or valve reliefs must be taken into account.
We all know the age-old maxim, "There's no replacement for displacement." It's the truth. That's why so many sanctioning bodies use displacement and compression rules to limit engine power. Of course, they sometimes do it in wily ways. For example, NASCAR's Late Model Stock rule book has no specific rule on compression, but it limits bore, stroke, and minimum combustion chamber volume. Many other sanctions use this method. So if you want to maximize compression-and thus power-you have to know what you are doing when determining combustion chamber volume.
Other sanctions may choose to limit overall cubic inches and compression. You have more options in bore, stroke, and chamber volume size, but if you are cutting your own valve reliefs in the piston tops, experimenting with different head gaskets, or relieving the combustion chamber walls to reduce valve shrouding, are you sure that your compression ratio is still within the limits? Or just as important, that you haven't lost critical compression?
Jeff Dorton of Automotive...
Jeff Dorton of Automotive Specialists demonstrates the proper procedure for cc'ing the combustion chambers on the cylinder heads. Before beginning, any changes to the chamber should already be made, including valve seats. Using the same valves you plan to run, coat the seats with a thin layer of grease and slide the valves into place. Spin them around on the seats a time or two to make sure the grease is evenly distributed, and wipe away the excess. This will ensure a good seal. If the valves and springs are already installed, this step is not necessary.
Thankfully, determining compression isn't difficult with a few simple tools and a calculator.
First things first-you've got to be able to determine displacement exactly. Displacement is simply the area swept by the top of the piston as it moves up or down the bore one time. It does not include any of the area above top dead center (TDC) such as the combustion chamber. The conventional formula for determining displacement is as follows:
Bore x bore x stroke x 0.7854 x number of cylinders = cubic inches
For example, consider a standard Chevrolet 350 with a 4-inch bore and 3.48-inch stroke. Plugged into the formula, it would look like this:
4 x 4 x 3.48 x 0.7854 x 8 = 349.849
For this formula, both bore and stroke are measured in inches. The third number (0.7854) is a constant used to convert to cubic inches. Bore size can be determined simply with a dial caliper (although an engine builder's dial bore gauge is usually more exact), and stroke can be pulled right off the crankshaft spec sheet. For calculating compression ratio, you need only the displacement of a single cylinder, so to determine that, all you have to do is strike the number of cylinders multiplied.
Now that we have displacement nailed, it's time to move on to compression ratio (CR). The CR is simply the volume of the cylinder and combustion chamber when the piston is at bottom dead center (BDC), divided by the volume of the cylinder and combustion chamber when the piston is at TDC. For all the importance given to compression ratio, the number is, at best, theoretical. It assumes that the valves close exactly at TDC or BDC, which will never happen with a performance cam. It also makes no considerations for volumetric efficiency, either for the good or bad. Still, compression ratio is useful for engine builders putting together an engine package and knowing the CR is a must to ensure legality.
Install the plug you plan...
Install the plug you plan to run in the completed engine. This may not seem important, but plugs with different electrode and nose designs can vary the volume by as much as 11/42 cc. Automotive Specialists tries to size all the combustion chambers within 0.2 cc's of the minimum, so you can see why half a cc is critical.
Several factors must be determined before calculating CR, including piston volume, deck clearance, gasket thickness, and combustion chamber volume. Piston volume is the negative area on the top of the piston. This is usually the valve reliefs or dish area. The manufacturer will give you this as part of the spec sheet. If you cut your own valve reliefs, you will need to make your own measurements. Deck clearance is the volume of the bore between the piston top at TDC and the top of the deck. Gasket thickness acts essentially as additional deck height, but when working the gasket into your calculations, you must use the compressed thickness.
The formula for calculating CR is slightly more complex than the formula for displacement, but is still straightforward. It is as follows:
(D + PV + DC + G + CC) (PV + DC + G + CC) = CR
CR = Compression Ratio
D = Displacement
PV = Piston Volume
DC = Deck Clearance Volume
G = Gasket Volume
CC = Combustion Chamber Volume
It is important to make sure each variable is the same unit; do not mix cubic inches and cubic centimeters. Since your displacement figure is probably already in cubic inches, it's easy enough to convert cubic centimeters to inches. Simply divide the number by 16.386.
Coat the deck of the head...
Coat the deck of the head around the edge of the combustion chamber with a thin coat of grease or Vasoline to provide a good seal between the deck of the head and the cover plate. If any excess gets inside the combustion chamber, be sure to wipe it out.
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...
Place the cover plate so that it completely covers the combustion chamber. Dorton saves a little cash by making his own plates. The only criteria is that it be flat, clear, have one hole for the burette, and several smaller holes to allow the air to escape when the chamber is filled with liquid.
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...
Fill the burette (you can get them from engine builders' supply houses such as Powerhouse Products or from medical supply houses) with mineral spirits to the zero mark. Then, using the fluid in the burette, fill the combustion chamber to the bottom of the cover plate.
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.