Checking Compression Ratio

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Car magazines throw around compression ratio numbers like 9.0:1 and 12.5:1 as though everyone knew what they were talking about. For those who don't understand what compression ratio is, it's simply the total volume of the cylinder and combustion chamber above the piston at the bottom of its travel (referred to as bottom dead center [BDC]), divided by the combustion chamber volume above the piston at top dead center (TDC). It sounds simple enough, although most people who build their engines never actually check the compression ratio to know for sure.

Why is knowing your true compression ratio that important? With today's lower-octane grades of gasoline, it's important not to exceed known maximum allowable compression ratios for various engines or you may experience detonation that will quickly destroy your engine.

Several factors dictate what the maximum compression ratio of an engine should be. As a general rule of thumb, engines with aluminum cylinder heads can tolerate one full point of compression more than an engine with cast-iron heads. The maximum compression ratio is also limited by combustion chamber design. Some cylinder heads such as small-block Chevys are more tolerant of higher compression ratios than big-block Pontiacs. When in doubt, consult an engine builder who specializes in your particular engine. In general, most engines running premium fuel can tolerate a 9.5:1 compression ratio with cast-iron cylinder heads and 10.5:1 with aluminum heads. The reason for this difference is that compression ratio equates to heat, and aluminum heads reject heat better and faster than cast iron.

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Combustion Chamber Volume

Ideally, you should check your compression ratio before any of the final engine machine work is done. If the engine needs to be bored oversize, this must be done so that the new pistons can be mocked up for measurement. Also, if the cylinder heads are going to be modified such as unshrouding the valves in the combustion chamber, this work needs to be done and the valves installed temporarily to measure combustion chamber volume.

The measuring process is accomplished by actually filling the combustion chamber with fluid from a graduated burret. A flat plexiglass plate is placed over a combustion chamber and fluid is released into the chamber through a hole in the plexiglass plate. A thin layer of grease seals the plate against leaks to either the cylinder head or cylinder block. When the combustion chamber is completely filled with no air bubbles showing, read the burret and write down the number.

Most burrets are graduated in milliliters (ml.), which are the same as cubic centimeters (cc.). All compression-ratio calculation formulas use ccs as a unit of measure. Just make sure you have enough fluid in the burret to start with. If your heads are supposedly 64ccs, make sure you start with at least 75ccs in the burret. You can use any thin fluid such as water (a drop of food coloring makes it easier to see what you are doing). For these photos, we used antifreeze because it has low viscosity and is easy to see.

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Above-Piston Volume

CCing the cylinder heads is easier than ccing the area above the piston at top dead center. If your pistons are flat tops or dished, and are below the deck surface of the cylinder block at TDC, you simply bring the piston up to the top of its travel (TDC) measured with a dial indicator and place the plexiglass plate over the top of the piston. The 464-cid big-block Buick used here for photos had 35ccs above the piston at TDC. However, if the piston has a large dome and any part of the dome extends above the cylinder block at TDC, you must cc the piston with it positioned exactly one inch down in the cylinder bore. Fill the area above the piston with fluid, then subtract this volume from the calculated cylinder volume at one inch and you get the piston dome displacement in ccs.

The final figure that you will need for your compression ratio computations is the head gasket's compressed thickness. You can do this by actually measuring an old gasket of the same type you are using or by consulting the manufacturer specifications for that gasket.

Time to do the math. Plug the necessary figures into the following formulas and you will compute your engine's compression ratio. You need the following information:

> cylinder head combustion chamber volume in ccs

> compressed head gasket thickness in thousands of an inch

> gasket bore diameter in inches

> volume above the piston at TDC (negative volume if piston dome protrudes above the cylinder deck)

> engine bore diameter

> stroke in inches

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Cylinder Volume

First, calculate the cylinder volume in ccs by using the adjoining formula. For those who are intimidated by pi, another way to get the cylinder volume in ccs is to multiply the bore times itself (bore squared) x 0.785 x the stoke (in inches) x 16.387. Our Buick's bore diameter was 4.352 inches or 18.94 when squared and our stroke was 3.90 inches. So, its cylinder volume is 950.19 ccs: 18.94 x 0.785 x 3.90 x 16.387. Next, we did the same computation for the cylinder head gasket, which had a little larger bore and a 0.040-inch stroke (compress gasket thickness). It worked out to 12.2 ccs.

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Compression Ratio

Now we had all of the needed volumes to calculate the compression ratio. Our Buick turned out to be 8.97:1, which we got by adding 950.19, 12.2, 35 and 72 to get 1069.37, then dividing by 119.18. We were shooting for a 9.5:1 compression ratio. Our solution was to install thinner (0.020-inch-thick) head gaskets that had a volume of 4.92ccs compared to the composition gasket's 12.2ccs. That was enough to get us to 9.5:1. If we hadn't physically measured the compression ratio, we could have given up about 50hp. No self-respecting hot rodder could live with that.

Don't let the simple math intimidate you. Buy a burret and plexiglass plate and measure your engine before you have the machine work done.

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