What is compression ratio and how do you calculate it? One popular misconception is that pistons alone determine compression ratio; however, this isn’t true. Compression ratio comes from not only piston dome or dish features, but also stroke, bore, and combustion chamber size. Compression comes from piston travel from bottom dead center (BDC) to top dead center (TDC) with both valves closed. Cylinder volume (displacement) is squeezed into the area above the piston. Compression ratio is cylinder volume at BDC versus cylinder volume with the piston at TDC. For example, if cylinder volume with the piston at BDC is 10 times more than it is with the piston at TDC, then the compression ratio is 10.0:1, or simply 10:1.
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Five basic factors affect compression ratio: swept volume, piston dome,clearance volume, head gasket volume, and combustion chamber size.
Swept Volume
Swept volume is the amount of air (or volume) the piston displaces during its journey to the top of the bore; hence the word “swept.” If you enlarge swept volume by boring the cylinder oversize (or increasing stroke), you increase compression ratio.
Piston Dome
You may also increase or decrease compression ratio by changing the piston dome. If you “dish” the piston (giving it a concave shape), you lose compression. This is common with stock pistons, which are often dished to reduce compression. A good example is the 351C-2V with its dished pistons and open chambers. To raise compression ratio, the piston is “domed” (giving it a convex shape, similar to that of the combustion chamber). This reduces clearance volume at the top of the bore. When you reduce clearance volume, you increase compression ratio. Whenever you go to an aftermarket head, keep combustion chamber size in mind. The new combustion chambers can wind up larger than your stock chambers. If you desire greater compression, you can make adjustments with proper piston selection.
Cylinder Volume
Cylinder volume is calculated using a simple formula. For example, using a standard 351C bore and stroke (4.000 x 3.500 inches), the numbers work out like this:
Cylinder Volume = .7853982 x bore2 x stroke
When you apply this formula, you come up with 43.982 ci per cylinder. Multiply this number by eight and you have 351 ci. Truth is, you have 351.858, which is closer to 352 ci.
If you bore the same cylinder to 4.030 inches, you have 44.644 ci per cylinder, which comes out to 357 ci.
If you take a standard 4.000-inch bore and overbore it by .030 to 4.030 inches, compression increases by a fraction of a point. If you have a compression ratio of 10.0:1, compression increases by less than a point with a .030-inch overbore.
Even valve reliefs are considered part of swept volume; checked with a graduated cylinder and fluid. The larger the valve reliefs, the lower the compression ratio.
You might be inclined to ask why you should dish a piston with an increase in stroke. The answer is pistons are dished to keep compression conservative.
You reduce swept volume with a domed piston to raise compression because you’re reducing volume above the piston when it reaches TDC.
Clearance Volume
You compute compression increase (or decrease) by calculating the clearance volume, which is the area above the piston when it reaches TDC. It is important to understand that the piston doesn’t always reach TDC flush with the block deck. In most applications, the piston comes within .005 to .020 inch below the deck surface. This is called piston deck height, which affects compression because it determines clearance volume at the top. If you have a lot of clearance volume, you have less compression. The greater the piston deck height, the lower the compression ratio.
The following is a formula for calculating clearance volume.
Clearance Volume = .7853982 x bore2 x deck height
Again, let’s look at our example 351C engine with a 4.000-inch bore and 3.500-inch stroke. Let’s say it has a piston deck height of .015 inch below the block deck. Using the formula works out to a clearance volume of .188 ci, or just a fraction of the cylinder’s 43.98 ci. If the deck height increased any amount, compression would drop. If deck height decreased any amount, compression would increase.
Next is to figure in the piston’s role in all of this. Remember that if you dish the piston, you lose compression. If you dome the piston, you increase compression. Most piston manufacturers give the specifications for a piston. If it is dished, the manufacturer tells you how much. Likewise for a domed piston, you learn how much—in cubic centimeters.
You can use the following formula to convert cubic centimeters into cubic inches.
Cubic Inches = cubic centimeters x .0610237
Back to our 351C engine. Let’s say our 351 has dished pistons with a volume of 4.00 cc (or .244 ci). This lowers the compression ratio because you have more clearance volume above the piston. If you dome the piston by the same amount, you increase compression accordingly.
Cylinder Head Gasket Volume
The next factor in compression ratio is cylinder head gasket volume, which contributes to clearance volume above the piston. The thickness of the head gasket affects compression ratio. The thicker the head gasket, the greater the clearance volume. This lowers compression. The thinner the head gasket, the lower the clearance volume, which increases compression. To fi gure the head gasket volume (displacement), use the following formula.
Cylinder Head Gasket Volume = .7853982 x bore2 x compressed thickness
Again our 351-ci engine with a 4.000-inch bore. You have a cylinder head gasket that is .040 inch thick. You take .7853982 x 4.000 inches to the second power x .040 inch to arrive at .502 ci of clearance.
Combustion Chamber Size
Combustion chamber volume is the actual size of the chamber in cubic centimeters. Chamber size for closed-chamber 351C-4V heads runs approximately 62 to 64 cc. Chamber size for open-chamber 351C heads runs 72 to 77 cc.
Combustion chamber volume is figured with a graduated scale using fluid. You meter fluid into the chamber and figure how much fluid is used. You get this figure in cubic centimeters. Our sample cylinder head has 64-cc chambers.
Here’s how to turn cubic centimeters into cubic inches:
Combustion Chamber Volume in Cubic Inches = cubic centimeters x .0610237
Based on the formula, at 64 cc, you have 3.90 ci of volume in the chamber alone.
Compression Ratio Calculation
Now, you have all of the information needed to compute compression ratio in your 351C engine. Use the following formula.
Compression Ratio = (cylinder volume + clearance volume + piston volume + chamber volume + head gasket volume) Ă· (clearance volume + piston volume + head gasket volume + chamber volume)
The math for our 351C engine, with its 4.00-inch bores and 3.50-inch stroke, .020-inch deck height, .040-inch head gasket thickness, 64-cc chamber heads, and 4.000-cc dished pistons works out like this:
(43.982 ci + .188 cc + .244 cc + 3.90 ci + .502 ci) Ă· (.188 ci + .244 ci + .502 ci + 3.90 ci)
48.816 Ă· 4.83 = 10.10:1
These figures are added as you measure space between piston dome and block deck with the piston at BDC. You can even take this to the extreme by measuring the volume above the top ring around the piston’s circumference because that also counts.
Written by George Reid and Republished with Permission of CarTech Inc
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