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You'd think that the pistons listed for a 10.5:1 compression ratio would actually give you 10.5:1. But it's usually not that simple. Perhaps that's why so many car crafters have a foggy or incomplete understanding of compression ratios. To clear things up, this story will define what compression ratio is, let you know how to alter it, and show you how to calculate it for any engine.
Throughout the story we'll use the example of a typical 350 Chevy (4.000-inch bore, 3.48-inch stroke) with a 0.015-inch deck height, a head gasket with a 4.100 gasket bore and 0.038-inch compressed thickness, 76cc heads, and pistons with 4.5cc valve reliefs--and you'll see what these numbers mean as we go.
What Is Compression Ratio?
Remember what happens during the compression stroke of the four-stroke cycle: Both the intake and exhaust valves are closed so no air can escape, and the piston moves upward from bottom dead center (BDC) to top dead center (TDC) so that the air/fuel mixture in the cylinder is compressed into the combustion chamber. Compression ratio is the relationship of cylinder volume (or displacement) with the piston at BDC to cylinder volume with the piston at TDC. If the volume of the cylinder with the piston at BDC is 10 times greater than the volume of the combustion area with the piston at TDC, then 10 units of volume get squeezed into 1 unit of space, and the compression ratio is 10.0:1.
There are five factors that affect compression ratio: cylinder swept volume, clearance volume, piston dome or dish, head-gasket volume, and chamber volume.
Cylinder Swept Volume
The swept volume of the cylinder indicates how much air the piston displaces as it moves from BDC to TDC. Increasing the cylinder volume without making any other changes will increase the compression ratio because it enlarges the cylinder volume without increasing the combustion chamber volume. In other words, the piston will have to cram more air into the same amount of space. Cylinder volume is calculated using the bore and stroke of the engine with this formula: Cylinder volume = 0.7853982 x bore2 x stroke
On a standard 350 Chevy, the bore is 4.00 inches and the stroke is 3.48. Apply the formula, and you'll find that one cylinder is 43.730 ci (multiply this times eight cylinders and you get 349.84, which is rounded to 350 for total engine displacement).
If you overbore our sample 350 from 4.00 inches to 4.020 inches and make no other changes, the compression ratio will increase from 8.84:1 to 8.90:1 because the volume of the cylinder has increased. When overboring an engine, the percentage of gain in compression ratio decreases as you add clearance volume and increases as you remove clearance volume, as we'll describe next.
Clearance Volume
Clearance volume is determined by the distance from the cylinder block deck to the top of the piston flat (not counting any dishes or domes) when the piston is at TDC. In many engines, especially 350 Chevys found in cars, the pistons don't come all the way up to the height of the deck--they can be anywhere from 0.003 to 0.020 inch below it. This amount is known as the piston deck height, and it affects compression ratio because it affects the volume of air in the combustion area when the piston is at TDC. If the piston is farther below the deck, then clearance volume is increased and the compression ratio is reduced. If the piston is closer to the deck, clearance volume is reduced and compression ratio is increased.
Here's how to calculate the clearance volume once you know the piston deck height: Clearance volume = 0.7853982 x bore2 x deck height
In our sample 350 with a deck height of 0.015 inch (meaning the top of the piston is 0.015 inch below the deck of the block), the clearance volume is 0.188 ci.
If the deck height of our sample engine was increased to 0.020, compression would drop from 8.84:1 to 8.75:1. If the deck height of our sample engine was decreased to 0.003, compression would increase from 8.84:1 to 9.05:1.
Piston Dome
Note that clearance volume does not take into account any pop-up domes or sunken-in dishes on the head of the piston. These configurations also increase or decrease volume in the combustion chamber and affect the compression ratio. The manufacturer's catalog will list the displacement in cubic centimeters of the dishes or domes on the piston, but we've found that it's not consistent whether they express the cc's of a dish as a positive or a negative number. For the purposes of calculating compression, we prefer to view the cc's of a dish as a positive number because a dish adds volume to the cylinder (and reduces the compression ratio); a dome is a negative number because it subtracts volume from the cylinder (and increases the compression ratio).
Another confusion with piston designations is that they're listed in cubic centimeters, but we use cubic inches to calculate compression ratio. You can convert to cubic inches with this formula: Piston dome or dish in cubic inches = cc's x 0.0610237
Since our sample engine uses pistons that have 4.5cc dished valve reliefs in them, then they increase the volume of each cylinder by 0.275 ci. If we changed to pistons with a dish of 22 cc (1.34 ci) and made no other changes, then the compression ratio would drop from 8.84:1 to 7.58:1. If we used pistons with a dome of 12 cc (0.73 ci), then the compression would increase from 8.84:1 to 10.56:1.
Head-Gasket Volume
Head-gasket volume is determined by the compressed thickness of the gasket. A thicker gasket adds volume and reduces compression; a thinner gasket reduces volume and increases compression.
A gasket's compressed thickness is listed in the manufacturer's catalog and ranges from 0.051 inch to 0.015 inch. Also, the gasket bore is often larger than the engine bore; a 4.100-inch gasket is common. In our example, we assumed a head gasket with a 4.000-inch bore. Once you know the compressed thickness and gasket bore, here's how to calculate the volume that the gasket will add to the combustion area: Head-gasket volume = 0.7853982 x gasket bore2 x compressed thickness
In our example with a 0.038-inch thickness and 4.000-inch bore, the gasket adds 0.478 ci to the volume of the cylinder. If we used a thinner 0.015-inch gasket and made no other changes, the compression ratio would increase from 8.84:1 to 9.27:1.
Chamber Volume
The volume of the combustion chambers is the final factor in determining compression ratio. The larger the chamber, the more volume is added to the cylinder and the lower the compression ratio; smaller chambers reduce volume and increase the compression ratio.
For small-block Chevys, chamber sizes range from around 58 cc to 78 cc. However, the volume of the chambers can vary greatly depending on the type of heads and valves used, the amount the heads may have been milled, the number of valve jobs that have been performed, and any custom chamber grinding that has been done. Manufacturers of cylinder heads will tell you the range of sizes of the chambers in their heads, but for any used or custom-machined heads, the only way to know the size of the chambers is to have a machine shop check. Once this number is known, here's how to convert it from cubic centimeters to cubic inches: Chamber volume in inches = cc's x 0.0610237
Therefore, the 76cc chambers in our 350 have a volume of 4.638 ci. If we were to use cylinder heads with 58cc chambers and make no other changes, the compression ratio would increase from 8.84:1 to 10.72:1.
Add It Up
Once you have all the information listed above, youre ready to calculate the compression ratio of the engine youre building. First you add up the volume of the cylinder with the piston at BDC, then divide it by the volume with the piston at TDC. Heres the formula:
Compression Ration =
Cylinder vol. + clearance vol. + piston Comp. vol. + gasket vol. + chamber vol. divided by Clearance vol. + piston vol. + gasket vol. + chamber vol.
Apply this to our example of the Chevy 350 with the 3.48-inch stroke, 4.00-inch bore, 0.015-inch deck height, 0.038-inch head gasket with a 4.000-inch bore, 76cc heads, and 4.5cc dished pistons, and here’s what it looks like:
43.730 + 0.188 + 0.275 + 0.478 + 4.638 divided by 0.188 + 0.275 + 0.478 + 4.638 =8.84:1
This engine has an 8.84:1 compression ratio. When using this formula, don't forget that the displacement of domed pistons should be expressed as a negative number.
