How To Calculate Your Engine's Compression
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 four-wheelers 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 with a 0.015-inch deck height, a 0.038-inch head gasket, 76cc heads, and pistons with 4.5cc valve reliefs-and you'll hear 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 cubic inches (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 (not counting any dishes or domes) when the piston is at TDC. In many engines, especially 350 Chevys found in 4x4s, 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 cubic inch.