Compression Ratio Formula

  • Item #: Compression Ratio
  • Condition: New
  • Compression Ratio:
  • This formula comes in handy when you're designing your engine and you need to know what Compression Ratio you're going to end up with when choosing the stroke of the crank, the final bore size and the cylinder head chamber size. Compression Ratio or CR is described as the amount of space a cylinder has when the piston is at BDC (Bottom Dead Center) versus the amount of space there is when the piston is at TDC (Top Dead Center) and is indicated in ratio form. It is one of the more difficult numbers to get absolutely accurate. That can only be done if the engine is disassembled and you can measure the volume of several critical areas, i.e., the compressed gasket volume, the deck height clearance volume, etc. You need five different measurements. They are described below. For the sake of keeping it simple we'll give you some averages to work with, with respect to some of these numbers. But first we'll give you the actual formula. We'll use a typical 350 Chevy engine.
  • Compression Ratio = Cylinder Volume + Deck Clearance Volume + Head Gasket Volume + Piston Dish or Dome Volume + Cylinder Head Chamber Volume / Deck Clearance Volume + Head Gasket Volume + Piston Dish or Dome Volume + Cylinder Head Chamber Volume
  • 1. Cylinder Volume:
  • The cylinder volume is the actual volume of just the cylinder with the piston at BDC which is calculated by the bore and the stroke times 0.7853982 (the number to calculate the area of a circle.)
  • Example:
  • Cylinder Volume = 0.7853982 X Bore X Bore X Stroke
  • Cylinder Volume = 0.7853982 X 4.0" X 4.0" X 3.48"
  • Cylinder Volume = 43.73 CI
  • 2. Deck Clearance Volume:
  • Deck Clearance Volume is the space left by the piston from the top of the piston to the top of the block deck surface. Although this number can range from 0.003" to 0.020", we're using 0.015" as an average.
  • Example:
  • Deck Clearance Volume = 0.7853982 X 0.015" X Bore X Bore
  • Deck Clearance Volume = 0.7853982 X 0.015" X 4.0" X 4.0" = 0.188 CI
  • 3. Head Gasket Volume:
  • The Head Gasket Volume is the space that is left by the diameter and compressed thickness of the gasket, that can range from 0.015" to 0.051". We're using 0.030" thick gasket for our calculation.
  • Example:
  • Head Gasket Volume (HGV) = 0.7853982 X 0.030" X 4.0"X 4.0" = 0.376 CI
  • 4. Piston Dish or Dome Volume:
  • The Piston Dish or Dome Volume is the amount of material which either cuts into the top of the piston like valve reliefs or protrudes upward like a higher compression dome. This will actually need to be measured but we'll use a flat top piston with 0.275 CI valve reliefs for our calculation.
  • Example:
  • Piston Dish or Dome Volume = 0.275 CI
  • 5. Cylinder Head Chamber Volume:
  • The Cylinder Head Chamber Volume is the combustion chamber of the cylinder head. Although this number can vary depending on the condition of the head, we're using 64CC heads for our calculation. This number will need to be converted to CI by multiplying the 64 CC by 0.0610237.
  • Cylinder Head Chamber Volume = 64 CC X 0.0610237 = 3.905 CI
  • Final Calculation of Compression Ratio:
  • Example:
  • Compression Ratio = Cylinder Volume + Deck Clearance Volume + Head Gasket Volume + Piston Dish or Dome Volume + Cylinder Head Chamber Volume / Deck Clearance Volume + Head Gasket Volume + Piston Dish or Dome Volume + Cylinder Head Chamber Volume
  • Compression Ratio = 43.73 + .188 + .376 + .275 + 3.905 / .188 + .376 + .275 + 3.905
  • Compression Ratio = 48.474 / 4.744
  • Compression Ratio = 10.21:1
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