Compression ratios, or C/R, are a sort of dark art. There are dozens of myths surrounding them, and only now, with computer generated modeling of airflow and the combustion process, are scientists and engineers beginning to fully understand how they work.

Regardless, knowing how to calculate your engine's C/R is especially important when you're seeking to improve performance.

There are dozens of simple formulas out there, but they are often a good percentage off from the actual C/R. And being slightly off in your calculation could mean the difference between an engine that purrs like a kitten and an engine that detonates and sends a piston through your crankshaft.

Point being, you don't fuck around with compression ratios. You must precisely measure each of the specifications, then perform the correct calculation. Here's what you'll need to know.

Measurements

  • Cylinder Bore
    This is the diameter of the cylinders of your engine block. Since you'll most likely have a machine shop bore or hone your cylinders, it is best that you have then perform this measurement down to the thousandth of an inch (or, to a hundredth of a millimeter) and give you that number.
  • Crackshaft Stroke
    This is the distance the piston travels from the top to the bottom of the cylinder. You measure this from the centerline of the crankshaft to the centerline of one of its arms. Then double it. Or, you could have the machine shop measure this for you as well.
  • Combustion Chamber Volume
    Combustion chamber, in this instance, refers to the bottomside of the head, where the valves are sitting. One can measure this with some Play-Do, but that's not extremely accurate. You want extremely accurate. Have the shop measure this. And while they're doing that, why not have them do a port and polish and 5-angle valve job?
  • Crown (-) or Dish (+) Volume
    Some pistons are not flat-topped. They may have extrusions, known as crowns, or indentations, known as dishes. While the shop guys are calculating the volume of your combustion chambers, have them figure out how much volume is reduced by the piston crown, or increased by the piston dish.
  • Head Gasket Thickness
    When you buy a head gasket, it will typically state how thick it is. Just look on the box. If you're still not sure, use a micrometer to gauge thickness.
  • Head Gasket Bore
    This is the diameter of the cylinder holes in the head gasket. Typically, they are NOT exactly the same size as your cylinder bore - So measure them.
  • Deck Height
    This is the distance between the crankshaft centerline (when mounted in the engine block) and the deck. The deck is the flat surface which the cylinders are embedded in.
  • Compression Height
    The compression height is the distince between the centerline of the piston wrist pin and the top of the piston. If the piston has a crown or dish, you do not count it - You only count the major flat portion of the piston.
  • Connecting Rod Length
    This is the distince between the centerline of the piston wrist pin and the centerline of the crankshaft.

Math

Okay, kiddies, break out your calculators, a pencil, and some paper. We're gonna calculate compression ratios! Wheeeee! First of all, make sure your measurements are all the same standard - You don't wanna be multiplying millimeters by inches and end up with some wacky number. Due to the precision involved, I prefer to work in millimeters. And I'm American. Stuff that in your pipe and smoke it.
  1. Determine Cylinder Volume
    The formula for calculating the volume of a cylinder is:
    ( π * ( r 2 ) * h )
    Radius is one-half of the cylinder bore. Height is equal to the crankshaft stroke. Work this formula out and you now have the displacement of an individual cylinder.
  2. Determine Combustion Chamber Volume
    This combustion chamber volume is very different from that listed above. It takes into account the size of the head combustion chambers, the volume contained in the head gasket, the volume between the top of the deck and the top of the piston, and the volume subtracted/added by the piston crown/dish. Therefore, determining this has several steps of its own.

    1. Determine Head Gasket Volume
      You use the same formula for head gasket volume as you did for cylinder volume, since they're both cylindrical in nature.
    2. Determine Volume Between Deck and Piston
      This one's trickier, but not much. You know the diameter of the cylinder already, so you can easily find the radius. We know deck height. But how far from the deck is the piston? Simply add the Compression Height to the Connecting Rod Length. Then subtract that number from the Deck Height. You've now got the height portion of this equation. Use the cylinder formula and you'll have this measurement.

    Now, you have Head Chamber Volume, Gasket Volume, Deck to Piston Volume, and Crown Volume. Add them up! This gives you the volume of gasses after the piston has reached top dead center of its stroke.
  3. Determine Compression Ratio
    You're almost home. Now, compression ratio, as stated in the previous wus, is the ratio of the volume of the air present in the cylinder at bottom dead center compared to the amount at top dead center. At BDC, the volume is the total between Cylinder Volume and Combustion Chamber Volume. At TDC, it's just the Combustion Chamber Volume. That leaves us with this formula:
    ( (Cylinder Volume) + (Combustion Chamber Volume) ) / (Combustion Chamber Volume)
    Do the math and voila! You've got your compression ratio!
For a gasoline-based engine, you should end up with a number somwhere between 7 and 13. Typically, compression ratios are displayed as ratios, and therefore, if your number was 8, you would say you have an 8:1 compression ration.

This concludes the lesson on how to calculate compression ratios. Go forth and hot rod.