Calculating Engine Compression - The Power of Squish

At first glance, the concept of engine compression is simple enough:It's the ratio of the volume of the combustion chamber and bore when thepiston is at bottom dead center (BDC) versus when it is at top deadcenter (TDC). Easy as pie.

That's fine in concept, but in stock carracing, where the competitors are so equal, a car that is giving away afew tenths of a point of compression can be at a significantdisadvantage. A droplet of race fuel has a specific amount of energycapacity that is released when burned, so it only makes sense that themore fuel you can squeeze into a given area and burn efficiently, themore power you can produce. Increasing the compression effectively doesthe same thing because you are packing the same amount of air and fuelinto a tighter bundle, so when the spark ignites the burn, it packs thatmuch more punch.

Of course, that's the reason many racing series limitcompression--because they know that's one of the surest ways to limitpower. That's fine, as long as everybody is following the same rules,but are you sure you are taking advantage of all the compression you areallowed? If you don't know how to precisely calculate your enginecompression, there is no way you can know.

The Basics

Before you can calculate compression, you have to know your engine'sdisplacement. The easy answer is just to list the displacement listedfor your block in the catalog, but we need more precision than that.Displacement is defined simply as the area swept by the top of thepiston as it moves up or down the cylinder bore one time. It does notinclude any area above TDC, meaning the combustion chamber. Let's take astock Chevy 350 as an example. It has a four-inch bore and 3.48-inchstroke.

The total displacement is eight times 43.73 or 349.84. If youare racing a four-cylinder engine, you would multiply by four instead ofeight. That strange number--0.7854--is simply a constant that convertseverything to cubic inches.

The formula for calculating displacement for one cylinder is:

Cubic Inches = Bore x Bore x Stroke x 0.7854

In our example, the calculation works out like this:

4 x 4 x 3.48 x 0.7854 = 43.73 cubic inches

Calculating Compression Ratio

Calculating the displacement is the easy part. All you need to know for that is bore and stroke. To understand the compression ratio, you also need to take into account all of the area that remains in the combustion chamber when the piston is at TDC. Easy enough--that's just the combustion chamber volume, right? Unfortunately, that's only part of the equation. There are more things inside the combustion chamber that can affect compression ratio than Richard Petty has cowboy hats. Chamber design, seat depth, valve pockets, piston dish, ring depth, gasket thickness, the list goes on.

The calculation for determiningcompression ratio works out likethis:

CR=(D + PV + DC + G + CC) / (PV + DC + G + CC)

CR = Compression Ratio

D = Displacement

PV = Piston Volume

DC = Deck Clearance Volume

G = Gasket Volume

CC = Combustion Chamber Volume

Variables that affect piston volume are domes, dishes (which includevalve pockets), and the side clearance (which also includes the openarea inside the top ring groove that isn't occupied by the ring). If youare purchasing off-the-shelf pistons, the manufacturer can tell youthese volumes. For instance, a 5cc dome on the top of a piston willincrease compression ratio versus a flat-top piston. Likewise, largevalve pockets will decrease compression. We'll get into the calculationslater.

Less critical--but still important--when it comes to understandingcompression is the piston's side clearance. Side clearance is the areabetween the side of the piston and the cylinder bore. It extends fromthe top edge of the piston down to the top ring. A little-known factabout most racing pistons is that the diameter of the piston above thetop ring land is smaller than the rest of the piston. The reason is thispart of the piston receives so much heat from combustion, and extra roomis required to allow for expansion. Because racing pistons try tominimize the ring depth, the volume of the side clearance is minimal.Most sanctioning bodies that attempt to regulate compression ratio willallow 1 cc for piston side clearance.

Deck clearance volume isdetermined by the distance between the top of the piston at TDC and thedeck of the block. Normally, on racing engines, the piston at TDC isbetween 0.005 inches and 0.020 inches below the deck. It is possiblethat the piston has a zero deck height (even with the deck of the block)or even extends above the block, in which case it should be given anegative value in our equation. You can measure deck clearance using abridge and dial indicator with the piston at TDC. Be aware thatpistons--especially a cold piston in a cold bore--can rock on its pin, soalways measure along the axis of the piston pin.