The Importance of Compression Ratios and how to Measure Them

March 11th, 2011

Text and Photos by Sam Logan

Since race pistons in domestic V-8 engines move up and down in excess of 100 times per second, replacing them is a normal part of the racer’s routine. NHRA Top Fuel and Funny Car teams replace them after every race pass and every second qualifying pass. Pro Stock teams replace them after every 40 passes approximately, and weekend warriors replace them every 12 to 18 months, sooner if their engines are nitrous-assisted. At piston replacement time questions of spec changes usually arise—especially the topic of compression ratios.

Diamond Piston’s Ron Beaubien explains, “An engine’s compression ratio is calculated by taking the total swept volume (with the piston at bottom dead center) and dividing it by the total compressed volume (with the piston at top dead center). For example, if the total swept volume of a 632-ci big-block Chevrolet is 1380.34cc and the total compressed volume is 86.69cc the compression ratio would be stated as15.92:1.”

To find the engine’s total swept area in cubic inches the following formula can be applied: 0.7854 x bore diameter x bore diameter x stroke length x the number of cylinders. To convert cubic inches to cubic centimeters multiply by 16.39. Using a burette is the best method of measuring the compressed volume (chamber volume plus piston volume).

Compression ratios are often influenced by rulebook regulations, and engine efficiency can also be a deciding element in their composition. Success isn’t always best achieved by calculating using high compression ratios, though. When the cylinder is over-compressed “pumping loss” is induced (i.e.: it takes horsepower to squeeze the cylinder’s contents).

Keith Wilson of Wilson Manifolds, who for 30 years has distinguished himself with the leading racing teams in attaining better air-fuel distribution and cylinder filling, notes, “Efficient cylinder filling allows us to keep the piston crowns as flat as possible—we try not to shoot our fire over the hillock!” The Fort Lauderdale induction specialist continues, “when you examine an intake port of an assembled motor with valves adjusted, cam degreed etc., and you observe the intake valve cracked open, it is imperative the mixture flows out and around the valve rapidly into the cylinder during those early moments of valve lift. The incoming air mixture must not be impeded by an awkward shape on the piston. Any encumbrance will harm the engine’s ability to produce power.”

In addition, overly zealous ignition timing is not always recommended either. As Chuck Lawrence of Jon Kaase Racing Engines contends, “Earlier firing of the ignition causes the engine to work harder, as the piston is rising on its compression stroke it has to overcome the premature downward forces of the expanding gases.”

Calculating compression ratios accurately is important for at least three reasons. “First,” says Bob Fox, head of Diamond, “pistons are often requested with compression ratios higher than are physically possible to provide. Second, some sanctioning bodies stipulate strict limitations on compression ratios, and if they are not calculated precisely, the racer could either squander power he could legitimately generate, or unwittingly get caught cheating. Third, if the race engine is designed to meet strict specifications, including operating on a specific race fuel, having the compression ratio calculated properly is worth doing.”

Yet, when piston makers or suppliers request the information necessary to make their pistons to the correct specifications, invariably crucial data is omitted. Failure to complete the custom piston information form is usually the biggest difficulty they face. And why does this wearisome problem recur so often? It is difficult to say, however, it is known that the two chief problems pertain to block deck height and chamber volume.

Chamber volume is measured by inverting the cylinder head on the workbench (complete with two valves and a spark plug installed), placing a piece of thick acrylic plastic (with a ¼- or 3/8-inch-diameter hole in it) over the combustion chamber, filling a 100mm burette graduated in cubic centimeters with a colored liquid and transferring the liquid to the combustion chamber. This measuring process is duplicated to establish the piston volume.

Block deck height is measured from the crankshaft centerline to the block deck, usually with some form of caliper. Knowing the exact block deck height is crucial because it is used to verify four vital measurements: half of the stroke dimension, rod length, compression height and the piston-to-deck dimension.

The piston-to-deck dimension is the measurement from the flat of the piston to the deck surface: Is it to be positioned at zero (flush with the block deck surface) or placed down the bore by a small amount? Most engine builders request the piston-to-deck dimension to be .005- or .010-inch down the bore. This small fudge factor gives them the ability to take a skim cut off the decks at a later date if necessary.

The compression height of the piston, also known as compression distance, is measured from the centerline of the piston pin to the flat on the top of the piston.  Once these dimensions are established accurately, the piston will be positioned at the precise height in the cylinder and the compression ratio will be exactly as desired. However, when some of these vital dimensions are omitted—perhaps the spaces are left blank or they contain the word “stock”—grief usually follows.

For example, a racer has a desired compression ratio of 11.9:1 and assumes the block deck height to be stock. Let’s assume 10.720 inches represents stock, but at some time in the past, and unknown to the present owner, the block was sent to a machine shop where the decks were “cleaned up” and the height is really 10.700 and not 10.720 inches. As a result, the .020-inch difference in compression distance will cause the piston to sit higher in the bore, resulting in a much higher and unwanted compression ratio of around 12.5:1.

Savvy piston makers with experience in different race engine categories will tell you that compression is a most intriguing topic, and that having more is not always to your advantage. “When better cylinder head and induction manifold designs prevail,” says Bob Fox, “less compression is needed because they accomplish better cylinder filling. Therefore, it compresses more air in a given area. But if the cylinder head and the induction system are less efficient, more compression is needed because there is less air in the cylinder.”

The question then becomes, how much air are we drawing into the cylinder? But in the meantime, here is how to provide Diamond and other piston makers the vital information often missing from the piston order form.


Diamond Pistons

Dept. DR

23003 Diamond Drive

Clinton Township, MI 48035


Jon Kaase Racing Engines, Inc.

Dept. DR

735 West Winder Ind. Parkway

Winder, GA  30680

770.307.0241 (http://www NULL.jonkaaseracingengines

Wilson Manifolds

Dept. DR

4700 N.E. 11th Avenue

Oakland Park, FL 33334

954.771.6216 (http://www NULL.wilsonmanifolds

Ernie Elliott, Inc.

2367 Elliott Family Parkway 3

Dawsonville, GA 30534


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