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Putting theSQUEEZE OnThe Importance of Compression Ratios and How to Measure Them

BY SAM LOGAN

Since race pistons in domesticV-8 engines move up and down inexcess of 100 times per second,replacing them is a normal part of aracer’s routine. NHRA Top Fuel andFunny car teams replace them afterevery race pass and every secondqualifying pass. Pro Stock teamsreplace them after every 40 passesapproximately, and weekend warriorsreplace them every 12 to 18 months,sooner if their engines are nitrous-assisted. At piston replacement time,questions of spec changes usuallyarise, especially the topic ofcompression ratios.

Gibtec Piston’s Rob Giebas explains, “Anengine’s compression ratio is calculated bycomparing two volumes in the cylinder. One

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is the swept volume plus the clearance volume.The other is the clearance volume only. Thelarger the first volume and the smaller the second,the higher the engine’s compression ratio”

Swept volume, also called displacement, is thevolume a piston displaces in a cylinder as it movesfrom its lowest point (bottom dead center) to itshighest point (top dead center). Displacementdoes not include the clearance volume, which isthe volume above top dead center. By dividingthe combined swept and clearance volume by theclearance volume only, the compression ratio canbe calculated.

For example, if the swept and clearance volumeof a 632-ci big-block Chevrolet is 1380.34cc(cubic centimeters) and the clearance volumeonly is 86.69cc, the compression ratio would bestated as 15.92:1.

To find the engine’s displacement in cubicinches the following formula can be applied:0.7854 x bore diameter x bore diameter x strokelength x the number of cylinders. To convertcubic inches to cubic centimeters multiply by16.39. Using a burette is the best method formeasuring the clearance volume (chamber volumeplus piston volume above top dead center).

Compression ratios are often influenced bycompetition rule books. Also, engine efficiency is

The inability tomeasure theblock height andthe piston andchamber volumesare the two mainimpediments toachievingaccuratecompressionratios. The mostaccurate methodfor measuringpiston andchamber volumesis to fill thespaces with acolored liquidadministeredthrough acalibratedburette.

a deciding element in theircomposition. Highercompression ratios aren’t alwaysthe ones best calculated tosucceed. When you over-compress the cylinder, youinduce a pumping loss since ittakes horsepower to squeezethe cylinder’s contents.

In addition, overly advancedignition timing isn’t alwaysrecommended either. As ChuckLawrence of Jon Kaase RacingEngines contends, “Earlier firingof the ignition causes the engineto work harder, as the piston isrising on its compression strokeit has to overcome thepremature downward forces ofthe expanding gases.”

Calculating compressionratios accurately is important

for at least three reasons. “First,”says Giebas, “pistons are oftenrequested with compressionratios higher than are physicallypossible to provide. Second,some sanctioning bodiesstipulate strict limitations oncompression ratios, and if theyare not calculated precisely, theracer could either squanderpower or unwittingly getcaught cheating. Third, if therace engine is designed to meetstrict specifications, includingoperating on a specific racefuel, having the compressionratio calculated properly isworth doing.”

Yet crucial data is oftenomitted from custom pistoninformation forms. The twomost common difficulties areblock deck height and chambervolume. Chamber volume ismeasured by inverting thecylinder head on the workbench(complete with two valves and aspark plug installed), placing a

To measure piston volume, first use adegree wheel to attain two objectives:set the piston (complete with rings andattached to the crankshaft with yourlongest rod) at top dead center andsquare the piston with the deck.

Apply a thin line of grease to seal thetop edge of the piston to the cylinderand place a thick acrylic plate over thecylinder. The plate requires a ¼-inch or3/8-inch-diameter hole drilled where thecylinder meets the piston.

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Fill a 100ml burette graduated in cc’swith a colored liquid to the zero mark(some builders use mineral spirits,others rubbing alcohol with a greentint). Then transfer the liquid to thepiston cavity.

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piece of thick acrylic plastic (with a ¼-inch or3/8-inch-diameter hole in it) over the combustionchamber, filling a 100mm burette graduated in cc’swith a colored liquid and transferring the liquid tothe combustion chamber. This measuring process isduplicated to establish the piston volume.

Block deck height is measured from thecrankshaft centerline to the block deck, usuallywith some form of caliper. Knowing the exactblock deck height is crucial because it is used toverify four vital measurements: half of the strokedimension, rod length, compression height andthe piston-to-deck dimension.

The piston-to-deck dimension is themeasurement from the flat area around the pistonperimeter to the deck surface. A decision must bemade whether to place the piston at zero (flushwith the block deck surface) or place it down thebore by a small amount? Most engine buildersrequest the piston-to-deck dimension to be .005inch or .010 inch down the bore. This small fudgefactor gives them the ability to take a skim cut offthe decks at a later date if needed.

The compression height of the piston, alsoknown as compression distance, is measured fromthe centerline of the piston pin to the flat area onthe top of the piston. Once these dimensions areestablished accurately the piston will be positionedat the precise height in the cylinder, and thecompression ratio will be exactly as desired.

However, when some of these vital dimensions areomitted—perhaps the spaces are left blank or theycontain the word “stock”—grief usually follows.

It’s time to crunch the numbers. The burette delivered 16.8cc. Next, we measurethe volume of the combustion chamber

The first step in preparing to measure combustion chamber volume is to seal it byapplying a layer of grease to the mating surfaces of the inlet and exhaust valves.

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Install thespark plug next.

Beforeplacing theacrylic top plateover thecombustionchamber, a thinline of grease isapplied. Takecare not to applytoo much, or itwill enter thechamber to thedetriment of themeasurement.

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Let’s assume the racer has adesired compression ratio of11.9:1 and believes the blockdeck height to be stock. Let’sfurther assume 10.720 inchesrepresents stock. But atsometime in the past, andunknown to the present owner,the block had visited a machineshop where the decks were“cleaned up” and the height is,in fact, 10.700 inches and not10.720 inches. As a result, the.020-inch difference incompression distance will causethe piston to sit higher in thebore, resulting in a much higherand unwanted compressionratio of around 12.5:1.

Savvy piston makers withexperience in different raceengine categories will tell youthat compression is a mostintriguing topic, and thathaving more is not always toyour advantage. “When bettercylinder head and inductionmanifold designs prevail,” saysGiebas, “less compression isneeded because theyaccomplish better cylinderfilling. Therefore, it compressesmore air in a given area. But ifthe cylinder head and theinduction system are lessefficient, more compression isneeded because there is less airin the cylinder.”

SOURCESGibtec Pistons333 West 48th StreetDenver, CO 80216303.243.3340Gibtecpistons.com

Jon Kaase Racing Engines, Inc.735 West Winder Ind. ParkwayWinder, GA 30680770.307.0241JonKaaseRacingEngines.com

Press the plate down onto the grease, and fill the burette with theliquid until it reaches the zero mark.

Dispense the colored liquid from the burette into thechamber. If an air bubble occurs, tilt the cylinder headto bleed the air through the fill hole.

This is the condition for which we’re aiming.

On readingthe burette, welearned ourcombustionchamber volumeis 51.7cc. Thisvalue plus 1cc toaccount for thevolume aroundthe top ringlandand the pistonvolume (in ourcase 16.8 plus)are the figuresthe pistonsupplier needs.

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Block deck height is the measurement from the crankshaft centerline to the block deck.Usually this measurement is captured by some form of dial gauge caliper. Here, Chuck Lawrenceof Jon Kaase Racing Engines uses a steel ball as an aid to obtain an accurate reading.

Then he deducts the diameterof the ball and adds half thecrank main’s journal diameter. Ona Kaase Boss Nine engine, thediameter of the main’s journal is3.193 inches; therefore, toestablish his block deck height,he takes his dial gaugemeasurement, subtracts the steelball measurement and adds1.5965 inches.

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