TECH EXCLUSIVE: BUILDING PRESSURE
Building horsepower involves paying attention to detail. Without following that simple initiative, one cannot expect to end up with a powerplant that is able to traverse the quarter-mile in the shortest amount of time
One of the more important aspects involved in the production of horsepower involves the calculation of the compression ratio within a cylinder’s combustion chamber. In extremely simple terminology, compression ratio is the difference in the volume of the cylinder and cylinder head combustion chamber with the piston at BDC (Bottom Dead Center) divided by the volume of the cylinder/combustion chamber combination with the piston at TDC (Top Dead Center).
Diamond Piston’s Ron Beaubien explains, “For example, if the total swept volume of a 632-cubic inch big-block Chevrolet is 1380.34cc (cubic centimeters) and the total compressed volume is 86.69cc, the compression ratio would be stated as 15.92:1.”
Before measuring the volume of a piston dome, the piston must be placed down in the bore a certain amount to allow the flat acrylic plate to clear the dome.
What it all comes down to is the ability to build the maximum amount of pressure within a cylinder. In some categories of competition, such as when it comes to the Super Stock and Stock class cars, compression ratio is often dictated by sanctioning body rules. However, engine efficiency is usually the deciding element in the choice of just “how high”. Higher compression ratios are not always the ones best calculated to succeed. When you over-compress the cylinder gases you induce a “pumping loss;” after all, it takes horsepower to squeeze the cylinder’s contents. It’s here where piston dome height comes into play.
Keith Wilson of Wilson Manifolds, who for thirty years has distinguished himself with the leading racing teams in attaining better air-fuel distribution and cylinder filling, comments, “Efficient cylinder filling allows us to keep the piston dome as flat as possible—we try not to shoot our fire over a hill.
“When you examine an intake port of an assembled engine,” the Fort Lauderdale induction specialist added, “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 dome. Any encumbrance will harm the engine’s ability to produce power.”
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, head of Diamond, “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.”
In addition, overly zealous ignition timing is not always recommended either. In most cases, firing the spark plug BTDC as we often do is necessary in order to complete the combustion cycle. Cylinder pressure is what the key here is and having the maximum amount of pressure occur at a point roughly 15-degrees ATDC (After Top Dead Center) has been found to be ideal situation.
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.”
Regardless of the reasons, of which there are many, calculating the compression ratio is one of those “detail” items that can make or break an engine combination. Calculating compression ratios accurately is important for at least three reasons.
Next you calculate the volume of a flat piston in the bore. For example, if the piston was placed down a 4” bore by .250” the formula would be bore x bore x .7854” x .250” which equals 3.142 cubic inches. Multiply that by 16.39 to arrive at the cc’s which in this case equals 51.5 cc’s. Finally, if your measurement with the burette is, say, 38cc’s it would mean the dome volume is 13.5 cc’s (51.5 – 38).
“First,” says Fox, “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 manufacture 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 with such rapidity? It is hard to say, since it can lead to frustrating repercussions for the racer.
There are several areas where care in measurements must be taken to accommodate the piston manufacturer. The first is cylinder head 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 to seal the combustion chamber, filling a burette graduated in cc’s with a colored liquid and transferring the liquid to the combustion chamber through a small hole in the plate.
Second, block deck height must be ascertained and it 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 other vital measurements such as piston-to-deck height, piston compression height, and crankshaft stroke and rod length.
Next you calculate the volume of a flat piston in the bore. For example, if the piston was placed down a 4” bore by .250” the formula would be bore x bore x .7854” x .250” which equals 3.142 cubic inches. Multiply that by 16.39 to arrive at the cc’s which in this case equals 51.5 cc’s. Finally, if your measurement with the burette is, say, 38cc’s it would mean the dome volume is 13.5 cc’s (51.5 – 38).
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 slightly down the bore. This small fudge factor gives them the ability to take a skim cut off the decks at a later date if needed. The thickness of the head gasket to be used is also a required number on the piston order form.
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. By carefully supplying your piston manufacturer with the proper numbers, you can then be assured of receiving a piston that will perform the job intended and help to produce a powerplant which will assist you in your quest for quick elapsed times.
Fuel car teams typically replace the pistons after every race pass and every second qualifying run, while Pro Stock teams replace them after approximately 40 passes. ADRL Pro Extreme champion, Frankie Taylor, replaces his pistons every six to nine months. Fortunately, most weekend warriors will replace a set of pistons on a less frequent basis, but doing so in any case can lead to compression ratio changes.
The importance of compression ratio in an internal combustion engine cannot be overstated. Equally important when ordering custom pistons is the ability to correctly supply the manufacturer with enough necessary dimensions to assure you receive exactly what your engine combination requires.
