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Compression ratio, as any engine builder will tell you, is one of the most important design characteristics of any engine. Given the octane of the fuel you’re planning to use and the operating conditions, a compatible compression ratio is crucial if you don’t want the motor to come unglued.

Compression ratio isn’t hard to handle: it’s the ratio between the cylinder volume above the piston at bottom dead center (BDC) and the volume above the piston at top dead center (TDC). It might seem that at TDC all we need to know is the combustion chamber volume. Not so: the compressed thickness of the head gasket must be considered along with the area around the piston from the deck to the top compression ring. The piston may have valve reliefs cut in; if so, the volume of these must be added to the calculations; if the piston features a dome, the dome volume must be subtracted. Sound complicated? It’s not all that bad!

The tools necessary for this measurement job are about the same ones used when "cc’ing" cylinder heads. You’ll need a 100cc burette (a glass cylinder graduated in cubic centimeters, with a valve at the bottom) and stand, available at medical supply houses. A six by six inch piece of flat Plexiglas with a hole in the center is next, and a couple of bottles of isopropyl alcohol (rubbing alcohol). We add a little machinist’s dykem to the alcohol to give it color, making the graduations in the burette easier to read. Another item, which should be on the list, is the all-American handy dandy pocket calculator. Working with figures with six or more digits and keeping track of the decimals can be a pain without one. Accuracy is extremely important: a mere 3cc error is enough to throw your final figure off by a half point, say from 12:l to 11.5:1.

The other tools required are a couple of relatively simple formulas. The first is that used to find the volume of a cylinder. V=πr2h, where r is the radius of the base of the cylinder h is its height, and π is that old familiar constant, Pi, about 3.1417, in our example, we’re going to be using inches for the radius and height measurements, so we’ll have to remember to convert our found volume from cubic inches to cubic centimeters by multiplying it by 16.387

The other formula needed is an outgrowth of the definition of compression ratio, which is equal to the volume BDC divided by the volume TDC, as mentioned earlier. When all of the factors involved in a combustion chamber are separated so we can measure them, it looks like this:

CR = __SV+CCV+HGV+DHV+EDV__

CCV+HGV+DHV+EDV

CCV+HGV+DHV+EDV

Where CR = compression ratio

CCV = combustion chamber volume

HGV = head gasket volume

DHV = deck height volume

EDV = effective dome volume

SV = swept volume

CCV = combustion chamber volume

HGV = head gasket volume

DHV = deck height volume

EDV = effective dome volume

SV = swept volume

(Note that the EDV will be a negative number, to be subtracted rather than added, if the piston has a dome. It will be a positive number if the piston has valve relief depressions.)

What we need to do, then, is measure each of the five (CCV, HGV, DHV, EDV, and SV) and plug them into the formula for our answer.

COMBUSTION CHAMBER VOLUME(CCV)

We’ll read this figure directly from the 100cc burette. Install a spark plug of the type you’ll be using in the head, and coat the intake and exhaust valve seats with a light layer of grease. You don’t want any of the alcohol to leak out during the process. A light coat of grease around the edges of the combustion chamber is next: then cover the chamber with the Plexiglas plate, pressing down so the grease around the chamber seals evenly with the Plexiglas. Fill the chamber through the hole in the Plexiglas, making note of the exact number of cubic centimeters of alcohol drained from the burette in doing so. In our example, let’s say we get 70cc=CCV

HEAD GASKET VOLUME (HGV)

Let’s assume also that our engine will be fitted with head gaskets that measure .035 inch thick when torqued to factory specs. Out bore hole measures 4.150 inch diameter (most head gaskets don’t have perfectly round holes, so you’ll have to do a little "guesstimating" to get this figure). Half the diameter is the radius, 2.075. Plug these figures into the old V=Pi r2h, (that’s pi are squared) with 2.075 for r, and .035 for h. The radius squared comes to 4.036, so we have-------- 3.1417x4.036x..035=.473456 cubic inches.

Multiplying by our conversion factor of 16.387 gives us an HGV of 7.75cc.

DHV, deck height volume:

The deck height of an engine is the distance from the top of the piston at TDC to the cylinder head mating surface (the block deck). You'll need to measure it precisely with a depth mike or similar tool. Then plug it into the cylinder volume formula as h, with r as half your bore diameter. In our example the piston is .020 inch down the hole; it's not uncommon to have as little as .005 inch deck height on some all out race applications. The cylinder bore in our example is 4.150 inch --- calculators ready? Radius squared is again 4.306 so 3.1417x4.306x.020=.270563 cubic inches, multiplied by our conversion factor of 16.387 nets a DHV of 4.43cc.

EFFECTIVE DOME VOLUME (EDV)

This figure includes the volume of the dome and/or valve reliefs, plus the area around the piston above the top compression ring. We’re going to go after this one with the burette, using the block with a piston installed. Situate the block so the deck is level, and be sure to coat the top compression ring with grease. Now rotate the crank until the top of the piston is exactly one inch down the hole, using a depth mike if possible. Coat the deck around the edge of the cylinder with a light layer of grease, seat the Plexiglas snugly on top, and again fill with the burette, noting the amount of alcohol needed to fill her. You might lay some newspaper on the floor under the engine, so you’ll hear it drip if you’ve got a leak alcohol evaporates so quickly that you could go without realizing you had a leak until it was too late.

The reading we get with our test engine is 210cc. We have to compare this figure to the volume of the cylinder one inch deep to get EDV. Using the formula again, (Pi), the bore of 4.150 inch for r and 1 for h, we get 3.1417x4.306x1=13.5281 or 221.66cc. This figure we must subtract from our burette reading, giving us a negative number for EDV, due to the dome on our piston: 210-221.68= -11.68. Remember that if you have an engine with flat top pistons and a couple of valve reliefs, the direct readout from the burette is going to be greater than the calculated one-inch cylinder volume. Subtracting from the burette reading gives you a positive number for EDV in such a case.

SWEPT VOLUME (SV)

This value is actually the displacement figure for a single cylinder. There are numerous ways of calculating the swept volume, but we have found our old faithful cylinder volume formula to be the most accurate. The radius is half the diameter as before, and h is the stroke figure which is determined by the crankshaft you are using. Our stroke here is 3.75 inches, so we have 3.1417x4.306x3.75=50.7306 cubic inches; for the last time multiply by the 16.387 conversion factor to get 831.32cc. We now have all the figures we need to calculate the compression ratio.

CR = __SV+CCV+HGV+DHV+EDV__

CCV+HGV+DHV+EDV

CCV+HGV+DHV+EDV

= __831.32+70.0+7.75+4.43 - 11.68__

70.0+7.75+4.43 - 11.68

= __901.82__

70.50

= 12.79

We round this off to 12.8:1 for our final determination of the compression ratio.

All of this should have made it clear that there’s a lot more to ordering pistons than walking into your local speed shop and telling the counterman that you want a set of pistons with a certain compression ratio. All piston manufacturers try to do the right thing, but the aren’t mind readers You have to give them all the right information on your engine.

Don’t tell a piston maker that you are going to use a cylinder head with 70cc combustion chambers and then change your mind and use a head with 110 cc chambers instead. If you plug 110cc into our formula instead of the 70 cc used, the final answer drops from 12.8:l to 8.5:1.

So, take your time and make all calculation as accurate as possible.

HAPPY MOTORING GUYS!! -- *Bill Cole*

This page was created on December 2, 2000 and revised March 5, 2004

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