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We will use this section to provide you with basic math formulas that can be directly related to vehicles and performance.

Engine Displacement
Compressed Head Gasket Volume
Mean Piston Speed

Displacement of an Engine

Computing engine displacement is basically computing the volume of a cylinder. However, please note that the standard formula used for computing the displacement of an engine does NOT take into account piston configuration (such as indentations for valve clearance, high dome hemi pistons, if the piston does not come all the way to the top of the cylinder, etc), combustion chamber volume, head gasket thickness or several other factors. Therefore, the formula does not compute the actual volume of one of your cylinders.

What you get when when you compute displacement is the SWEPT VOLUME of a cylinder or an ideal volume. It is measured from BOTTOM DEAD CENTER (BDC) to TOP DEAD CENTER (TDC). You may want to remember the terms BDC and TDC as we will be using them in many of the explanations, especially when we get into discussions of camshafts and camshaft timing. We will also be using the terms, BEFORE and AFTER TOP DEAD CENTER (BTDC, ATDC) and BEFORE and AFTER BOTTOM DEAD CENTER (BBDC, ABDC).

The basic formula for computing the volume of a cylinder is:

This translates into the following for engine computation:

Let's try an example. An early model vertical twin Triumph has a bore of 2.795 inches and a stroke of 3.228 inches and two cylinders. Substituting those values into the above formula, we get:

This gives us a cubic inch displacement of 39.611 or, as it is commonly rounded UP to, 40 cubic inches.

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Compressed Head Gasket Volume

Another part of displacement, if you wish to find out what the displacement truly is, is to compute the volume of a compressed head gasket. Typically the formula calls to use the bore of the engine but I find it much more realistic to use the measured diameter of the head gasket. This can vary from the measurement of the bore by several thousandths.

If you are "blueprinting" an engine (a term we will get into soon), it may be a wise idea to buy several head gaskets and use a set that have the closest diameter measurement. This will insure that your combustion chambers have a much more similar volume, a factor critical to optimal performance.

Click Me to compute YOUR Head Gasket Volume

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Mean Piston Speed

Piston speed is a very important figure to know for your engine. It is the rate at which at which the piston moves up and down within your engine. Piston speed is usually measured in feet per minute (FPM).

In it's simplest form, piston speed is simply how far a piston travels in a complete revolution of the crank during a specific time period. Bear in mind that the piston moves down once and up once so the distance traveled is two times the stroke. It should also be noted that this is "average" piston speed. Piston speed itself is based on RPM, the higher the RPM, the higher the piston speed.

The formula for finding piston speed at any given RPM is:

                                 2 x stroke in inches x RPM
         PISTON SPEED IN FPM = ------------------------------
                                          12
    

One other very important thing to remember is that your piston is not always traveling at the same speed, even for a given RPM. As the piston nears Top Dead Center (TDC) and Bottom Dead Center (BDC), it actually travels slower. I know, I know. But it's a trigonometry thing. As a matter of fact, at either point, TDC or BDC, your piston actually STOPS!

Compute your piston speed at a given RPM

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