Mechanical Compression Ratio

October 15, 2010 | By: Iron Works

J&P Cycles® has partnered with Ironworks Magazine to provide great content from their publication for our readers. For additional, great content like this, make sure to check out Ironworks at www.iwblogger.com. The article below is a continuation of an article first featured on Monday’s blog.

Mechanical Compression Ratio

Let’s start with the most basic form of compression ratio, Mechanical compression, sometimes referred to as Static compression. Mechanical compression is a very basic measurement of the engine compression ratio without the camshaft dynamics being used in the calculation.

Mechanical compression is based solely on the volumes of the following components:

Swept volume of the cylinder
Combustion chamber volume
Swept volume of the head gasket
Deck height / quench volume
Piston dome volume

The equation for calculating the Mechanical compression ratio for the engine is the same one used to calculate the Corrected compression ratio. Only the numbers used in the equation will change. Use the following equation to calculate the engines mechanical compression ratio.

3.1416 x Bore x Bore x Stroke
4

This equation is used to calculate the swept volume of both the cylinder and the head gasket. To simplify the equation, the thickness of the head gasket will be used to replace the Stroke portion of the equation. Some engines will have a deck height that is used in the calculation; this area is the distance that remains in the cylinder, from the top of the piston deck to the top of the cylinder once the piston has reached TDC. This volume must be added to the combustion chamber volume. To further simplify things we will call the volumes of the combustion chamber, the deck height, head gasket, and piston dome the net combustion chamber volume. It is important to understand, in regard to the piston dome, that if the piston is a raised dome style piston that the area of the raised dome will be subtracted from the net combustion chamber volume and if it is a reversed dome that the area of the reversed dome will be added to the net combustion chamber volume.

The volume of the combustion chamber in the cylinder head is often an Advertised volume, sometimes referred to as a Nominal volume. This is commonly found with an as-cast combustion chamber. Combustion chamber volumes can vary somewhat in a cast cylinder head due to conditions known as core shift and shrink. CNC’d combustion chambers are machined from either a casting or billet and are normally exact in size, but to be sure of the numbers that are used in the equation you should always fluid check the combustion chamber volume. Fluid checking the combustion chamber of the cylinder head is the only true way to know the actual combustion chamber volume. This is very important with cylinder heads that have been modified or worked on since valve changes, valve jobs, addition of compression releases, etc. will alter the volume of the combustion chamber from its original manufactured number.

Now that we know all of the necessary volumes for our Mechanical compression calculation we will use the following equation to exactly figure the mechanical compression ratio of the engine:

SWEPT VOLUME OF THE CYLINDER +
NET COMBUSTION CHAMBER VOLUME
NET COMBUSTION CHAMBER VOLUME

Now that we understand just what Mechanical compression really is and how it is calculated, let’s use the equation to find the mechanical compression of our “Article Engine.” For this article we are going to say that our engine is a Twin Cam 95ci engine (3.875” bore & 4.000” stroke) with a set of cylinder heads that have an advertised 95cc combustion chamber, and a set of advertised 10:1 raised dome pistons. Here is how the cylinder swept volume is calculated:

3.1416 x 3.875 x 3.875 x 4.000
4

This equates to 47.173085 cubic inches (CI’s), now this must be converted to cubic centimeters (CC’s) so that it can be used in the equation, so multiply the cubic inches by 16.387 to convert.

47.173085
x 16.387
773.02534 cc of swept volume

Next we need to calculate the swept volume of the head gasket, for this engine we are going to figure that there is a zero quench and that the entire volume of the cylinder is being used; this means that only the head gasket swept volume will be used in the calculation. Now for the head gasket swept volume.

3.1416 x 3.875 x 3.875 x .040
4

This equates to .4717308 cubic inches so now this also must be converted to cubic centimeters.

.4717308
x 16.387
7.7302526 cc swept volume

We know that the cylinder heads have an advertised combustion chamber volume of 95cc and that the head gasket being use has a volume of 7.7302526cc. The manufacturer of the piston states that the raised dome portion of the piston has a 16cc volume and now we will figure the net combustion chamber volume that will be used in the calculation.

(Advertised Chamber)
+7.7302526 (Head Gasket)
102.7302526cc
-16cc (Advertised Piston Dome)
86.7302526cc

Now that the swept volume of the cylinder and the net volume of the combustion chamber is known, the actual mechanical compression ratio can be figured by using the equation stated in the beginning of this article. Let’s see what our “Article Engine” has for Mechanical compression.

773.02534 + 86.7302526
86.7302526

This calculates to 9.9129838:1 which will be rounded up to a 10.0:1 Mechanical compression ratio for the “Article Engine.” In the next article we will discuss “Corrected compression ratio and what it means, what it does, and how to find it.  IW

*RESOURCE

DELKRON Inc.
Bedford, Ohio
440-786-8820/866-335-5766
www.delkron-mfg.com

Story by Steve “Posie” Pfaff, Delkron

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