Setting Compression Ratio

If you are going to be building your own engine, then there is one sometimes overlooked portion of it that is essential in getting the utmost longevity and performance out of the engine, and that is setting the compression ratio. In order to set the compression ratio, there are a few things you will need to know. First off, is the cubic centimeters (cc) in the combustion chambers of your heads, and you will need to know the deck height, as well.

First, let's discuss the compression ratio. Compression begins when the piston is at Bottom Dead Center (BDC) on the downward (intake) stroke. This is when the piston has pulled the maximum air/fuel mixture into the cylinder. Now, as the piston moves up into the cylinder on the upward (compression) stroke, you will be compressing that air/fuel mixture, until the piston has reached Top Dead center (TDC) when the mixture has been compressed as much as possible.

This compression has a ratio, which can be anywhere from six- or seven-parts-to-one on a stock engine, all the way up to 16- or 17-parts -to-one in race-only engines. The higher the compression ratio, the more heat will be generated, which is why, with today's less-than-perfect fuels, most builders of street engines keep the compression down below 9-1/2-to-one, or lower.

One of the most important aspects of the compression ratio formula is measuring the volume of the combustion chamber. A number of VW Trends' advertisers carry a "head cc'ing kit" that includes all the necessary pieces to do this. All you need to do is add water, or cooking oil, or some other liquid, and do it according to the instructions. Stay away from liquids that have a high rate of evaporation (like alcohol) or are flammable. Also, bear in mind that this test must be done with the valves in and seated, and with spark plugs, so the fluid you use won't leak out.

The chart below will give you a good idea of what to expect from a stock, dual-port cylinder head, that has been flycut, in the way of ccs. These should be close, but check the ccs in the combustion chamber anyway, just to make sure.

To the deck height measurement there are numerous VW Trends advertisers that carry a "deck height tool." Deck height is the measurement between the top of the piston, at Top Dead Center, to the top of the cylinder (barrel) with the barrel firmly against the case surface. Many engine builders state that the deck height should not be less than 1.5mm (or 0.060in.), for most street applications, or the piston may interfere with the valves when they are open, causing catastrophic engine failure. All three items (compression ratio, deck height, and the ccs in the combustion chamber of the head) can be "tweaked" to affect the other.

For example, if you need/want more compression, less ccs in the head may be in order. The less deck height you run, the higher your compression will be. By knowing all these formulas you can use one to offset the other, to get your desired compression ratio. As well, knowing all separate cylinders measured deck height is important if you want a balanced, smooth-running engine, just as it is when you "cc" the separate combustion chambers in the heads.

For a practical application, we paid a visit to VW Paradise, in San Marcos, Calif., where one of the resident engine builders, Travis Even, ran through the procedure while building an engine for a customer. VW Paradise does this on every engine they build, whether it is an stock 1600 single port or their very-own record holding "Paradise Express" A/Dragster.

VW PARADISE
1510 Grand Ave.
San Marcos, CA 92069
(760) 744-9140
www.vwparadise.com