Calculating Your Engine's Compression Ratio
Setting the proper compression ratio (CR) for your needs is a very important aspect of building a properly running engine. It would be foolish to even think that I could cover even a slight bit of what there is to know about CR and the air cooled VW engine. So I won't even try. People have written whole books on the subject and most that is known is debated by those knowledgeable in such matters. I am but a simple enthusiast who has run a motor with too high of CR and experienced first-hand the "joys" of running such a beast. I now prefer the conservative approach to CR and you will see that this page is slighted in that direction on purpose.
What IS commonly agreed on is that higher CR generally yields higher levels of horsepower (HP). But this power is NOT free. Let's face it; if it was possible to safely run high CRs while maintaining the engine's longevity, EVERYONE WOULD BE DOING IT and there would be little to debate. Let's look, for a minute, at a few of the costs of this seemingly "free HP".
- High CR engines will typically run significantly higher operating temps. In a daily driven air cooled VW, higher temps can spell disaster in a hurry especially if you live in warmer climates. Excessive heat is probably your engines #1 enemy in the battle for longevity.
- High CR engines usually require higher octane fuel to run correctly without pre-ignition (knocking and pinging). Have you've checked the price of fuel lately?? Buying the higher octane fuels or needing to run boutique additives is EXPENSIVE, not to mention a pain.
- Pre-ignition often associated with higher CRs can melt pistons and generally wreak havoc on your engine. Engines cost big $$$.
What is debated, is the safe compression ratio to run on a daily driven air cooled VW. I tend to side with conservatism. After reading some of what Gene Berg and some other respected VW folks had to say about it over the years, I came to the conclusion that MY magic number for this engine would be between 7.0:1 to 7.25:1. That should enable it to survive on the low-grade pump gas that is available today while still giving me some good, usable power without overheating.
There are a few things you will need to calculate prior to calculating your CR, and they are as follows
- The volume for one cylinder (in cc). The formula for this is:
Bore (mm) X Bore (mm) X Stroke (mm) X 3.1416 = Engine cc. To get the volume for 1 cylinder, divide the Engine cc by 4
Here's my motors numbers: 90.5mm X 90.5mm X 69mm X 3.1416 = 1775 / 4 = 443.75cc
- The deck height volume (in cc). The formula for this is:
Bore (in.) X Bore (in.) X Deck Height (in.) X 12.87 = Deck Height volume in cc
Here's my motor's numbers: (1mm = .03937") 3.563" X 3.563" X .060" X 12.87 = 9.80cc
- Compression chamber volume (in CCs). To find out your true combustion chamber volume, you must CC your heads. To find out how to do this click here.
Here's my combustion chamber's true volume: 50cc
Now that you have all the necessary numbers, It's time to calculate what your un-corrected CR will be.
- Compression Ratio. The formula for this is:
(1 Cylinder volume cc + Head cc + Deck cc) = CR
(Head cc + Deck cc)
Here's my motor's numbers: (443.75 + 50.00 + 9.80 = 503.55cc) / (50.00 + 9.80 = 59.80) = 8.42 or 8.42:1 CR. That's quite a bit higher than my desired 7.25:1 (or less).
So now, how do I lower that to my desired number?? Well there are several different ways of adjusting this, all of which will either manipulate your deck height or combustion chamber volume. You could:
- Shave a little off the tops of all your pistons with the help of a machine shop (deck height).
- Add cylinder shims (deck height).
- Add copper head gaskets (deck height).
Shaving the tops of the pistons is the most costly and least used of the three methods. Most engine builders use cylinder shims under the cylinders, copper head gaskets, or a combination of those two. Cylinder shims are the most popular and the cheapest. For my engine, I am going to be using a combination of both cylinder shims AND copper head gaskets since I want the added cylinder sealing the copper provides as well as the protection against any possible future head damage caused by the steel cylinders being mounted directly to the aluminum heads.
Cylinder shims are typically available in .010" thickness increments ranging from .010" - .040" & .060". You can stack these to tailor your deck height perfectly. If you need much thicker shims and don't want to stack them, you can order custom made shims from most high performance VW engine builders or machine shops. Remember that when you order your shims, to order them for the correct size of cylinders.
Head gaskets, on the other hand, are usually available in .040" and .060" thick nesses.
Now I need to play with the numbers again to determine how thick the gaskets/spacer combination needs to be in order to tame my CR into the 7.0:1 - 7.25:1 range. I found that if I added .060" under the cylinders, my CR dropped to 7.37:1. Near my target range, but still a little high for me. If I added .080" under the cylinders my CR dropped to 7.08:1; PERFECT!! What I will do on my engine is split that .080 in half and use .040" cylinder shims along with .040" copper head gaskets. For my engine that should be the hot ticket!!
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Revised: July 08, 2009 .