Compression Ratio and Compression
by Ron Van Ness
Here are some data on the relationship between compression ratio and compression. The Bosch Automotive Handbook gives the following formula:
Compression = (Compression Ratio minus one) to the 1.1 to 1.2 power multiplied by Atmospheric Pressure
Given Atmospheric Pressure at sea level = 14.7 psi, we have the following:
| Compression Ratio | Compression (psi) | Range (psi) |
| 6.5 | 96-114 | 18 |
| 6.6 | 98-116 | 18 |
| 6.7 | 100-119 | 19 |
| 6.8 | 102-121 | 19 |
| 6.9 | 104-124 | 20 |
| 7.0 | 106-126 | 20 |
| 7.1 | 107-129 | 22 |
| 7.2 | 109-131 | 22 |
| 7.3 | 111-134 | 23 |
| 7.4 | 113-136 | 23 |
| 7.5 | 115-139 | 24 |
| 7.6 | 117-142 | 25 |
| 7.7 | 119-144 | 25 |
| 7.8 | 121-147 | 26 |
| 7.9 | 123-149 | 26 |
| 8.0 | 125-152 | 27 |
The Engine Volume of the VW Workshop manual gives the following specifications for these specific engine types:
"Compression (with engine warm, throttle open, all plugs out, gauge in plug seat and engine turned by starter):Engine code B, AE (up to 558 000) [Compression Ratio = 7.5]
New part: 114-142psi Wear limit: 100psi
Main difference between cylinders: --Engine code AE, AH, AK, AM [Compression Ratio = 7.3]
New part: 107-135psi Wear limit: 85psi
Main difference between cylinders: 28psi"
Knowing the compression won't help you pinpoint the CR. However, if you are presented with an "unknown" engine that tests out well on a leak down test, and you proceed to check the compression, you can at least tell if your CR might be extreme (e.g. if the leak down is good and compression measures at 150, you can conclude you have an engine with a high CR built in). For the most part though, knowing the compression will tell you very little about CR (as the table above shows), especially if you measure a compression of 110psi (could be a CR of anywhere from 6.5-7.3:1).
All this is bookish info, for your information. In the real world we aren't all at sea level, and though standard atmospheric pressure at sea level is 760mm on the mercury barometer (1013.2 millibars), atmospheric pressure can range from 950-1050mb according to some reference book I just had lying around here. This, the fact that we all don't have access to a leakdown tester and don't know how much we are losing past our rings/valves, and the wide range of compression for a given CR makes the whole business of relating CR to compression interesting only in an academic sense.
