Cranking Compression Pressure Estimate
What a compression gauge should read on a healthy engine, estimated from dynamic compression ratio.
| Cranking pressure (gauge) | 163.5 | psi |
| Absolute pressure | 178.2 | psi |
| Gauge pressure | 11.28 | bar |
- P absolute = atmospheric × DCR ⁿ
- P gauge = P absolute − atmospheric
When you need it
As a sanity check on a fresh build before you lean on it, and as a diagnostic on an engine that has developed a misfire or lost power.
How to take the reading
Engine at operating temperature, every spark plug out, throttle blocked wide open, battery charged or on a charger. Crank until the gauge stops climbing, which is usually about five compression strokes. Leaving the other plugs in or the throttle shut will give you low readings across the board and send you chasing nothing.
Consistency beats the absolute number
The estimate above tells you roughly what to expect, but the useful test is whether the cylinders agree with each other. Within about ten percent across the engine is healthy.
A big cam reads low everywhere, and that is normal — it is the odd cylinder out that means something. Two adjacent low cylinders suggest a head gasket between them. One low cylinder that comes up when you squirt oil down the bore points at rings; one that does not points at a valve not sealing.