Compression test calculator: is the variation between cylinders OK?
The absolute value depends on the engine, the gauge and cranking speed, so the most useful number is the difference between cylinders. Most manufacturers allow about 10 % (or a fixed number of bar) between the best and worst cylinder.
How to use it in a diagnosis
- Test with a warm engine, all plugs (or glow plugs/injectors) out, throttle wide open and a charged battery.
- Crank each cylinder for the same number of compression strokes (about 5–8).
- Enter the readings: cylinders more than 10 % below the best are flagged.
- For a low cylinder, add a teaspoon of oil (petrol) and retest: if it rises, suspect rings; if not, valves or head gasket.
Questions mechanics ask
What is a good compression reading?
Typical values are about 11–15 bar for petrol engines and 25–35 bar for diesels, but the manufacturer specification and the difference between cylinders matter more than the absolute value.
How much difference between cylinders is acceptable?
A common rule is no more than 10 % between the highest and lowest cylinder. Many diesel manufacturers specify a maximum difference of about 3–5 bar.
What does a wet compression test show?
Adding a little oil seals worn piston rings for a moment. If compression rises clearly, the rings or cylinder walls are worn. If it stays low, look at the valves or the head gasket.