Compression Testing Myths & Facts

[RotaryDiagnostics]

Often, the advice offered up by forum members and techs alike is that to get correct data during an engine compression test, it is important to always conduct the test on a fully warmed up engine. I conducted tests today specifically to attempt to answer the question of the impact of engine temperature on test results.

In short, the results of the tests indicate that the temperature of the engine has little, if any, effect on the actual compression numbers. Note the term "actual". By actual I mean the recorded numbers normalized to an engine cranking speed of 250 rpm in order to compare them with the specifications in the Mazda factory service manual.

Test conditions: The cold test was done on a 13b Turbo engine that had been idle for 2 days prior to the test. In other words, it was stone cold. Ambient temp was about 65 degrees. The warm test was done after a short road run with coolant temps rising to about 185 degrees and oil temps to around 170 degrees immediately prior to shutting the engine down. In other words, the engine was at normal operating conditions.

The setup was identical for both tests. Same engine, same plug holes, same tester system with the same Acquisition Units connected the same plug holes. Time between the tests was about 20 minutes. The warm test was conducted immediately after shutting the engine down.

So what exactly does the data reveal (refer to the attached images)? The max compression reading for rotor no.1 in the cold engine was around 126 psi compared to the max compression for the same rotor in the warm engine of about 129 psi. So there was a small difference in the recorded readings. However, the engine cranking speed for the cold engine was 237 rpm as compared with the warm engine cranking speed of 267 rpm. If the recorded readings are normalized to the same engine cranking speed, there will be little difference in the actual compression of the engine--cold or warm.

So conduct your test on a cold or a warm engine, but remember to consider engine cranking speed when evaluating the results. As a final caveat, by cold engine I mean an engine that has been run in the days prior to the test, not one that has been idle for weeks, months or years.