Compression Testing Myths & Facts
Greetings all,
I've been conducting frequent compression tests for several years now, mostly prototype testing or validation testing of the systems we build. That equates to hundreds of tests of our systems on various rotary engines. In reading threads on the various forums, I see a lot of advice on how to conduct a compression test. Some of the advice appears to be based on rotary lore that has been passed around on the forums for years.
This thread is intended to shed a little light on some of these myths and "facts" that I have tested at various times, mostly when testing prototype systems, but also testing systems prior to sale. I will add a new and different myth or fact to this thread periodically, so check back from time-to-time.
The first "fact" is: "Fully depress the accelerator pedal and crank the engine...". That is the instruction in the Mazda 2nd gen service manual. Well, is that true? And if so, just how much difference does it make? Is it so insignificant that one should not really worry about it?
The answers are yes it's true, it makes a huge difference and one should worry about it. Referring to the graphs below, you will see that the difference between depressing and not depressing the gas pedal can result in about a 20% difference in the readings. For example, peak pulses on Rotor 1 were about 100 psi with the gas pedal not depressed (first image) and 121 psi with the pedal depressed (second image).
All traces in these graphs were obtained from the same 13B Turbo engine, during the same test approximately 30 seconds apart, using the same tester. Incidently the engine was not warmed up for this test (another myth/fact).
So there is no doubt that when conducting a compression test. it is critical to have the gas pedal depressed for the duration of the cranking session.
The question of whether or not it matters if the engine is cold or fully warmed up for a test will be discussed in our next post. Check back.
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