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Compression Testing Results

This page is here to give you a rough outlying of how your compression test results should be presented.CompressionTesterSmall
What you need to know and what to ask for.

Why can’t I use a piston tester?
To start with we must point out that it is not possible to test a rotary engine (accurately) using a standard compression tester. This is due to the construction if the rotary engine, with 1 pair of spark plugs servicing 3 individual combustion chambers in the engine, a piston tester cannot read and old the individual compression figures.

Rotary Compression Tester
This is where a specialist rotary compression tester comes in. There are a range of testers available to specialists however what splits them apart from a piston tester is their ability to be able to log and interpret the individual compression figures from the rotary engine. As well at telling the operator the cranking speed of the engine during the test.

Why do you need the cranking speed?
The cranking speed in a rotary engine is extremely important when converting your results in to a meaningful set of results that can tell us how healthy your engine is.
This is because in a rotary engine as your cranking speed goes up the ability for your engine to produce compression increases at a linea rate.

Mazda rotary compression test results should be normalized to pressures produced at 250rpm.

What this means is if your results where read at 285rpm then the compression figures would be lower when normalized to the standard 250rpm. And obviously visa versa (if your had a starter cranking slower than 250rpm)

What should be my results look like.
Well there are different ways that results can be presented. Depending on if your test has been done with a genuine Mazda tester or an alternate rotary tester.

The below results are an illustration of how Essex Rotary (and many other specialists) presents compression test results. The bold figures are the important information you need to have

CompTestResults1So what does this all mean
The above results are for an RX-8 and have been normalized to the standard 250rpm
Each rotor should have 2 sets of results one for each chamber, in this example the figures in the “[680,830]” tell you the minimum and maximum pressures for a healthy engine.

There is also figures for “Rotor Chamber Difference [<150]” this is the differential between the minimum and maximum pressures produced on each face of the respective rotor (in this case they could not exceed 150kPa difference to remain within acceptable spec.

Similar to the above chamber difference but “Rotors Difference [<100]” tells you the difference between the highest and lowest figures across the two rotors. In this example they should not exceed 100kPa.

Both rotor difference and chamber difference are not always included in compression results from other specialists.

Basic results
As we have outlined above some of the information is not always included however some is absolutely essential to understanding your engines health (with regards to compression)

The basics you need to know when getting your results are.

  • Have my results been normalized to 250rpm already
  • what unit of pressure are my results in*
  • What was the actual cranking speed during the test**

*varying units of measurement can be used it is normally kPa however others may use PSI, Kg f, BAR.
**whilst the actual cranking speed is not essential information it can help when understanding what your engine is actually doing. However it is important that you have this figure if your results have not been normalized already.

The below example illustrated the bare essential information you need. Make sure this is the absolute minimum you are provided with. If your tester (garage mechanic, dealer or specialist) cannot provide you with this information then your test has probably not been done with the correct testing equipment (and your probably paying for a test that’s worthless)



We hope this information goes a long way in helping you to understand what you should expect to see as a set of compression test results


Huge thanks to Aston at Essex Rotary for installing R-Spec oil and water temp gauges into a RX8 “air vent mounted” pod. I bought the pod some while back via an RX8OC group buy but never got around to installing it myself. He did a superbly neat job in splicing into the car’s wiring and hiding all the cabling out of sight, even connecting in a spare power socket outlet under the dash for any future installs. Really pleased with how the gauges look and operate. Thumbs up to Aston for advisin…
Malcolm CaseCase Computer services