Compression Tests

All engines, rotary or not, require compression to work.  Compression is created during the compression cycle of an engine which happens after the fuel/air mixture has been sucked in and all ports (intake and exhaust) have closed, sealing off the combustion chamber. If there are any leaks out of this chamber then the maximum compression at ‘top dead centre’ will suffer a reduction and the engines efficiency (ie: ability to work to full capacity) is reduced by equal measure.

It is well known that rotary engines suffer from compression problems in time. These issues can be the result of various individual problems, ranging  from out-and-out seal failure to general wear & tear.

We often hear people using such terms as ‘rotary tips’, ‘tip failure’ and ‘tips wear out’ when describing a rotary engine, but what they are trying to describe is the lateral wear of the apex seals. These seals are seated at the three apex’s of each rotor within the engine. In truth this kind of wear is common and is a major problem in many engines we are asked to rebuild. Quite often though, there are other forces at work too which mean a full engine rebuild is often not only necessary but also the most economical option for repair too – even before apex seals have worn beyond their serviceable life!

Rotary Compression Testing

A compression test is the first port of call when trying to diagnose almost any running issue with a rotary engine. We often joke that ‘anything from a blown bulb to a lumpy idle engine’ is in need of a compression test first and foremost.  We have seen and heard of people wasting, literally, thousands of pounds in some cases, changing parts that don’t need changing and diagnostic tests that are heading in completely the wrong direction; simply because the owner’s chosen repairer doesn’t know any better. So, for the sake of a few quid we always advise a compression test first. If it reveals nothing detrimental, brilliant. If it does show that the engine is not so well though, then at least you will know exactly what is wrong and which problem to tackle next without wasting money ‘beating around the bush’ .

It is important to remember, that a rotary compression test differs greatly from that of a conventional piston engine compression test. Both in the way that it is carried out and also by the equipment required to conduct the test. It’s the latter of which that means in almost every case you will need to head to either a known rotary specialist or your local Mazda dealer. (Note if visiting your local Mazda dealer; ring ahead first! As not all Mazda dealers now carry the equipment required for rotary compression testing.)

Third-party compression testing equipment is available to the market of course, however the accuracy of such cannot be fully known or guaranteed.

Here at Rotary Revs, we use a genuine Mazda Compression Tester, purchased at great expense, in order to provide our customers with the most accurate of test results. 

Once your compression test is complete, your results should consist of six numbers (one per rotor face) plus a cranking speed. Most specialists will provide you with the six results already normalised to 250 RPM. This is important because the cranking speed has a direct impact on compression test results and any changes to the standard RPM rate can make your results look significantly better (or worse) than they actually are!

If you have any additional questions or concerns about compression testing, contact us to speak with one of our rotary specialists or get in touch to book your compression test at our garage. Our compression testing service costs just £59.99 and can be carried out within the hour, while you wait.