How do you compression test a rotary?

[RealizE]

This is just something i am curious about so i know what im doing when looking to buy a new (to me) car.

So, as the topic states - whats involved? I read somewhere, that the testing procedure is quite different to that of a piston engine, is this the case? Also what should the compression be for an atmo engine in good nick?

Help me out as i have no idea whatsoever :confused:
Cheers

Edit: Also, if there is anything else to look for when buying an old rotary powered car please dont hold back :)

[Dazz]

You need a special compression testing unit for a rotary. Most of the good workshops have them, and they measure all three chambers seperately as the engine rotates.

But basically, if you pull out the plugs, and crank the engine over (with the starter mtor not by hand) you should hear 6 nice strong even pulses. If they do not sound strong or even then there is some form of abnormal or uneven wear inside the engine.

When buying old cars it's all pretty much the same. Look in the radiator to make sure it has water, has coolant, and is basically clean.

Check the oil to make sure there is no browny sludge in it.

Check for blue smoke when the engine starts from cold.

Basically check the usual things you do when looking at any car.

[rxtasy]

Check for blue smoke when the engine starts from cold.

Although this does not necessarily indicate a problem for a rotary. Oil injection and the shape of the chamber mean that a small amount of oil can be trapped and will be burnt off next time you start the car.

If the smoke doesn't go away after a minute or two then you definitely have a problem. If it smokes regularly then the seals are probably worn and will need replacing soon. However it is totally normal for a rotary to smoke on startup once in a while. A compression test is the best indicator of the health of the engine.

It is also possible to use a "normal" compression tester to get a "guide" to the compression. But only a dedicated rotary engine compression tester will give you accurate numbers.

This is all discussed regularly on the rotary mailing lists, so a search on the web should find you something.

[KING_MAZDUP]

How much does it cost to get the seals replaced?

[FastEHHolden]

Having never spannered on rotors before I can't see why you can't use an ordinary compression tester. Remove the plugs, disable the ignition and test away. If you have a Crank angle sensor disconnect it.

12A = 665 kpa (1974 rx4) 12A 1979-85 = 590 kpa
13B = 590 kpa (including turbo)

[rxtasy]

FastEHHolden,

I'm by no means an expert, but I think its got to do with the three chambers of the rotor - on a piston engine you only have one chamber per spark plug to worry about.

I *believe* the dedicated rotary testers will record each chamber separately, whereas with a piston engine compression tester the best you can do is watch the guage and estimate that the three pulses were at a, b and c.

Would that sound about right?

[RX22NV]

The compression tester will only tell you the highest compression from each one as they hold a peak value, until you press the button to let the air go.
One dodgy seal will affect 2 chambers (the adjacent chambers)

The rotary ones will record the values for each chamber, just like rxtasy was getting at.

Replacing seals in a rotary usually involves a full rebuild. As the plates and housings must come apart, which are sealed together by rubbers/gaskets.
A seal kit from Mazda will set you back about $1400, while the gasket kit is about $300.

Be careful when you see smoke from a turbo rotary, often the turbo seal will be a bit sus. Look at the turbo for signs of oil, leaking from housings etc.

If possible, take the intercooler hoses off and look inside - this is a dead giveaway.


Look under the radiator cap, if there is milky white substance. If this is evidence, it is possible that the motor has a leaky oil/water seal and is letting water into the oil.

The same with the radiator, if it looks like brown/grey/muddy under the cap, it could mean that there is a dud water/oil seal and is letting oil or combustion into the water.

It doesn't mean that it is dead, one of my cars had a 12a turbo that used to get the milky whita under the cap and have browny deposits is the radiator. It ran fine for a year or so. Then I sold it. It ran well until the new owner put 20psi boost through it and blew it up

[FastEHHolden]

that makes sense about the leaky apex seal affecting 2 chambers, so I can see a case for the dedicated rotary comp tester...