Compression Testing
 

Recently I have seen a few threads with videos of compression tests done by "rotary shops". These were done with a piston compression tester which will not yield any real results besides showing that it is blown (0 compression). Trying to determine that an engine has low compression with a piston tester is a bad idea. The cranking speed of the engine plays a huge role in capturing the correct compression.

Here is the newest edition of the Mazda Rotary Engine Compression Tester, that we have had for several years. Notice that in the first picture the compression is low 6.7, 6.6, 6.8 (around 95psi) with the rpm reading at 175. The Mazda tester has a 250 RPM normalization button, in the next picture you can see that the compression is perfect 8.5,8.5,8.6 (around 125psi)

https://cimg6.ibsrv.net/gimg/www.rx7...01ba980089.jpg

https://cimg7.ibsrv.net/gimg/www.rx7...e7596431f3.jpg

So are you saying you will rent this out to solve the compression test problems of piston testers??? ~__~

No he's showing how much of a difference the proper RPM makes in determining compression...

I'm sure a Mazda compression tester like that runs an arm and a leg. There is a vender that just popped up that has something similar that's around $350.

Thanks for this banzai racing!

Matt

Just checking since I haven't found a rental yet but they do sell for over $2K.

Not renting out our tester sorry, we would have to get a $2500+ deposit

I agree piston tester reading is kind if iffy, had a test told me I have a blown motor, 2 years later my motor is still running strong so go figure.

Piston testers will really only tell you if the engine has some sort of seal failure. The actual numbers that it yields should not be used. The recent video that I saw was a "shop" showing the customer they had low compression, which simply can not be determined with a piston tester since there is no way to capture the cranking speed. It is pretty easy for an unscrupulous "shop" to show 60-70 psi compression, if the engine we were testing above had compression of 105 @ 250rpm it would be down in the 70's @175rpm and this is if the test is done properly with the throttle held wide open. Leave the throttle closed and the compression drops even more drastically.

So how much would you charge (ballpark) to compression test my engine if I were to make the drive to Indy?

Just call your local Mazda dealer, they should have the same tester that we have. Gas alone from NC does not make sense.

We are 2 hours north of Indy, we charge $100 for a compression test which is just our 1hr minimum shop rate.

But gas is cheap and the drive through the smokies is fantastic! Seriously though, we have family in Carmel so this would likely be just a side trip to consider while visiting this summer. Maybe we can talk about some mods as well. I'll look you up.

Mazda tester is ideal but You can normalize your compression test with the service manual diagrams for altitude and cranking speed compensation. See page C-10 of the service manual.

The problem is that most people have no idea what their cranking speed is. In the example above the 75rpm difference is not detectable on the tachometer. Even at 175rpm the car has no problem starting.

This would look promising if the rpms could be shown I would think and has multiple rotor applications.
Model ReCT

https://www.rx7club.com/rotary-compression-tester-com-325/greetings-fellow-rotary-junkies-1098558/#post12048510

One of the new vendors and DOES show RPM

Matt

most of the so called aftermarket compression tester is really just a clone of the Mazda's unit. I seen a few.

For personal use, I think these aftermarket testers are ok. but some of them uses some lame ass pressure sensor that falls apart after couple of times.

what about using the rpm reading on the power fc? it seems pretty accurate. how would one regulate their cranking rpm to ensure that 250rpm is whats happening?

Problem there is that the ECU needs to be disabled to prevent fuel and spark. It also does not solve the problem for anyone with a stock ECU.

FSM instructions are to pull the circuit opening relay and unplug the igniter.

Why not unplug the coil harness and take the EGI relay out, that would stop the spark and fuel and then you can read the engine RPM on the commander?

Yes something along those lines. When I deflood, I unplug the coil pack harness and unplug the the fuel pump in the trunk. I remove the spark plugs too of course and I get an rpm reading on the power fc. What im interested in is how to regulate cranking rpm.

Pull the EGI relay and the PFC gets no power at all, commander will not even light up.

That is a lot of work to deflood , when you just need to pull a relay.

You can't regulate cranking RPM, that is why there is a normalization table in the FSM for cranking speed and altitude. page C-10

My rule of thumb is taking the car to the track and measuring it's trap speed. If that numbers makes you happy then rotor on.

Of course this doesn't help when buying a used car but I prefer this test to any other.

That is a great way of determining RWHP, but not very useful for compression.

Yes but if car is running strong. It's running strong is my point.

So you would buy it if the trap speed is good, but then idles like crap and pulls no vacuum..... interesting.

I realize that I suggested people reading this thread use the compensation curves in the service manual, but it's not entirely obvious how to do that. So I went ahead and made a Microsoft Excel tool to assist. I am going to make a separate thread to spread the word.

The tool is available here:

https://drive.google.com/file/d/0B_j...ew?usp=sharing

It uses the cranking speed (from some source besides the tachometer in the instrument panel), compression reading in psi, and altitude that can be easily looked up online (whatismyelevation.com). The compensation formula came from determining the linear slope of the curves in the service manual diagrams.

This isn't as good as using a real Mazda compression tester but it's a step towards a more accurate understanding of engine health.

https://www.rx7club.com/attachment.p...1&d=1460811694