TIA for any help, looking to buy a 2005 Shinka with 20k miles but asked for a compression test be done. The results came back as;

Rotor 1 compression : 91 PSI - 215 RPM
Rotor 2 compression : 100 PSI - 215 RPM

I called the shop, explained I was looking for 3 numbers and an RPM per rotor and they went on to try to explain why that's not possible, but I got lost along the way.

 

It's not possible if they're using the wrong equipment, yes. Rotaries require a rotary compression tester that most shops don't have. That rotor 1 isn't looking so good, but it's still passing. 20k miles? wow.

 

They are likely using a normal compression tester and not a rotary specific tester. Those results are useless.

 

These cars if they sit for a long time (1+ year or more) without being run can easily lose compression. That’s why mileage is meaningless.

 

Complete numbers attached, testing altitude 607... Thoughts?

 

Above Mazda minimum. Acceptable range, engine shouldnt cause you any issues for now.

 

I've found another 8 with numbers;

7.3 7.1 7.3 RPM 281
7.5 8.0 8.0 RPM 283

Obviously these numbers are higher, but have a bigger differential. Which is the better engine?

 

Normalized assuming test done at sea level to 250 RPM (in PSI):

Rotor 1 : 101.4, 98.5, 101.4
Rotor 2: 102.9, 110.5, 110.5

Mazda allows for a maximum of 20 PSI difference between rotor faces so you are well within spec. Compression in this car I would classify as above Mazda minimum spec. Engine still good so long as the test was performed on a hot engine. Also, this engine is in better shape than your first one.

 

What altitude is this one at?

You can plug in all the numbers at this site: http://foxed.ca/index.php?page=rotarycalc

 

551 elevation. To use that calculator, do I have to first change the figures from kg/cm to psi? Then input those numbers into the calculator?

 

Yes, the input has to be in PSI, but the output can be set to the four units in the drop-down list.

 

551 feet wont really change the numbers that much. Still a decent engine just make sure those readings were taken while the engine was hot. Or, alternatively, if it was done by anyone worth their salt they would have provided a readout sheet.

 

1st test – 607 altitude

Kpa

636 622 632 RPM 208

657 633 631 RPM 208

Kpa to psi

92.24 90.21 91.66

95.29 91.81 91.52

Normalized/converted kg/cm2

7.59 7.42 7.54

7.84 7.55 7.53




2nd test – 551 altitude

Kg/cm

7.3 7.1 7.3 RPM 281

7.5 8.0 8.0 RPM 283

Kg/cm to psi

103.83 100.99 103.83

111.51 113.79 113.79

Normalized/converted kg/cm2

7.16 6.96 7.16

7.65 7.81 7.81


I'm trying to compare the two and likely confused by all the different units used, but isn't the first one the better choice overall?

 

If that’s the case you want to go with the better overall car. Engines are very similar.