My RX8 was in the dealership and had to have the fuel pump replaced. I have done compression tests before on my 300zx would report reading such as 130PSI #1, 135PSI #2 Cylinder.......but when they Tech did the write-up of the repair, he wrote the following section

"Run compression test on both rotors. Had 7.2 7.2 7.3 on #1 rotor cold and had #2 was 7.7 7.7 7.8 cold. Ran compression test hot too had marginal readings."

My question is what are these 7.2, 7.2, 7.3 numbers rated in... obviously not PSI?

What would a bad compression number look like using the above methodology?

 

If they had used the correct tool (WDS) and procedure to check the compression they should have reported reading from a screen similar to the attached WDS image. Apparently, they are using an older/different tool.

And what do they mean by: "Ran compression test hot too had marginal readings"? Are they suggesting that you had poor enginecompression? Time to consider consulting another dealer in my opinion.

 

I took my used 2004 RX8 to a mechanic. One of the things they did was a compression test and here are the results:

Rotor #1 @ 212 RPM: 7.7/7.3/7.5
Rotor #2 @ 216 RPM: 7.6/7.5/7.2

They used a Mazdaspeed tool that is calibrated/certified every year.

Mazda minimum specification is 7.0 Kg/cm^2
Extrapolated to 250 RPM, the engine compression is over 8.0 Kg/cm^2

They told me that these numbers are very good.

 

if they used the old Mazda compression test tool (specifically designed for rotary engines), then those are very good numbers.

FWIW, for the older rotaries, at a minimum the compression numbers should be 6, so 7's are more than ok :D

 

The unit they use is kgf/cm^2. Below are the specs on determining if its good.
8.5 standard, 6.9 minimum
1.5 difference in chambers and 1.0 difference b/w rotors.


*********************
Compression pressure (kPa {kgf/cm2 ,psi} [rpm])
Standard 830 {8.5, 120} [250 rpm]
Minimum 680 {6.9, 98.6} [250 rpm]

Standard difference in chambers Within 150 {1.5, 21.8}
Standard difference in rotors Within 100 {1.0, 14.5}

 

yeah those are k numbers those are good

 

Jaguar,
The only figures they neglected to give you were the cranking rpms. These are vital because the readings are dependant on cranking speed like ZZ8 illustrated. This is what worries me when dealers are eager to replace a motor and they may not have the most experience and diagnostic skills as regards rotary engines.
Paul.

 

true good point paul. thats one of the things i question abotu the engien replacements as well i.e.- did they do the comp test correctly?- we'll never know.

 

Your compression is right where it should be. Any lower and u are losing power and any higher and u would be damaging your internals, but over time of course. When he did the test did he remove the spark plugs and plug the hole with a pressure tester gauge, usually they do it with one and crank the engine and the dial will tell u how much pressure built up in the compustion chamber at TDC.

 

?????

 

What's that Paul?

 

I was just trying to figure what Priscilla meant by " Any higher and you would be damaging your internals", but I was trying to ask that ina slightly discreet way.
Paul.

 

It's cool paul,
U see compression can be bad if it is to high, I would rather have a car suffuring from low compression then compression that is to high because the inturnals of the engine are really at a strain. That is all I ment.

 

care to explain Paul? popcorn anyone?

 

Extra butter on mine Zoom.

 

hahaha very funny :D
But u know what I meant. Sure it is a rare case when compression is to high and if compression is low, well we all know what the problem is leak somewhere.

 

I try to keep my internal strain down through nutritional means, I guess it's probably similar.
Paul.

 

Funny guy? Anyways like I was saying, too much compression would cause a wide range of problems and a loss of hp and parts. U have to agree with that.

 

Here are my compression results. I think they are very bad, but the dealership says that mazda tech line won't let them do anything until they can verify my power loss problem. Can anyone tell me how bad these numbers are if at all?

1ST ROTOR ........................2ND ROTOR......MAX MIN
CHAMBER 1 644 KPA.........599 KPA...........ROTOR 1 79.6
CHAMBER 2 723 KPA.........612 KPA...........ROTOR 2 22.5
CHAMBER 3 717 KPA.........590 KPA...........MAX ROTOR 1&2
...................RPM 217.........RPM 212..............111 KPA

 

I love the forum, hahaha!

 

guyz, do i need to replace the engine when it had lost compression??

 

And just how will these compression reading change (lower to what level) if you live at 5000 feet.....not sea level?

 

Just got a copy of my compression check and was wondering if anyone knows how to read it. It lists the following:


Rotor 1
657KPa. Chamber 1
704KPa. Chamber 2
652KPa. Chamber 3
264RPM. Engine RPM

Rotor 2
519KPa. Chamber 1
590KPa. Chamber 2
631KPa. Chamber 3
266RPM. Engine RPM

51.9KPa max. rotor 1 - min. rotor 1

112KPa max. rotor 2 - min. rotor 2

72.7KPa max rotor 1 - max rotor 2

Car is slow as molasses.

 

Well, the compression check didn't say my car is slow as molasses - I added that part myself.

 

See the attached image for the shop manual page related to compression test readings. According to the manual, the ideal readings at 264-266 RPM should be between the high 800's maximum to around 700 minimum. The differences look OK, however. So, it seems a little strange to me that the absolute readings are not up to snuff, but the differences are well within spec. Smacks of a worn out engine with no single point failure. (Or a compresion test done with insufficient oil film in the rotor housings to provide adequate compression.)