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?

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

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

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.

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?


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.

????? :eek:

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.

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.

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.

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.
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.)

So, it seems a little strange to me that the absolute readings are not up to snuff, but the differences are well within spec.

Thanks Go48.

I wasn't too happy with the low readings myself, however, I just received a call from my service center (My 8's in the shop due to a lack of power). They informed me I have a clogged cat and need a new one.

Would the increased backpressure affect the #s like that? Also, would driving around with the cat clogged and the compression so low the have any long-term effect on the engine?

well you shall find out once your car's back with a new cat....

Thanks Go48.

I wasn't too happy with the low readings myself, however, I just received a call from my service center (My 8's in the shop due to a lack of power). They informed me I have a clogged cat and need a new one.

Would the increased backpressure affect the #s like that? Also, would driving around with the cat clogged and the compression so low the have any long-term effect on the engine?
I doubt that there will be any long-term effects from a clogged cat assuming the car was not operated for a very long time with that condition. The clogged cat would certainly have a negative effect on engine performance, though, as you have experienced.

Not sure about the effect of a clogged cat on engine compression readings. My gut feeling is that a clogged cat would not have a significant effect, but that's just a guess.

well you shall find out once your car's back with a new cat....

...and I will share the information with the world.

Another thing I'm slightly curious about is - that to get proper compression readings the throttle has to be wide open. With the electronically operated throttle on the rx8 how do we know for sure that floored pedal during cranking actually translates to fully open throttle plates, since the ecu has ultimate control over them?

Another thing I'm slightly curious about is - that to get proper compression readings the throttle has to be wide open. With the electronically operated throttle on the rx8 how do we know for sure that floored pedal during cranking actually translates to fully open throttle plates, since the ecu has ultimate control over them?

I would think that they are pressing the pedal to the floor just to keep fuel from being injected into the motor, sort of like the deflooding procedure. I could be wrong though and it actually be a factor in the compression check.

Having the throttle not wide open definately would affect compression numbers due to the signifiant intake restriction a closed throttle would impose on the system. That's the main reason why you're supposed to hold the gas to the floor for the test. As far as preventing fuel, you really should have the egi fuse removed to do the test to prevent the injectors from firing at all otherwise you've got a pretty dangerous situation.

Ok, makes sense.

They just told me the test was performed on a cold engine because it wouldn't start. Sigh. Though I don't believe anything they say, I'll bite...

Any ideas on how a cold engine would affect the numbers?

I think you also have to make allowances for altitude, a somewhat major factor in our area, and ambient temperatures, probably not as big a factor, but a factor. At @ 5,280 feet I think one may be looking at as much as an 18% drop off in maximum numbers, so depending a bit on Jaguar's battery condition, oil viscosity etc., i.e. rpm's turned by the starter, a 7.2 could be reported as low as 5.9 (7.2 less 18% or 1.296) As I recall, on piston engines, compression tests and leaks downs are supposed to be done on warm motors, and the tech needs to know what he's doing ~ i.e. throttle wide open etc.

I got another compression check done, this time on a warm engine. According to my understanding, the numbers should be much higher than they are, due to the higher engine RPM of ~306.

Rotor 1
713kPa - Chamber 1
730kPa - Chamber 2
737kPa - Chamber 3
305RPM - Engine RPM

Rotor 2
746kPa - Chamber 1
739kPa - Chamber 2
732kPa - Chamber 3
307RPM - Engine RPM

24.2kPa max. rotor 1 - min. rotor 1
13.8kPa max. rotor 2 - min. rotor 2
8.65kPa max. rotor 1 - max. rotor 2


Thoughts anyone?

I confronted the technician and mentioned that my numbers should be much higher due to the higher RPMs, and suggested that if he read the graph correctly I was still under minimum spec. He countered by telling me the new engines only read about 800kPa, implying I should be content with my numbers. When I pointed out that those #'s were for a cranking speed of 250RPM, and therefore not comparable to mine (with cranking speed of ~305RPMs), he didn't seem to understand.

I'm planning on taking this to another dealer. How concerned would you guys be if this was your car?
Thanks in advance.

I would be really concerned unless there really is a logical explination to why the numbers are low, like ALL new engines have less compression than older ones. But who know, you are right you might just want to get a 2nd opinion on this one.

Also, how long has the tech been a Mazda tech? I know that the one at my dealership he has been a Mazda Master tech since 1970 or something like that, so I repect and value his opinion on my car, maybe a good thing or maybe not, I guess time will tell with dealing with him.