Basically I want to know how long this engine has before it dies. Its a 93' Base Model. Currently has ~87,000 miles on it with 36,000 on the new engine (No documentation on the rebuild).

6.7-6.8-6.9 on rotor #1
6.0-6.1-6.3 on rotor #2

 

You can't really put a number of miles on any engine, although those numbers are starting to get marginal. If the seals are really worn out then the end maybe near, but if the housings were reused then it may not have sealed all that well to begin with. So, you may be looking at a "slow death", that is, compression just gets lower and lower but you never actually blow an apex seal, then it will become increasingly hard to start and flood easily, power will suffer, mileage will suffer, idle quality may suffer, but it'll still run. It would be up to you when to decide its time to finally go for the rebuild. If you're not having any problems with flooding or hard hot starts at this point, then I wouldn't worry about it.

 

To add to what Nathan said, your numbers are a bit low but they are consistent (at least within chambers). BTW, most engines die of coolant seal failure, not apex seal failure. The compression test does not guarantee anything, even with good numbers.

 

Please excuse a newbie's question, but my '94 manual says compression ratio should be 9.0, minimum compression reading should be 100 psi with less than 21 psi difference. So how can 6.1-6.9 C.R. be anywhere near good? Or are these numbers chamber pressure, in kgf/cm^2 units (spec being 7 minimum with 1.5 max difference)?

---- Bill

 

My mechanic told me 7 was factory minimum so I would guess the later.... I've never seen anyone test 9.0, maybe after my rebuild I will get lucky?

 

Since "compression ratio" is really the volume ratio of maximum chamber to minimum chamber volume, it's fixed by the dimensions of the engine. And you can't really read anything but pressure with a compression test, so I would agree it's probably the pressure in kilogram force per square centimeters, but what a weird unit for American gauges. Must be a Mazda special gauge calibrated in those units.

---- Bill

 

I agree, a ratio does not measure pressure It is a specialty mazda compression tester as it has to test all three sides of the rotor from one point unlike a piston engine....

 

The factory new spec for compression is 120psi or 8.5kg/cm^2. In '94 the minimum spec was reduced from 100psi (7.0kg/cm^2) to 85psi (6.0kg/cm^2). THE NEW SPEC IS NOT, NOT NOT NOT NOT 9.0! That is the compression ratio, NOT THE SAME THING. Please repeat this to everyone who asks because this is the biggest misconception out there. 8.5 is new, 6.0 is minimum, and the test must be done with the engine warm or the results will be about 1.0kg/cm^2 higher. Oh, and the reduced '94 minimum spec is the generally accepted minimum, not the higher '93 spec.

 

Nathan,

Thanks for the info. However, my '94 manual says the minimum is 100 psi, not 85 psi (7.0 kgf/cm^2, not 6.0). Where did your lower number come from?

---- Bill

 

That is strange, it must have been a later edition of the '94 manual, this change is pretty well documented, but I don't know the exact version that they changed it in. I personally have a '93 so its the 7.0 number in mine as well, so I can vouch personally for the change.

 

Nathan,

My copy of the '94 Workshop Manual is copyrighted 1993, "Printed in the USA (8/93)," and is part number 9999-95-018S-94. The companion 1994 Body Electrical Troubleshooting Manual is Part No. 9999-95-085F-94. I cannot believe they would change the manual itself in the same year; too expensive. Maybe a technical bulletin was issued some time in 1994?

---- Bill

 

I have seen 7.0 in every single source. I have never seen 6.0. I had my compression tested and I was just over 6.0. About 1,000 miles later my engine grenaded due to a corner seal failure. So I fell 6.0 is too low.

 

85 is what a second get rx7 shoudl see for compression test or 6 kg/cm2

for an fd its 100 psi or 7 kg/cm2

compression ratio is a static number that doesn change.
the compresisontaht u are chekcign is dynamic compression at 250 rpm or so while cranking the starter

 

i just did mine with a piston engien compression tool wihout the valve and i got 95 or ~6.7kg/cm2 psi on all sides of both rotors. is this good/bad?

what was teh highest u guys have seen?

 

i just realized i am forgetting to do somethign ill eb back with updated #s

 

6.7 on all faces is pretty good. Its more important to have consistant numbers than to have high numbers. I've had people claim to get low 9s which is basically impossible (the manual states 8.5 is the new spec) when the test is done on a properly warmed up engine. Doing the test on a cold engine has been shown to raise the overall numbers by almost exactly 1kg/cm^2, so make sure the test is done WARM, this is the proper procedure. I agree with Rikki that most engines bust because the apex seals break, but I disagree with the statement that they don't wear out. I think maybe what he is saying is that the "failure" mode for 3rd gens is almost always the rapid variety rather than the soft failure you see in n/a rotaries (ie the compression just slowly goes down and down until the car won't run anymore, but the seal never breaks). However, on n/a rotaries you will see examples of seals that just plain wear out. I mean, maybe you can still get it to start and if you keep driving it, it will eventually break, but that doesn't count in my book. Also, don't overlook the fact that as they wear down they are more likely to break, so really breaking an apex seal without pinging (which happens) is sort of like wearing it out. Either way, compression is not the ultimate indicator, there is none, nor does it tell the whole story. However it is AN indicator of engine health. If you're getting something like 7.5, 5.5, 5.5 on one rotor, you're telling me that engine has just as long to live as a car with 7.5 across the board? The seal may not be broken but its still bad news.

 

you're supposed to hold the throttle all the way open while u do a compression test right?

 

As long as the chambers are about the same then your
engine is fine. By looking at your no# the chambers are about even.

I had the same compression no# some time back and was still pulling 114 MPH thru the traps.

jc.

 

It would appear the expert has spoken. So tell me Rikki, how long have you owned your FD, and how long have you been around and rebuilding rotaries?
Just curious.

 

A compression test is one of, if not, the most important thing you can check on a rotary car before you buy it. A compression test is a very good indicator of what kind of condition the motor as in. Every rotary specialist, including Mazda, recommends having a compression test performed before buying a car and/or to see what condition the motor is in. This, of course, is nothing brand new.

At 9,934 miles I had my car tested by the local Mazda dealership. The results are as follows:

Rotor No. 1 = 9.1, 9.0, 9.0 @241 RPM.

Rotor No. 2 = 8.7, 9.0, 8.9 @241 RPM.

The test was performed with the engine warm.

I called a few rotary specialists to get second opinions on my results and they all agreed that these results are very good. By talking to them, I also learned that compression results can change very frequently. For example, if I drove another 100 miles my results might very slightly. This is due to the fact that seals and springs on the rotors are consantly shifting around.

Hope this proides some insight.

 

Nice thread....cleared up alot about compression for me. Thanks!

 

Those numbers can be fine man, whats your RPMS at? There is a chart in any ROTARY manual that shows the compression chart.

 

This is not a flame but I wish people would stop getting so excited about a 6.5 vs a 7.2 or whatever.

Different testers get slightly different results. So my number on my car of say... 7.0 does not necessarily directly compare with another guy's 7.0. Perhaps if we both used the same tester it would have come out differently.

My own testing has shown that a simple mistake like the person running the test not holding the throttle open can cost you 0.5. The engine being cold can cost you up to another 0.5 or so.

What I look for on _my_ tester is a nice number around 7 is pretty good. You want to see even results across all 6 chambers, that's the most important I think.

I've seen cars as low as 4.0 on my tester that start and run fine (yeah that one is getting worn out) and my current "high scorer" on my tester is Rikki's car at about 8.0 or so... so there is a WIDE range (at least on my test gear) that indicates that the car should run.

So everyone, stop sweating some stupid shop manual specs or wondering if a 5.9 means your car will die next week but a 6.0 means it will last 12,345 more miles.

If you really want to know how your car is doing, test it every few months with the same test box under similar conditions and keep a log of it. In this way you can see if there is any trend.

Regards,
Brian

 

Well, for one thing if you have nice numbers it could mean you have a really nice healthy engine.