R3tr0grade

Evening folks, I purchased an FD earlier this summer with 49k miles. I'm up to 51k ish and will be storing it soon as I live in New England, so I decided to compression test today. This is my first FD, but I'm not new to rotaries. I have a Twisted Rotors TR-01 V3.0 tester.

Some background on the car before I go into the numbers I got. It was owned by the original owner in California until about 2018. I do know it sat for about nine years in his garage after he could no longer drive it. A dealership bought it from him (I assume his estate) and got it ready for sale. Next guy bought it and had it shipped to WA State, he did all the basic FD maintenance that I planned to do to it when it arrived - aftermarket AST, removed pre-cat, changed fuel filter etc. He owned it for about a year and traded it to a dealer out there. It sat in their showroom for 6 months until I found it and started the due diligence process since I was buying it sight unseen. I had it inspected by a local Mazda dealer who performed a compression test using the old OE Mazda tester, the numbers they got were in the mid-high 80's so I knew it was probable that I would have to get a rebuild within the not so distant future, but still pulled the trigger.

I've been driving it pretty regularly over the last couple months and have just done basic maintenance (oil, plugs, air filter etc.) and the water pump when it failed a couple weeks ago (still not sure if it was the gasket or pump itself but replaced both.)

Also maybe worth mentioning, my first tank of gas I ran Royal Purple Max Clean (their version of seafoam basically.) After that I've been premixing on every tank - half oz per gallon of gas.

Fast forward to today, I took the following steps using the tester I mentioned earlier:
- Started the car and let it get to operating temp - I actually let it keep going for about 10 minutes total concerned that I would get inflated numbers if it was too cold
- Shut it off, removed EGI Main Relay
- Jacked up the car and removed both leading plugs
- Cranked it while holding the clutch and gas at max

Got the following numbers which have already been run through the calculator on foxed.ca to compensate for elevation etc:
- Front: 114 113 110
- Rear: 111 115 112

Original numbers on the tester were all 115 - 120 around 275 RPM with a barometric correction factor of 1.000 before I ran them through the calc. Also, paid attention to the temp gauge and it remained at the mid-way mark the entire time I was cranking. Even with the correction, these indicate an insanely healthy motor. Am I missing something? I expected to maybe pick up a few PSI since I've actually been driving the car and presumably clearing out carbon buildup or sticky seals, but this seems like a huge increase.

Thoughts? I know others here use the same tester. If the engine had cooled down too much (which I don't believe it did, plugs were very hot to the touch) what would the PSI inflation realistically be? I don't want to be lulled into a false sense of security with these numbers if I'm missing something. I do intend to perform another test later this month before it actually gets mothballed until 2021.

Thanks again, cheers

arghx

iTrader: (3)

I'm a little confused. What tester were these latest numbers from?

Does the engine smoke on start up or deceleration at all?

Molotovman

iTrader: (49)

It's feasible, regular maintenance and driving have caused the compression to increase again or the tech who did the test at Mazda didn't do it correctly.

Pete_89T2

iTrader: (4)

^I think the odds of the 2nd part there are more likely - most Mazda dealers these days have zero rotary experienced techs. That said, I do know from personal experience that good maintenance and more frequent driving tends to improve your compression numbers.

I purchased an RCT 5.2 compression tester at DGRR 2018, and have been seasonally checking my FC compression ever since. My findings were that the compression numbers coming right out of my car's winter semi-hibernation are noticeably lower than they are when I re-test after a few weeks of frequent driving. I also found that my compression improvement can be accelerated a bit by running a can of Seafoam on an almost empty gas tank and driving the snot out of it - I figure the lack of frequent use & full heat cycles during the winter months causes more carbon buildup, and the side/apex seals are probably getting sticky. The seafoam and/or frequent driving & heat cycles frees things up and your compression numbers improve a bit.

R3tr0grade

@arghx The test at the dealership earlier this year was done using the Mazda OE compression tester. I don't have it written down anymore, but they got between 5.7 - 6.1 x100 kPa so 80's in PSI. I have a Twisted Rotors TR-01 V3.0, with which I got the second set of numbers. I have already corrected them for elevation and my battery's cranking RPM via the calc on foxed.ca. There are no problems with the way it starts, runs or drives. I'm just trying to stay on top of compression so I don't get surprised.

@Molotovman @Pete_89T2 I've thought about that too, I've also had my fair share of experience with Mazda techs who want nothing to do with rotaries. I've been cautiously optimistic about these numbers over the last few days, though your responses and experiences put me a bit more at ease. If I find they've dipped significantly next spring, I'll try seafoam in the tank next season and see what effect it has. I can't steam clean through the vaccuum lines in my community - the fire department is on the corner and I don't think that would go over well lol.

Molotovman

iTrader: (49)

Perhaps someone could have mistaken "8.0" kpa on the Mazda tester for 80 psi. 8.0 kpa x 100 is ~116psi which would be more in line with the numbers you are seeing.

R3tr0grade

Unfortunately that's not the case. Since I couldn't be there in person, they sent me a video of their tester after they had run it on each rotor and both were in the 5.7 - 6.1 range on all faces. One rotor was a bit higher than the other, but they didn't specify which.

I was the one who did the conversion to PSI, since I am more familiar with testers like my TR-01 and the similar ones from rotarycompressiontester.com etc.

arghx

iTrader: (3)

if it's of any help, my car in the high 50k range mileage had compression in the high 90s psi according to the Mazda tester (after doing psi conversion). It had smoke on cold start up and elevated oil consumption. It still drove fine. It was the original Mazda engine from 1995.

It's possible you just had some carbon build up on the first test. If you have zero smoke and expected oil consumption then it's more likely to actually be a higher compression, healthy motor.

j9fd3s

iTrader: (3)

this one time at the dealership we had a car come in that needed a compression test (it had a piston engine) and all the techs gabbed their testers and had a go, and the results were wildly different. same engine same day, different testers was about a 20psi variation.

additionally like Pete says, the compression varies with how the engine is running. this other time at a different dealership we had an Rx8 come in running on 1 rotor. we did a compression test and 1 rotor was ok, the other was in the 5's, bad. we called Mazda they told us to replace the ignition system, and we wondered how that would fix the engine, but it was under warranty so why not. put it all in, it starts running on both rotors, and after a long warm up, retest compression and it was good on both rotors.

so what you're looking for is not really the peak number, but the difference between chambers, and rotors. there is a spec for that in the FSM

arghx

iTrader: (3)

I can tell you from years of lab work that pressure measurement tools are really supposed to be calibrated regularly. When you start comparing equipment which were all calibrated on different schedules (or most likely, never calibrated since they were built), you can get a lot of variation.