Pete_89T2

iTrader: (4)

So this year at DGRR, I decided to splurge and buy myself one of these bad boys...


Finally got around to using it this weekend to test the engine in my '89T2, Great tool BTW - super easy to use, high quality & rugged construction, it's accurate and does everything the official Mazda compression tester does. Now on to the test results...

Front Rotor:
111, 88, 99 PSI (values corrected for altitude & normalized to 250 RPM by the RCT v5.2 tester)

Rear Rotor:
114, 114, 109 PSI (values corrected for altitude & normalized to 250 RPM by the RCT v5.2 tester)

Compression specs from the '89 FSM say the minimum is 589 KPa or 85 PSI or 6.0 Kg/cm^2, @ 250 RPM, so in that regard both rotors "pass", BUT there is also the differential compression spec (i.e., difference between chambers) of 147 KPa or 1.5 Kg/cm^2 or 21 PSI, which the front rotor fails.

Background on the motor - It's a fairly new build, has just over 10K miles on it now. Build started out with all new or almost new parts; S4 housings, S5 plates and S5 rotors/rotating assembly. Used all new Mazda OEM apex, side & corner seals, Mazda competition apex seal springs & oil control rings/springs. It also has a pretty aggressive street port using the Pineapple racing 13BT intake & exhaust templates. With my current turbo & ECU setup, after the engine was broken in it dyno'ed at ~ 360 RWHP / 330 Ft-Lbs torque @ 6500 RPM. That's with the ECU limiting boost at 14.7psi / 1 bar for self protection.

What do these results indicate, a single "loose" side seal? Is it worth tearing it down now for what should be an "easy" rebuild (i.e., clean. inspect, new hard & soft seals & reassemble) or can I expect to get many years of trouble free use out of it as-is? I'd add that the car starts easy hot or cold (easier cold though) and it still has plenty of power.

Thanks!

Mlammert

Personally I would say no, it's not worth tearing apart but then again I'm far from an expert. Have you tried calling any of the local rotary builders in your area and asked their opinion? My own car had something of the same problem and the mazda only shop guy I took it to for the compression test said to just drive the wheels off it.

mazdaverx713b

iTrader: (91)

You may just have a sticking seal. Car on can do a lot of nasty things to these engines. Might be worth a Seafoam treatment.

Pete_89T2

iTrader: (4)

^That's really what I'm hoping it is. Never tried the Seafoam treatment on a rotary, that might be worth a shot.

Given that only 1 chamber is reading low, it must be a side seal (one sticky/bad apex seal would take out 2 chambers). While it's by no means conclusive, when I pulled the trailing plugs to do the comp test, I noted that the rear rotor plug came out looking brand new, while the front rotor plug had more carbon deposits. Not fouled, but much dirtier. That carbon could just be the result of the weak chamber itself (i.e., weak chamber = less complete combustion, more unburned HC's = more carbon deposits), or it could be a symptom of too much oil getting into the combustion chamber on the front rotor. That could be due to either: (1) Weak oil control rings, or (2) Something is buggered up with my OMP's oil delivery (i.e., too much premix is somehow getting to the front rotor).

I'm pretty confident the oil control rings are fine because the engine doesn't burn any measurable amount of sump oil between changes. I'm also using an RA OMP adapter, so zero sump oil should be consumed to lube the apex seals - I have a separate oil tank providing gravity fed TCW3 premix lube through the stock electronic OMP, and stock plumbing & oil injectors. An AEM Infinity ECU manages the stock electronic OMP, using a 2D load (MAP) & RPM based delivery scheme similar to how the stock ECU manages it. With the OMP map I'm currently running, my premix oil consumption averages about 1~2 ounces per gallon of gas consumed for normal street driving, and a bit more when I'm flogging it, which is right about where it should be. I no longer bother adding any additional premix to the fuel. However, it took about a year and maybe 6~8K miles of driving & closely monitoring my premix consumption to get that OMP map in the AEM Infinity tuned right - the initial "default" map AEM provided would dump stupid rich amounts of premix, so it's possible that a the lengthy OMP tuning process could have caused a sticky side seal.

j9fd3s

iTrader: (3)

you might try to compression test it again after new spark plugs for a while.

here is a story, its 2006, at the dealership, and Mike #2 pulls in an Rx8, and its obviously running on 1 rotor. he compression tests it, and one rotor is like 9's, and the other rotor is 5's, obviously bad.

he calls Mazda, because unlike the 6's, there were steps with the Rx8, and they tell him to put coils/wires/plugs in it and recheck

we order the parts, wondering how that would fix the engine, he puts them on, fires it up and runs it for a while, and then retests it.

it passes, the "bad" rotor comes up to the 9's...

Pete_89T2

iTrader: (4)

^Interesting story. I was actually thinking of dropping in some fresh plugs & driving the snot out of it (aka - Italian tune-up method) to try to blow out the carbon and then doing a retest to see if anything improves.

BTW, my test numbers were missing from my original post for some reason, I edited them back in.

Akaviri

iTrader: (1)

People have had good results with distilled water into the running engine through the intake manifold. It steams out the carbon pretty effectively and harmlessly. Gotta keep your revs up to keep it from stalling out so it's easiest to have a buddy help.

lduley

iTrader: (1)

I agree with everyone, I'd run seafoam or water through the intake then run the **** out've it, then recheck it, I got a feeling you got a sticky seal also

KNONFS

iTrader: (14)

I am no expert, and I am basing my opinion on exactly that, opnion. With that out of the way, if the car starts fine (cold or hot), idles fine, and drives like day1, then I wouldn't open the engine for a rebuild, at least not yet. Once you start seeing "issues", then I would consider opening the engine.

Pete_89T2

iTrader: (4)

Update - Turns out it was just a sticky side seal. I had a full tank of gas when I did the first compression test almost a month ago, so when that burned down to about 1/8 of a tank, I decided to do the Seafoam treatment. Opted to dump in a full can of Seafoam into the tank which seemed like the safest de-carbonizing option, and proceeded to drive the snot out of her till the tank was almost empty. Then I filled the tank again, and have been driving the the car normally since then. Today I did another compression test, and it looks like that sticky side seal is free again. Here's the results:

Front Rotor:
101, 89, 96 PSI (values corrected for altitude & normalized to 250 RPM by the RCT v5.2 tester)

Rear Rotor:
116, 114, 114 PSI (values corrected for altitude & normalized to 250 RPM by the RCT v5.2 tester)

Compression specs from the '89 FSM say the minimum is 589 KPa or 85 PSI or 6.0 Kg/cm^2, @ 250 RPM, so in that regard both rotors "pass", This time around the front rotor also passed the differential compression spec (i.e., difference between chambers) of 147 KPa or 1.5 Kg/cm^2 or 21 PSI.

My front rotor is probably built a bit "looser" than the rear overall - maybe I could have done a better job setting the side seal clearances, but at least the 3 chambers are all consistent now. Might try a few more de-carbonizing cycles with the Seafoam to see if I can get a bit more improvement.