mdpalmer

iTrader: (24)

Hey guys, I've read a bit on compression tests for rotary engines with a standard piston engine compression tester. I want to get your opinion on my FD's engine health. There's some uncertainty in the cranking RPM, as my tach didin't budge past the lowest tick mark.

The testing parameters were like this:
Warm engine (coolant temp at time of readings was 90degC);
Front rotor first, Leading plug;
Rear rotor second, Leading plug;
Cranking RPM = ? I looked at the tach myself and it hardly moved. I’m not sure if the engine was cranking at the standard 250 RPM it’s supposed to for the test (you can correct for cranking RPM, but I couldn’t even read it on my tach);
Throttle wide open.

Anyway, check out the videos and let me know what you think. My references are:

http://www.rotaryresurrection.com/3r...ion_check.html
http://www.fd3s.net/compression_test.html
http://www.aaroncake.net/rx-7/blown.htm
http://www.asashop.org/autoinc/dec98/techtips.htm

THE VIDEOS ARE HERE:

FRONT ROTOR
REAR ROTOR

JDawG707

ouch, it looks like a bad engine or its not cranking fast enough, but sounds like it is.

i just did mine and i got 102 all the way around, but it was with a snap on Compression tester

JHew84

iTrader: (9)

The cranks sound weak, not really fast enough. I'd try it again with your car hooked up to some jumper cables. Although i'm not sure that will change your compression readings, unfortunately...

[edit]I guess Mr. Landers dissagree's and thinks that weak cranks will produce low numbers... He knows his **** so i'd do what I suggested and try it again hooked up to another car...[/edit]

- James

mdpalmer

iTrader: (24)

Yah, I dunno. I'm still unsure as to the cranking RPM. I think it may be cranking slower than 250 RPM (Mazda standard). I've been driving this this thing for the last 8000 miles like I stole it, no problems at 10-12 psi on stock turbos. WHat makes the snap-on compression tester special? Mine is just a generic one I got from PepBoys....

JDawG707

it dousnt make it special lol, but did you take out the bleeder valve? it wont really do much since max all your seeing is 30, but that low of compression your engine should be running lol Is it a fresh engine or anything?

JHew84

iTrader: (9)

nothing makes it special, better build quality maybe, a price tag 5x higher than what you paid . As long as your tester isn't leaking pressure or something like that it's a pretty straight forward tool and will work fine...

Apathy

I dont think the bleeder valve was out b/c you can see him push it to reset the psi... Take the bleeder valve out, make sure the engine is warm, and do it w/ jumper cables... If you only had 30psi on the front rotor, the thing would barely move let alone boost properly...

Gadd

Your not doing it correctly. You need to remove the schrader valve from the bottom of the tester and do not hold the relief valve on the side of the tester open. Also put the car on a charger to make sure the battery is full up before tested.

That being said, the rear rotor does not look good.

Paul

mdpalmer

iTrader: (24)

I thought about taking that valve out, but I if you look at Kevin Lander's procedure, he doesn't say anything about it. He says to look for bounces to 30-35 psi (I'm assuming with the schrader valve installed).

Banzai Racing's procedure calls for removing the valve and looking for bounces in the 85+psi range: http://www.banzai-racing.com/ (How-tos --> Compression Test).

Another thing I forgot to mention is that when I tested the rear rotor (I did the front one first) the motor was cranking over noticably slower.

I'll see if I can try both ways when I get home tonight. I've read that a special tool or needle nose pliers can be used to remove the schrader valve. Any comments on the easiest way to get that thing out?

Also, the engine is a rebuild of 8000 or so miles use, used housings/hard parts, new seals all around.

Any more comments?

dubulup

keep driving it like you stole
oh and I belive the test should be done on the trailing...not leading.

http://www.fd3s.net/compression_test.html
"Remove the front and rear trailing side spark plugs."

GoodfellaFD3S

iTrader: (213)

How many miles total are on the rotor housings? Also, I agree that you need to get more juice to the battery to get the rpms up.

For reference, my FD with about 10k on the motor and 60k on the rotor housings made right at 100-102 psi a while back.

felix_is_alive

For real ??
Crap i have been using the leading for years what would be the difference ??

Gadd

You can get a schrader valve wrench at any hardware store, auto parts or department store, or you can buy valve cap with removers built into them. If your holding the relief valve open, your releasing the pressure before the gauge can see it. I don't think testing from the trailing or leading plug makes any difference, the rotor is past TDC before the seal passes the trailing hole.


Oh, and as dubulup said "keep driving it like you stole it"

Paul

slo

iTrader: (1)

Using a timing light on the trailing coil, one with the display that shows rpm, and a charged battery my engine was doing around 210 RPM. With another running car and jumper cables hooked up it hit 250's. That brought the compression on a piston tester up from 90 to just over 100. But then its pretty hard to read it accuratly when the graduation is 5PSI.

mdpalmer

iTrader: (24)

Hey Rich, the front housing has around 80k miles while the rear has around 100k (says the engine builder from what I recall). Now that I think about it, I should have got new housings when the motor was out. Thanks for the data point

EDIT: Oh yeah, about "driving it like I stole it"... c'mon, I didn't buy this car to just cruise! I bought it to haul ***!!!!! It makes me so sad to see how many people are so scared to actually push the thing for fear of breaking something; to me that defeats the purpose of having an FD

RotaryResurrection

iTrader: (8)

There are 2 totally different sets of results you can see when using a standard piston tester. IF you leave the valve stock, but just hold it in, you'll see bounces in the 30-35 range. If you completely remove the valve, the bounces may go higher. Regardless, neither of these "bounce" numbers are accurate as an actual value, just a reference for comparison with the other 2 "bounces" on the same chamber. All 3 should be about the same as far as your eye can discern.

Then the final test with the valve sealed shows you the highest number for the chamber. And since our previous test showed that all 3 faces were pretty equivalent, then we can also assume that this one final number is also equivalent for all 3 faces.

Usually the leadings are changed more often and are easier to get in and out. Also the leadings are easier to get to IMO because you dont have to work around the damn knock sensor and coilpack plug on the FD like you would if you were pulling trailings. You will see about 5psi difference between the trailing and leading, sometimes less. Numbers are never absolute regardless of testing procedure, however.

For whatever reason, rotary owners obsess about compression. When a group of car guys gather in a parking lot somewhere the mustang owners are discussing cams and rearend gears, and the rotary guys are discussing compression numbers.

Either you have enough to run, or you don't. Tell me when the last time was, honestly, you checked compression on your piston engine daily driver? Why would you, right? I mean, as long as it starts up and runs fairly well, you have no reason to. Then why the hell are we so concerned with it in the rotary? If you have a severe problem, then fine, check it for troubleshooting purposes. IF you're just checking it to sleep better at night, then you're just setting yourself up for disappointment, as the output is rarely surprisingly high.

And the rear on that engine read quite low because cranking rpm was way down, you could clearly hear it. Compression falls way off as rpm drops below 225. The front sounded completely normal to me. I think that the back would also be pretty normal, though a bit less than the front (again this is normal for a rotary) if the cranking rpm were the same.

RotaryResurrection

iTrader: (8)

Well, I guess I straight killed this thread.

JHew84

iTrader: (9)

Haha, the truth hurts . It is a good way to look at it, if it's not broken there's no need to go chasing flaws just to get yourself worked up. Reminds me of the cummins dodge guys, those trucks are notorious for lift pump failure, half of the guys on the forum are testing it every other week, stressing about any weird fluctuations, etc. It's no use worring about what might happen, just enjoy what you have and keep good care of it.

S8nana

i'd be more worried if i didn't hear even pulses...just like mine...got a chipped seal in the front...rebuild time...

mdpalmer

iTrader: (24)

Yeah, I hear you. I never worried about compression on this motor since it ran just fine. The only reason I'm checking it and making videos is so I can show potential buyers of the car that the engine has fully documented-no bullshit compression readings. When I get back into an FD in the future, I would hope that the seller would do the same for me.

Thanks to everyone who contributed to the conversation here. I will most likely get around to taking additional readings in the next week using the given advice (fully charged battery with jumped connection to running car, schrader & non-schrader valve readings).