fi addict

im looking at an engine
78, 81, and 82 about....
i have pictures of the test
these numbers seem good for a sitting engine. what are your thoughts?
and what do you think? good for the 20b miata project?

RETed

Under what conditions were the test done?
Under Mazda guidelines, those numbers would be "borderline".


-Ted

fi addict

90-120 is good from what i know but this is a 20b that has been sitting on a shelf for a bit (3-4 weeks atleast) and had atleast a month of downtime in shipment and a lil more time in a yard.
another one i found had 60-58-70 or something...worse!
still waiting on another...
what should i inquire about the testing?? cold vs hot? how long it was running? I wouldnt expect it to be on more then 3-5 minutes

HWO

I'd be suprised to see much more psi outta a motor that is sitting on the shelf, remember its cold as the antarctic and the seals wont be all that well lubed.

i'd personally consider anything over 50psi cold, shelf compression to be sweet, as long as its all even, even per phase, and per rotor

RETed

http://fc3spro.com/TECH/FAQ/compress.html
Under those conditions?


-Ted

HWO

He STATES the engine has been sitting, the pictures show and engine on the ground, which obviously hasn't been running. Your 'tech' info you link to talks about compression testing a HOT engine, not a COLD, hasn't bee run for months engine.

as for the TB being hard open, it makes no difference. Do a compression test of an engine with the pedal to the floow, then do it again with it closed, there is no difference in the readings.

Cold, sitting compressions are ALWAYS significantly lower than what the engine will get when its hot. I have compression tested cold engines from japan which show 40-40-40- 40-40-40 and once run up in a car then tested hot show 100+ on all phases.

Also, how on earth are you supposed to crank a rotary to 200+rpm with the starter motor? Have you ever cranked your engine over and watched what RPM your Haltech registers on the Engine Data Page? most engines read 120-180 when you are cranking them, with 2 12V 11 plate batteries in parrellel i have had fresh engines up to 240-300rpm cranking, but have never seen it possible with 1 battery.

Cranking speed, Port size etc All play a key role in compression test numbers.

Evil Aviator

It's too bad there isn't something like a Factory Service Manual where one could look up the proper compression check procedure.

HWO

the think is Evil, a factory service manual would only spell out how to compresson check an engine in a car after it has been brought up to operating temp. Not a cold engine which hasn't seen life for a few months because its been sitting on a shelf at an engine importers

Evil Aviator

You can't perform an accurate compression check on a cold engine that has been sitting. The best you can do is cycle it around by hand, and if you hear a pop for each apex, then at least you know it's not completely blown. The only way to tell if it is in GOOD shape is to perform the check within the specified factory conditions once the engine has had some running time for the seals to start working again.

peejay

and if it's pumping 78-82psi then it's probably damned strong.

I had an engine that did a whopping 25psi on the compression gauge if it hadn't been running for a few months. Yes it was in the winter too. Ran juuust fiiiine....

RETed

I would think common sense would kick in at this point.  Getting a little engine oil into the chambers is not very hard to do, and this will take care of any normal problems associated with an engine sitting for a significant amount of time.

I dunno where this case from, but my experience says otherwise.  We've consistently gotten anywhere from 10psi to 20psi difference with the TB closed versus open.

Bolt on spare bell housing...bolt on starter.  I don't think I have to go into the nitty gritty details to get a starter to crank over.  I can't confirm cracking RPM.  If it's getting close to 100psi, that's good enough for me - like I said, common sense has to kick in sometime.  I want to avoid wildly varying compression numbers in individual faces or zeros.  I did this to my 20B before I bought it.  Since history was questionable, we just poured a few tablespoons of engine oil into the chambers from the spark plugs holes, rotated by hand...hooked up bell housing and starter and hit the battery.  Compression checkers showed 100psi even on all 3 faces for all 3 rotors.

Bullshit porting will significantly change compression numbers - I'd like to see proof of that.  Having the TB closed versus open will be a much more significant change of compression numbers.

Why you busting my ***** over such details?  I'm not going to go into every single scenario possible, but yet you want me to defend my generalizations over every conceivable scenario?  I gotta assume the original poster has some common sense, and we're not dealing with some idiots with no brains.  My compression check write-up was done to deal with generalizations and not every single fricken situation you might run into out there.  I'm sure the original poster will come back in here to ask more questions if he has a problem with anything.


-Ted

HWO

Long duration intake ports will give less 'compression' on a guage than small ports will, same as doing the same 'compression' check on a piston engine with long duration cams.

I'm not busting your ***** I am simply saying your compression testing guide is all fine and dandy when the engine is running in a car, but when the motor is not in a car, has not been running recently, there is ALOT more to take into consideration.

How would you feel, if you wanted to know about checking compression numbers on a sitting motor, and someone gave a how to on compression testing a running engine in a car?, it'd be like someone asking you how to row a boat, and you replying with "buy a boat with a motor on it"

fi addict

here's the test... and i think we strayed away from the question...
good/bad? walk from another one?

Sponge Bob Square Pants

Wouldn't that only be true for a leaking / low compression engine?
On a mint condition and proper compression engine, the compression chamber is sealed off from the intake ports when the reading is taken, so opening/closing the throttle body is no difference.
Or is there something I'm missing?

RETed

So where is all this air that's going into the combustion chambers is coming from?


-Ted

Hastur

its mint mate. thats fine.

Hastur

its mint mate. thats fine.

smith88

were you testing the compression on the lower plugs in the picture, or was the engine upside down?

Does it really matter which set of plugs you test it from, or are the top ones just easier to get to?

Thx