i was wondering what are good compression results of how much compression shoud an fd motor out of an 93 mazda rx7 tt have and also the JDM spec too.thanks let me know

 

These are rough numbers, so take them with a grain of salt.

100+ is good
80+ Is runnable
79- is you are getting close to rebuild time

 

Note that just sticking a regular compression tester in the spark plug hole and spinning the starter will give you the best compression of the three sides of the rotor. One side could be completely shot and you wouldn't see it without a proper test unit. I recall Mazda having one that graphed the pressure so you could see how each side of the rotor measured up...

 

http://www.youtube.com/watch?v=hAE5h..._embedded#t=13


100 psi,105 psi, 115psi,105. im trying to get a motor and just want to know

 

85psi is minimum spec per 94' manual.

 

Piston compression gauge with the Schrader Valve removed

3 even bounces of 85 PSI or above is GOOD. Readings above 110 PSI is GREAT!
3 even bounces of less than 85 PSI: indicates that motor is tired, and indicates engine should be rebuilt before causing damage to rotors or rotor housings.
2 high and 1 low: indicates problem with side seal (stuck, cracked, broken, etc.) Engine needs to be rebuilt.
1 high and 2 low: indicates problem with apex seal (stuck, cracked, broken, etc.) Engine needs to be rebuilt.
NO bounces: either incorrectly installed gauge or destroyed rotor and housing.

 

so what can you guys tell me about the compression check in my video?

 

The motor is completely cold, so the numbers are misleading---higher than they should be.

Of course the throttle is probably not open, so that'll lower it as well.

That's a JDM motor, the coolant seals could be completely shot and you'd have no way of knowing until you start it up and get a smokescreen. I never have (and never will) recommend these.

You feelin' lucky ?

 

That they are not testing it properly, what they are showing is an accumulative test, which will only show the highest reading from the strongest face. In other words you could technically get the same readings from an engine with damage as long as one apex seal was intact on each rotor. What you look for in a compression test is three distinct numbers per rotor. It should bounce up to a number, then go back to zero, then back up to a number

Something like 100-0-100-0-100 on the gauge

That vid is showing 75-85-105 and never dropping to zero

And as Rich has said you do not know the condition on the coolant seal or the oil seals. We have pulled many J-specs apart to find incredible amounts of damage.

 

the needle should never drop to zero they are not turning over the motor enough times to see the bounce. The motor in the vid is good. if it had a bad seal instead of the number climbing to 115 it would drop to 0 because the motor would no longer be holding pressure. so if it does go 100-0-100-0-100-0 on the gauge that would be a blown motor. if they turned the motor over and it say went 75-95-110-95-110-95-110 that is a good motor. oh and if the face was bad and the gauge stayed at the previous number no matter what the pressure was inside the motor the needle would have only move 1/3 of the speed it did on the video. the motor on the video is like really good as far as i can tell.

 

The gauge should absolutely drop to zero after each pulse of the rotor. If the compression gauge maintains the pressure from the previous rotor face it is not going to give you an accurate reading on the next. The needle MUST bounce down to zero just like I described above. This is the only way to get three distinct readings from a piston tester. The schrader valve MUST be removed from the tester, to allow the pressure to drop.


http://www.banzai-racing.com/compression_test.htm

 

Not if you want to disguise numbers it shouldnt drop

 

This is how it should look, notice how it drops to zero after each pulse. The test in your vid is invalid and used to trick people as David has said.


http://www.youtube.com/watch?v=WTRJm1OAEXc

 

As others have said...

That test is not a good test - so it tells you very little
You do not know the condition of other key parts such as coolant seals

The motor might be fine.... might be mostly scrap - hard to say.

I would recommend rebuilding yours yourself or have a reputuable (as judged by members here) shop install an engine for you (or ship you one if you will do the install yourself).

B

 

Waaait a minute. Are you guys saying JDM stuff isn't automatically better, just based on it's JDM-ness ?