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#1
Michael Colangelo

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So I just did a compression test on my new-to-me 1997 Spec Miata.  Car has 51k miles and I believe this is the original motor.  Engine runs fine with no obvious signs of problems.  I followed the standard procedure for a compression test and these are the results:

 

Results (Cyl 1 / Cyl 2 / Cyl 3 / Cyl 4):
Dry:  134, 145, 130, 125
Wet:  148, 149, 138, 135

 

I don't yet have a shop manual but my understanding is that 182 psi is the "standard" value for this motor.  

 

What could cause all the numbers to be consistently low like this?  While cyl #4 is the weakest, the others aren't much better.  Maybe head gasket?  I don't know.  

 

What can you folks tell from the above results?

 

Thanks in advance.



#2
Tom Sager

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So I just did a compression test on my new-to-me 1997 Spec Miata.  Car has 51k miles and I believe this is the original motor.  Engine runs fine with no obvious signs of problems.  I followed the standard procedure for a compression test and these are the results:

 

Results (Cyl 1 / Cyl 2 / Cyl 3 / Cyl 4):
Dry:  134, 145, 130, 125
Wet:  148, 149, 138, 135

 

I don't yet have a shop manual but my understanding is that 182 psi is the "standard" value for this motor.  

 

What could cause all the numbers to be consistently low like this?  While cyl #4 is the weakest, the others aren't much better.  Maybe head gasket?  I don't know.  

 

What can you folks tell from the above results?

 

Thanks in advance.

 

The next step after getting suspect compression results might be to leak it down at or near operating temp.  That should tell you a lot more. 


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#3
Ron Alan

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Leak down and or dyno will also verify your compression results...or make you realize something was amiss! 


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#4
davew

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Don't get too upset over low numbers. Every gauge in the shop reads different. Battery charge (i.e. how fast the motor spins) will make a difference. Engine Temp, throttle open or closed. A compression test is a basic test. If they are even, you are probably ok. A leakdown test is much more of a diagnostic test. If you have compression leaking, it will tell you where. If leakdown numbers are good and the compression numbers are low, then I would look at gauge and or procedure.

 

dave


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#5
Michael Colangelo

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Thanks for the info, guys.  



#6
luvin_the_rings

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you could also have an engine with the lower comp pistons in it. we ran a whole season with the pistons from an automatic in it before we realized. Put a new engine, bam, second off our lap time. 


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#7
callumhay

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you could also have an engine with the lower comp pistons in it. we ran a whole season with the pistons from an automatic in it before we realized. Put a new engine, bam, second off our lap time.



Very interesting point..does anyone know how to tell the lower compression vs normal compression pistons on the Mazda web site? I went on there and took a look at the 1999 miata engine pistons...there are 2 part numbers BPZO-11SAO and BPZO-11SBO both standard diameter pistons . Is one low comp and the other normal?
Thanks
Cal

#8
Ron Alan

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Best of my knowledge...only 1.6 motors had different pistons. I dont think 1.8 automatic motors have different pistons...especially NB generation


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