Jump to content

Photo

Need Help Interpreting Compression Test Results

- - - - -

  • Please log in to reply
7 replies to this topic

#1
Michael Colangelo

Michael Colangelo

    Member

  • Members
  • PipPipPip
  • 272 posts
  • Location:Rosamond, CA
  • Region:Southern California
  • Car Year:1997
  • Car Number:94

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

Tom Sager

    Veteran Member

  • Members
  • PipPipPipPipPip
  • 1,455 posts
  • Location:Chicago Suburbs
  • Region:Central
  • Car Year:2001
  • Car Number:94

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. 


  • Michael Colangelo likes this
Donor - Made PayPal donation Bona fide - A bonafide Spec Miata driver We have a Winnah! - Won their 1st race... Congratulations! Make it Rain - Made Paypal donation of $100+

#3
Ron Alan

Ron Alan

    Veteran Member

  • Members
  • PipPipPipPipPip
  • 3,513 posts
  • Location:Northern CA
  • Car Year:1995

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


  • Michael Colangelo likes this

Ron

RAmotorsports

2014 NASA SM Western States Champion(Car owner)

2015 NASA 25HR E3 Champion(Car owner)

2016 NASA 25HR E3 Champion(Car owner)

2017 NASA 25HR E3 Champion(Car owner)

2018 NASA 25HR E3 Champion(Car owner)

2017 NASA NORCAL SM Series Director

2018 NASA NORCAL SM Series Director

Donor - Made PayPal donation Bona fide - A bonafide Spec Miata driver

#4
davew

davew

    Veteran Member

  • SMembers
  • PipPipPipPipPip
  • 2,297 posts
  • Location:Beloit, Wi
  • Region:Chicago
  • Car Year:1999
  • Car Number:72

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


  • Michael Colangelo and BNaumann like this

Dave Wheeler
Advanced Autosports, the nations most complete Spec Miata shop
Author, Spec Miata Constructors Guide, version 1 and 2.0

Building Championship winning cars since 1995

4 time Central Division Spec Miata Champion car builder 2012-2013-2014-2017

Back to Back June Sprints Spec Miata 1-2 finishes 2016 and 2017

5 time June Sprints winner in Mazda's

6 Time Northern Conference Champion Car Builder

2014 SCCA Majors National point Champion car builder

2014 SCCA Runoffs winner, T4 (Bender)

2014 Central Division Champion, ITS (Wheeler)

2013 Thunderhill 25 hour winning crew chief

2007 June Sprints winner, (GT1, Mohrhauser)

Over 200 race wins and counting.
www.advanced-autosports.com
dave@advanced-autosports.com
608-313-1230

Survive the 25, NASA Thunderhill - Survive the 25, NASA Thunderhill We have a Winnah! - Won their 1st race... Congratulations! Sponsor / Advertiser - Site sponsor / advertiser... support these guys! Donor - Made PayPal donation Bona fide - A bonafide Spec Miata driver

#5
Michael Colangelo

Michael Colangelo

    Member

  • Members
  • PipPipPip
  • 272 posts
  • Location:Rosamond, CA
  • Region:Southern California
  • Car Year:1997
  • Car Number:94

Thanks for the info, guys.  



#6
luvin_the_rings

luvin_the_rings

    Member

  • Members
  • PipPipPip
  • 176 posts
  • Location:Los Angeles, CA
  • Car Year:1990

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. 


  • Michael Colangelo and callumhay like this

#7
callumhay

callumhay

    Member

  • Members
  • PipPipPip
  • 131 posts
  • Location:tampa
  • Region:fl
  • Car Year:1999
  • Car Number:35

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

Ron Alan

    Veteran Member

  • Members
  • PipPipPipPipPip
  • 3,513 posts
  • Location:Northern CA
  • Car Year:1995

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


Ron

RAmotorsports

2014 NASA SM Western States Champion(Car owner)

2015 NASA 25HR E3 Champion(Car owner)

2016 NASA 25HR E3 Champion(Car owner)

2017 NASA 25HR E3 Champion(Car owner)

2018 NASA 25HR E3 Champion(Car owner)

2017 NASA NORCAL SM Series Director

2018 NASA NORCAL SM Series Director

Donor - Made PayPal donation Bona fide - A bonafide Spec Miata driver




0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users