Tech / General Engine Is your car making a strange sound or won't start? Thinking of adding power with a new combination? Need other technical information or engine specific advice? Don't see another board for your problem? Post it here!
Sponsored by:
Sponsored by: CARiD

How to do a compression test?

Thread Tools
 
Search this Thread
 
Old Aug 22, 2008 | 03:15 AM
  #1  
BladeX10's Avatar
Thread Starter
Member
 
Joined: Jul 2008
Posts: 279
Likes: 0
Car: 1991 RS v8
How to do a compression test?

I'm finally going to go buy a compression gauge tomorrow. Before i even put another cent into this car.. I want to run a compression test and see if my rings are shot or if i got a messed up gasket. Problem is.. I dont know what to do.

What would be the proper way of doing this? I got all the plugs out so far. What PSI is considered good? Tell me all i need to know about this.
Reply
Old Aug 22, 2008 | 04:24 AM
  #2  
Viprklr's Avatar
Supreme Member
20 Year Member
iTrader: (5)
 
Joined: Feb 2002
Posts: 3,462
Likes: 4
From: N. Illinois
Car: 92 GTA/ 00 TA
Engine: 383/350
Transmission: 700R4/T-56
Re: How to do a compression test?

There is no magical psi number that you are looking to achieve. Technically they're all supposed to be within 10% of each other.
With that said, i've seen an engine with 120 psi (what I consider low) run fine and burn nothing.

Do the test with a fully charged battery
Throttle blocked wide open
Ignition coil completely disconnected (ask me how I know)
and obviously with the tester screwed into a port.

Ideally the engine is supposed to be warm.

Start cranking. If needed, have another person watch the gauge for the initial compression spike. Higher is better. Crank till it rises no more. Record the number. Move on.
If you have some low ones pour some engine oil into the spark plug hole and re-test that cylinder. If it goes up...rings. If it stays about the same...valves or hg.
Compression tests are useful but a leakdown tester is the best.
Reply
Old Aug 22, 2008 | 10:30 AM
  #3  
fast82z's Avatar
Senior Member
iTrader: (1)
 
Joined: Jul 2008
Posts: 751
Likes: 2
From: NE Ohio
Car: 82 Z28
Engine: TT LS
Transmission: 4L80E
Axle/Gears: S60 3.54's
Re: How to do a compression test?

If you have a card for the cam, you can use a calculation to figure out what psi it should read. It just all depends on compression, cam profile, bore, stroke, rod length, and altitude. Here's a calc. if you like: http://www.rbracing-rsr.com/comprAdvHD.htm Has always worked for me; says I should be 91psi, and I get that as an average.
Reply
Old Aug 22, 2008 | 07:10 PM
  #4  
jv9999's Avatar
Senior Member
iTrader: (1)
 
Joined: Jul 2007
Posts: 1,141
Likes: 2
From: MA
Car: '87 IROC/'68 SS
Engine: 5.7L/350
Transmission: 700R4/Muncie 4-spd
Axle/Gears: 3.27 9 bolt / 3.31 12 bolt
Re: How to do a compression test?

You'll have tons of fun screwing that tester into the rear plug holes too!
Reply
Old Aug 22, 2008 | 07:28 PM
  #5  
BladeX10's Avatar
Thread Starter
Member
 
Joined: Jul 2008
Posts: 279
Likes: 0
Car: 1991 RS v8
Re: How to do a compression test?

I know i got my work cut out for me. Lol. I got pretty big hands.. so getting there trying to unscrew the plugs was enough work.. Also cut my hand a few times too
Reply
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
Elephantismo
Electronics
14
Feb 13, 2019 12:51 AM
FormulasOnly
TPI
95
Jul 23, 2018 08:47 AM
NZKnight
Tech / General Engine
6
Oct 15, 2015 02:47 PM
RedLeader289
Electronics
6
Sep 23, 2015 06:50 AM
WhteRbt
Tech / General Engine
2
Sep 21, 2015 09:48 AM




All times are GMT -5. The time now is 04:03 AM.