Compression test--good? bad?
Joined: May 2006
Posts: 3,881
Likes: 3
From: Jacksonville, Tampa & Tallahassee
It will yeild decievingly high comp numbers.
Other than trying to fool the would-be buyer of a tired motor, what might be the purpose of such a test?
Other than trying to fool the would-be buyer of a tired motor, what might be the purpose of such a test?
Clarification needed about the compression gauge.
Alright, I've attached a picture with two areas circled - 1 and 2. I want to see if we can clear this up once and for all - I think some are in the same boat as I.
Where's the shrader valve that you're supposed to remove? Is this a valve located in area "1" or is it the same valve you open to release the pressure at area "2"? I've been thinking both refer to the same, which may be why I get confused when you guys say remove the valve.
If they're the same, I still don't get it, because I would think holding the valve open would do the same thing. If not, and there's a valve in area that causes a restriction in area "1" it would make sense, because that restriction would hinder all the chamber pressure from registering as the motor spins when you're looking for even bounces.
When doing a compression test don't you also have to take into account the rpm speed of the engine at the time of the test. The higher it is the more you have to reduce the "nominal" readings.
Attachment 353517
Alright, I've attached a picture with two areas circled - 1 and 2. I want to see if we can clear this up once and for all - I think some are in the same boat as I.
Where's the shrader valve that you're supposed to remove? Is this a valve located in area "1" or is it the same valve you open to release the pressure at area "2"? I've been thinking both refer to the same, which may be why I get confused when you guys say remove the valve.
If they're the same, I still don't get it, because I would think holding the valve open would do the same thing. If not, and there's a valve in area that causes a restriction in area "1" it would make sense, because that restriction would hinder all the chamber pressure from registering as the motor spins when you're looking for even bounces.
Alright, I've attached a picture with two areas circled - 1 and 2. I want to see if we can clear this up once and for all - I think some are in the same boat as I.
Where's the shrader valve that you're supposed to remove? Is this a valve located in area "1" or is it the same valve you open to release the pressure at area "2"? I've been thinking both refer to the same, which may be why I get confused when you guys say remove the valve.
If they're the same, I still don't get it, because I would think holding the valve open would do the same thing. If not, and there's a valve in area that causes a restriction in area "1" it would make sense, because that restriction would hinder all the chamber pressure from registering as the motor spins when you're looking for even bounces.
Joined: Apr 2005
Posts: 3,785
Likes: 30
From: And the horse he rode in on...
The Compression Testing Post to End all Compression Testing Posts
For Referrence- https://www.rx7club.com/showpost.php...65&postcount=3
This post should be stickied.
https://www.rx7club.com/2nd-generation-specific-1986-1992-17/rotary-compression-tester-734840/
Here is jackhild59's methodology for uniform compression test results on a rotary engines
Fully charge the battery. Slow rotation will yield deceptively low results.
Use the bottom spark plug hole; take both bottom spark plugs out for the duration of the test.
Remove the EGI fuse: you don't want the motor trying to try to run.
The throttle should be held to the floor or the throttle plate tied fully open. Failure to do this will yield very low results.
You will need an assistant to operate the ignition (and throttle) or a remote start switch with the throttle tied open.
Ideally the test should be done while the engine is warm. Cold is ok, but warm numbers are what should count. If you test a cold engine, be aware the what looks like an unacceptably low number may actually turn out to be quite all right. There is no hard fast number that will be the difference from cold to hot. Warm will be consistent, cold may not be. If an engine fails the test cold, it may well pass the test when warm.
Crank the engine for about 10-15 seconds max for each test.
Bump Test:
This verifies that the apex seals are operating properly, ie. not broken or stuck.
Remove the Schroeder valve in the BOTTOM of the compression tester (NOT the side release valve) for the 'look for three uniform bump test'.
Leave the side release valve alone for this test.
Test each chamber front then rear, make notes! Those bumps should be even and somewhere around 60-70 psi minimum.
Compression Test:
This is to test the actual compression in the engine.
Replace the Schroeder valve in the BOTTOM of the compression tester.
Retest each chamber, this time allowing the pressure to pump up to the maximum psi. Write this number down.
PSI should be above 85 psi per the FSM. Lower than this and the engine is in need of a rebuild. This engine will be prone to flood. It may however run for quite sometime, especially if it is NA.
90 psi+ should be enough to run reliably;
95 psi + is great,
100psi+ is excellent.
NOTE: All pressures will tend to be lower with longer hoses and higher with shorter hoses. Experts agree that ideally, the hose should be as short as possible; Best would be if the hose were eliminated and the unit screwed directly into the sparkplug hole essentially with the gauge as closely coupled to the compression chamber as possible.
NOTE 2: In general, Turbo II's will be lower numbers than NA, S4's will be lower than S5's.
NOTE 3: This write up is a compilation from several authors, Reted, Kevin Landers, Aaroncake and others combined, clarified and hammered into one place. The intent is to create consistent documented methodology in one place so that the newbs and accomplished alike can agree on the results.
Here is jackhild59's methodology for uniform compression test results on a rotary engines
Fully charge the battery. Slow rotation will yield deceptively low results.
Use the bottom spark plug hole; take both bottom spark plugs out for the duration of the test.
Remove the EGI fuse: you don't want the motor trying to try to run.
The throttle should be held to the floor or the throttle plate tied fully open. Failure to do this will yield very low results.
You will need an assistant to operate the ignition (and throttle) or a remote start switch with the throttle tied open.
Ideally the test should be done while the engine is warm. Cold is ok, but warm numbers are what should count. If you test a cold engine, be aware the what looks like an unacceptably low number may actually turn out to be quite all right. There is no hard fast number that will be the difference from cold to hot. Warm will be consistent, cold may not be. If an engine fails the test cold, it may well pass the test when warm.
Crank the engine for about 10-15 seconds max for each test.
Bump Test:
This verifies that the apex seals are operating properly, ie. not broken or stuck.
Remove the Schroeder valve in the BOTTOM of the compression tester (NOT the side release valve) for the 'look for three uniform bump test'.
Leave the side release valve alone for this test.
Test each chamber front then rear, make notes! Those bumps should be even and somewhere around 60-70 psi minimum.
Compression Test:
This is to test the actual compression in the engine.
Replace the Schroeder valve in the BOTTOM of the compression tester.
Retest each chamber, this time allowing the pressure to pump up to the maximum psi. Write this number down.
PSI should be above 85 psi per the FSM. Lower than this and the engine is in need of a rebuild. This engine will be prone to flood. It may however run for quite sometime, especially if it is NA.
90 psi+ should be enough to run reliably;
95 psi + is great,
100psi+ is excellent.
NOTE: All pressures will tend to be lower with longer hoses and higher with shorter hoses. Experts agree that ideally, the hose should be as short as possible; Best would be if the hose were eliminated and the unit screwed directly into the sparkplug hole essentially with the gauge as closely coupled to the compression chamber as possible.
NOTE 2: In general, Turbo II's will be lower numbers than NA, S4's will be lower than S5's.
NOTE 3: This write up is a compilation from several authors, Reted, Kevin Landers, Aaroncake and others combined, clarified and hammered into one place. The intent is to create consistent documented methodology in one place so that the newbs and accomplished alike can agree on the results.
This post should be stickied.
Joined: Apr 2005
Posts: 3,785
Likes: 30
From: And the horse he rode in on...
I agree that it is impossible to tell the accurate pressure on each individual face. The test is still useful and should be considered as close as you can get to a uniform number to evaluate a motor.
To get better info you need an electronic tester-there are a couple on the market that do a dandy job.
Junior Member
Joined: Apr 2005
Posts: 48
Likes: 0
From: Lagrange, Kentucky
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