Compression test help!!
Thread Starter
Joined: Nov 2001
Posts: 7,303
Likes: 4
From: Delray Beach, Fl.
Compression test help!! What should a stock short block with 11.1CR show on a compression tester?
170PSI???
180PSI???
I know the cyl. should be with 15% MAX of each other but I need to know whats considered normal for a 10K mile motor thats been taken very well care of but ran a 100 passes.
JS
170PSI???
180PSI???
I know the cyl. should be with 15% MAX of each other but I need to know whats considered normal for a 10K mile motor thats been taken very well care of but ran a 100 passes.
JS
Joined: Nov 2001
Posts: 3,043
Likes: 0
From: Texas and Qatar
Re: Compression test help!! The compression ratio won't correspond directly to a PSI compression test. Too many factors come into play, one cam can have high PSI and another can have lower PSI on the same engine. There isn't a exact number to look for, just minimums.
The main thing to look for would be consistency between all the cylinders, they could be 160psi or all of them 175psi. With 10K miles it should be a fairly broken-in engine and have ideal compression.
Allot of race engine builders keep track of that stuff between races, compression tests and leakdown tests in a logbook. Its cheaper to freshen an engine than to repair them when something goes wrong, thats on engines that go 60 passes before being "refreshed" also.
The main thing to look for would be consistency between all the cylinders, they could be 160psi or all of them 175psi. With 10K miles it should be a fairly broken-in engine and have ideal compression.
Allot of race engine builders keep track of that stuff between races, compression tests and leakdown tests in a logbook. Its cheaper to freshen an engine than to repair them when something goes wrong, thats on engines that go 60 passes before being "refreshed" also.
Joined: Nov 2001
Posts: 3,043
Likes: 0
From: Texas and Qatar
Re: Compression test help!! More than likely your readings will be within 5-10psi with a compression test. Something like this is what you'll see:
#1: 165psi #2 170psi
#3: 175psi #4 165psi
#5: 170psi #6 170psi
#7: 175psi #8 170psi
Pull all the plugs and both bulk coil pack connectors and hold the throttle open when cranking (park/neutral). You will want the compression needle to jump up 4 times to take the reading, I sometimes just let it go about 6 times (most of the pressure will build on the first pump)
If you have a starter bypass button, you can energize the thick purple wire sticking out of the harness on the passenger inner fender with 12v and it will crank over without the key, be careful with that and a 6-speed car <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" /> It will save you some time for a complete comression test. I always do each cylinder 3 times before moving to the next one.
A leakdown test will tell you more about the condition of the rings and valve sealing if you suspect a problem.
#1: 165psi #2 170psi
#3: 175psi #4 165psi
#5: 170psi #6 170psi
#7: 175psi #8 170psi
Pull all the plugs and both bulk coil pack connectors and hold the throttle open when cranking (park/neutral). You will want the compression needle to jump up 4 times to take the reading, I sometimes just let it go about 6 times (most of the pressure will build on the first pump)
If you have a starter bypass button, you can energize the thick purple wire sticking out of the harness on the passenger inner fender with 12v and it will crank over without the key, be careful with that and a 6-speed car <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" /> It will save you some time for a complete comression test. I always do each cylinder 3 times before moving to the next one.
A leakdown test will tell you more about the condition of the rings and valve sealing if you suspect a problem.
Thread Starter
Joined: Nov 2001
Posts: 7,303
Likes: 4
From: Delray Beach, Fl.
Re: Compression test help!! Frosty,I want to make sure my comp. is good.
THEN I'll do a leakdown on the motor.
JS <img border="0" alt="[angel]" title="" src="graemlins/gr_angel.gif" />
THEN I'll do a leakdown on the motor.
JS <img border="0" alt="[angel]" title="" src="graemlins/gr_angel.gif" />
Thread Starter
Joined: Nov 2001
Posts: 7,303
Likes: 4
From: Delray Beach, Fl.
Re: Compression test help!! Thans Joe,So I should get a reading for all 8 and then determine if a few holes are weak.
Makes sense to me..
But what about the 15% variance between any holes?Does this sound normal for a stocker shortblock with ay 10K on it?
EG.
175PSI 5 cyl. and 150PSI for the other 3cyl.?
JS
Makes sense to me..
But what about the 15% variance between any holes?Does this sound normal for a stocker shortblock with ay 10K on it?
EG.
175PSI 5 cyl. and 150PSI for the other 3cyl.?
JS
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TECH Senior Member
Joined: Nov 2001
Posts: 10,341
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Re: Compression test help!! hehe um ok <img border="0" title="" alt="[Smile]" src="gr_stretch.gif" />
A leakdown test is more accurate but whatever floats your boat <img border="0" alt="[angel]" title="" src="graemlins/gr_angel.gif" />
A leakdown test is more accurate but whatever floats your boat <img border="0" alt="[angel]" title="" src="graemlins/gr_angel.gif" />
