Compression check guidelines

H

hd2002hd

Member
Jul 20, 2010
457
St. Louis
Boat Info
1996-Sea Ray 215 Express Cruiser/1996-Sea Ray 330 Sundancer/1998-Sea Ray 400 Sundancer/1996-Sea Rayd
Engines
5.7 Mercruiser/454's/454's w/V drive/120HP
So, what does a good compression test look like on a 1996 330 DA? :wow: I have seen posted anything above 140. Is this a good go by number? If so, what does great look like and is there a concern with to high of a number? The boat has about 635 hours on both 7.4's.
 
You want to see consistency between all 8 cylinders. It is better to see that all are at 140 vs. 7 are at 190 and one is reading 120. Someone will correct me if I am wrong but I think a good rule of thumb is all cylinders within 20% of each other. Do you have some numbers from your engines?
 
Great lakes pirate said:
You want to see consistency between all 8 cylinders. It is better to see that all are at 140 vs. 7 are at 190 and one is reading 120. Someone will correct me if I am wrong but I think a good rule of thumb is all cylinders within 20% of each other. Do you have some numbers from your engines?
Click to expand...

Make sure it is at operating temp when you take the test.
 
I do have the numbers. Port engine has 6 at 160# and 2 at 165#. Starboard has 2 at 160#, 5 at 165# and 1 at 170#.
 
Sounds like the compression is great on the boat. The highest I have seen on boats is in the 170's. Good luck on the purchase.
 
10-15% between the highest and lowest cylinders has always been the rule in the automotive world
 
Motor should be warm, throttle fully advanced. As stated above difference between high to low should be within 10%, the number itself is not so important (north of 100 psi I would say), just that they are within 10%. Your numbers seem good.
 
Larry said:
Motor should be warm, throttle fully advanced. As stated above difference between high to low should be within 10%, the number itself is not so important (north of 100 psi I would say), just that they are within 10%. Your numbers seem good.
Click to expand...
takes 80psi for burn to happen...120 to happen efficiently...135+ is good depending on compression ratio
 
I should have clarified the 100 psi number is a bare minimum for a functioning motor....

Sent from my DROID BIONIC using Tapatalk
 
What do you guys think of these numbers?

Cyl #1 - 105
Cyl #2 - 105
Cyl #3 - 115
Cyl #4 - 105
Cyl #5 - 110
Cyl #6 - 105
Cyl #7 - 90
Cyl #8 - 115

But the motor was cold and this is the first time it was turned over since I put her to bed for the winter
we had just finished replacing a ton of parts and wanted to do a compression test also
I think I might want to run it agian after getting to motor up to temp.
 
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Studman,
The engine definitely needs to be at operating temp with the throttle propped open before you can assess your readings. Also note that all of the plugs need to be removed while collecting the data. Sorry if I'm stating the obvious, but I did here of someone that ran the test while pulling and replacing one plug at a time.
 
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cmount1 said:
Studman,
The engine definitely needs to be at operating temp with the throttle propped open before you can assess your readings. Also note that all of the plugs need to be removed while collecting the data. Sorry if I'm stating the obvious, but I did here of someone that ran the test while pulling and replacing one plug at a time.
Click to expand...

Hey Cmount1,
My buddy who helped me with all the part exchange is a 30 year Mech. , but for cars, said he didn't think it would be a big deal to run the test cold but I disagree.
concidering it's been sitting cold for 5 months. I am going to get it started and warm her up to temp, then pull the plugs and run the test again
oh BTW we did have all the plugs out
Thanks for your imput
 
Heres the deal, if you can't get the engine started then there is no way to warm it up. Do a cold test and use 12-15% as a limit. If the engine has been sitting for awhile some mechanics shoot a squirt of oil into the cylinder to lubricate the rings. If you do this then make sure you turn it over a number of times before testing. This will raise the compression but it should be consistent across all cylinders. You can leave the plugs in and do it one at a time or take them all out and test. Again be consistent. Depending on how you test will determine the compression pressure. That is not as important as the ratio from one cylinder to te next.

The new electronic methods don't actually measure compression at all. They measure the length of time it takes each piston to go from bottom dead center to top dead center. This will be shown in percentage starting at 100% for the highest then every other cylinder is measured against that value. We don't even publish compression specs anymore. We only use the percentage values as many manufacturers do now a days.

Just my opinion
 
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Studman,

I never ran a cold test, I have 25 years experience and was taught to run the test hot. I would be interested to see what the difference may be when you run the test hot.

Oldskool,

I understand that in certain instances you may need to run a cold test but, all of the plugs should be removed. If there happens to be a leak between cylinders it will affect the pressure in the adjacent cylinder. Otherwise I agree you you.

I know current OBD-II systems when diagnosing a misfire measure the deceleration/acceleration rates between engine cycles to identify which cylinder has the misfire? They also use the same method to run a dynamic compression test when invoked by the user to identify a weak cylinder.

I may be restating the method you describe, just interested to know if it's determend differently.
 
I too would like to see the difference. I think #7 will still show weak. I will concede that plugs out is the best way to go but have done it both ways. You need to remove the plugs anyhow so plugs out would be the way to go.
 
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