Compression Numbers for 454s

B

Brian S Liewergen

New Member
Oct 24, 2020
5
Boat Info
1996 370da
Engines
Twin 454 MercruiserS
Hi All,

I'm new here, looking at purchasing a 370EC w/ twin 7.4s with ~450hours.

Compression test showed the below:
P
115
115
115
115
115
110
115
115

S
115
115
115
110
105
110
110
115

These seem on the low side to me (would expect 140ish? given the hours?). However they are fairly uniform. My guess is this test was done cold and potentially with a weak port battery.

Any concerns? Is uniformity more important than the actual compression number? Should I have them re run the test hot?

..sorry, I did do a search but found hundreds of topics on this. Thought I would make a fresh one for some quick feedback. It's beyond the end of season here in Chicago and I need to make a fairly quick decision on this boat.

Thanks!
 
Learned a bit more, sounds like the motors have been more or less sitting for 15 years. Explains the low hours (boat in question is a 1994). Any merit to the thought that they just need to be warmed up and ran hard for a bit to loosen things up? Maybe sticky rings/valves?

Of course seller says it "runs great".
 
Has not been tested warm, I've contacted the dealership selling and requested a re-test with the motors warm. Also, the port battery is reported weak. Requested they change that for the re-test as well. Waiting on a response.
 
Has it been sea trailed? Did they reach 4200-4400 rpm? If it did the engines are probably fine.
 
Brian S Liewergen said:
Has not been tested warm, I've contacted the dealership selling and requested a re-test with the motors warm. Also, the port battery is reported weak. Requested they change that for the re-test as well. Waiting on a response.
Click to expand...
Def needs to be warm for accurate numbers, also a good battery will help

I feel uniformity is the important thing. It’s a little low, especially for the hours. Mine had 750 hours and were 135/140
 
Not to high jack the thread but I also recently had a compression test done on 97 330DA w/approx. 700hrs I am looking to purchase w/twin 454. The compression numbers are as follows:

P - 160, 160, 150, 160, 130, 155, 155, 160
S - 155, 150, 145, 135, 150, 150, 150, 150

The compression test was done cold, and the mechanic recommended a leak down test in #3 on the port side, but he didn't say anything more than that on the report and I have't been able to speak with him.

Does anyone have any insight into why a leak down test would be needed on that cylinder only? Additionally, does the variance in the numbers across all cylinders concern anyone?

Thanks
 
Last edited:
ejlivas said:
Not to high jack the thread but I also recently had a compression test done on 97 330DA w/approx. 700hrs I am looking to purchase w/twin 454. The compression numbers are as follows:

P - 160, 160, 150, 160, 130, 155, 155, 160
S - 155, 150, 145, 135, 150, 150, 150, 150
..Thanks
Click to expand...

I recently bought a boat with a 454, low hours. The compression check was done with a warmish (not hot) engine, with full throttle and a fully charged batteries. The readings were all in the 145 psi range. I would suggest the 30 psi difference you are seeing is concerning. Could be a valve or ring problem or something similar.
 
Brian S Liewergen said:
Hi All,

I'm new here, looking at purchasing a 370EC w/ twin 7.4s with ~450hours.

Compression test showed the below:
P
115
115
115
115
115
110
115
115

S
115
115
115
110
105
110
110
115

These seem on the low side to me (would expect 140ish? given the hours?). However they are fairly uniform. My guess is this test was done cold and potentially with a weak port battery.

Any concerns? Is uniformity more important than the actual compression number? Should I have them re run the test hot?

..sorry, I did do a search but found hundreds of topics on this. Thought I would make a fresh one for some quick feedback. It's beyond the end of season here in Chicago and I need to make a fairly quick decision on this boat.

Thanks!
Click to expand...

The good news is they are all very close to the same. Bad news is they are low - for reference, on my 4.3 the last compression test I did was this past spring - cold engine it was right at 140 on all six cylinders. I wouldn't fear the worst at this point, if you were seeing one or two cylinders much lower than the others then yes. If it were me, knowing the engine has not been run much lately, I would take for a good run and then test it again warm.
 
The spec for 454 compression is lower than for small block 6 and cylinder engines. Around 130-140 for a healthy 454 would be very good. Small blocks spec around 170-180. The quoted reading are low but uniform and my return to normal if tested warm/hot with good batteries. Someone needs to make some effort to get better test conditions in order to make an informed decision otherwise the purchase price will be lower.
 
scoflaw said:
Leaky valve no doubt, common problem.
Click to expand...

I’d agree if there was one or two suspect cylinders but 32 valves all leaking the same amount?

I imagine that after a good warm up run you’ll see numbers closer to or within spec with one or two slightly below.

Heat it up, get oil all over everything and test again.
 
Soulshine said:
I’d agree if there was one or two suspect cylinders but 32 valves all leaking the same amount?

I imagine that after a good warm up run you’ll see numbers closer to or within spec with one or two slightly below.

Heat it up, get oil all over everything and test again.
Click to expand...

Agreed about the test procedure.

Garbage in - Garbage out.

If the compression test wasn’t performed correctly, the results are not accurate.

You may indeed have internal issues, but don’t assume it based on an improperly performed test.
 
Soulshine said:
I’d agree if there was one or two suspect cylinders but 32 valves all leaking the same amount?

I imagine that after a good warm up run you’ll see numbers closer to or within spec with one or two slightly below.

Heat it up, get oil all over everything and test again.
Click to expand...
I was responding to post #11
 
Agree something is suspect...Port numbers all 115 but one...then Starboard numbers basically similar....retest after engines are warmed up and use different gauge as well.
 
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