Compresson Test Results On My 5.7..Video Included..Also Performance Upgrade Questions

[jserr68594]

I bought a 2001 Mercruiser Vortec 5.7 250 HP 2 Bar. carb engine that has one low cylinder. The engine has 870 Hrs. It was purchased from the original owner who is well into his 60's and had the engine in a 25' Grady White that he used for fishing in salt water.

The compression test was done after running the engine for two minutes or so to warm the block up. I had the choke open on the carb, but did not have the trottle wide open. How would this affect the test results? The left side compression numbers are very consistent, while the right side has a definite low cylinder. Here are the readings:


Right Side Of Engine:
(2) 170
(4) 175
(6) 155
(8) 185

Left Side Of Engine:
(7) 185
(5) 180
(3) 185
(1) 185

I am not sure where to go at this point. Is it worth messing with, or use it as is? I did squirt tranny oil into the 155 cylinder and it only brought the compression up 5 pounds, so I am assuming it is in the valves and not in the rings. Could this be a head gasket? Am I correct to think this?

Should I have both heads redone, or just the side with low compression? Should I do anything with the shortblock as far as rebuilding is concerned? Is there anything I should check on the motor due to it's salt water use?

I am putting this is a 20' regal boat. I hoping to make 325 HP out of it. I plan on using a high-rise intake and a 4 Bar. carb. What else should I consider upgrading for performance? I do want the engine to hold up well and also make decent HP with it.

Here is a video of it running out of the boat: MVI 0673 - YouTube

Thanks for your input!

 

[Another retired guy]

Re: Compresson Test Results On My 5.7..Video Included..Also Performance Upgrade Quest

My vote goes to a burnt exhaust valve. Pressure is only just outside the 10% rule, but now would be the best time to fix it. Do the right side first. If resulting pressures are close to the left, then that should be ok.If they're significantly higher then do the left.
What oil pressure reading did you get?

 

[jserr68594]

Re: Compresson Test Results On My 5.7..Video Included..Also Performance Upgrade Quest

Would you just mess with one head or take both off to be checked over?

 

[Don S]

Re: Compresson Test Results On My 5.7..Video Included..Also Performance Upgrade Quest

If you want to know what is causing the problem, you go to the next step. A leak down test. That will tell you where that little bit of compression loss went.

PS: In the real world, you aren't going to get 325 hp out of a carbed engine in a boat. When you get to those HP ratings, you have cams that won't work (see reversion), and rpm's that won't work in marine engines.

 

[jserr68594]

Re: Compresson Test Results On My 5.7..Video Included..Also Performance Upgrade Quest

If you want to know what is causing the problem, you go to the next step. A leak down test. That will tell you where that little bit of compression loss went.

PS: In the real world, you aren't going to get 325 hp out of a carbed engine in a boat. When you get to those HP ratings, you have cams that won't work (see reversion), and rpm's that won't work in marine engines.

I guess it looks like I should remove the head regardless. I assume the problem will be obivous one it is apart, correct?

I was planning on using this engine as a guidline for mine: 5.7L MerCruiser 350 GEN+ 325 hp Crate Engine

They get 325 HP out of that engine supposedly. Are they being overly optimistic with the HP rating?

 

[Another retired guy]

Re: Compresson Test Results On My 5.7..Video Included..Also Performance Upgrade Quest

Would you just mess with one head or take both off to be checked over?

I guess it depends on what you want out of it, and how much time/money you have! If you're looking to bring it back to " new" condition then you'll have to tear it down and check everything ( that's why I asked about oil pressure). If you just want an engine to work well ( it is 11 yrs old ) without spending mega bucks/time, then just address the known problems.

 

[jserr68594]

Re: Compresson Test Results On My 5.7..Video Included..Also Performance Upgrade Quest

I guess it depends on what you want out of it, and how much time/money you have! If you're looking to bring it back to " new" condition then you'll have to tear it down and check everything ( that's why I asked about oil pressure). If you just want an engine to work well ( it is 11 yrs old ) without spending mega bucks/time, then just address the known problems.

I didn't have an oil pressure gauge hooked up to it unfortunately. I think I will pull both heads of and have them gone through and then inspect the main bearings and piston walls. If I see any signs of issues, go from there I guess.

 

[Another retired guy]

Re: Compresson Test Results On My 5.7..Video Included..Also Performance Upgrade Quest

I didn't have an oil pressure gauge hooked up to it unfortunately. I think I will pull both heads of and have them gone through and then inspect the main bearings and piston walls. If I see any signs of issues, go from there I guess.

Ah, a winter project. My experience - if you go looking for problems, you're going to find them

 

[84EdH]

Re: Compresson Test Results On My 5.7..Video Included..Also Performance Upgrade Quest

I would do the leak down test Don suggested before I pulled either head. It will help to identify where the compression is weak. Either intake valve, exhaust valve, past the rings, into the cooling circuit etc. You may not "see" what is wrong with the heads off, unless it is something obvious.

As far as the carbed 325hp crate engine, to match it, you would need to change the cam, maybe the pistons, maybe the carb, rocker arms etc. My experience is that I had a Merc 350 rated at 260hp. After many years, I replaced it with a new 5.7 carb, high rise manifold, roller cam, that was rated at 300hp. It runs great, but I dont feel that much difference in the performance of the boat. So like all this boat stuff.....what are you after and how much time and money do you want to spend ?

 

[tpenfield]

Re: Compresson Test Results On My 5.7..Video Included..Also Performance Upgrade Quest

I did not see any cooling water running to the engine in the video . . . (True ?) , but there appears to be a sea water pump on the lower starboard side. . . .

So, maybe a new one of those as well. . .

I would do a cylinder leak test as your next step before you start loosening bolts, etc. It could be something that a sea foam treatment could fix.

If you are concerned about cost, you will quickly get into expenses if you pull the engine apart, and that may or may not be necessary. It is best to do some more testing, etc. at this point.

 

[Bondo]

Re: Compresson Test Results On My 5.7..Video Included..Also Performance Upgrade Quest

I didn't have an oil pressure gauge hooked up to it unfortunately. I think I will pull both heads of and have them gone through and then inspect the main bearings and piston walls. If I see any signs of issues, go from there I guess.

Ayuh,.... It's a 10 year old Saltwater motor,.... I'd run it til she pukes, then replace it.... The #s ain't that bad...

What yer proposin' is basically a Rebuild....
If that's the path ya choose to take, I'd be startin' with a NON-Saltwater motor...

 

[MarkSee]

Re: Compresson Test Results On My 5.7..Video Included..Also Performance Upgrade Quest

The #s ain't that bad...

5 of the cylinders have the compression of what my 7.4 has after a full valve job which was done when 1 exhaust valve was compromised that resulted in a compression of about 100 for 1 cylinder.
2 of your other cylinders are still pretty good so you really want to do all that because of 1 cylinder being a bit low and not even knowing yet what the cause is?

We actually took a sea trial before doing the compression test and could not even tell that 1 cylinder had an issue.

Mark