Compression test on 7.4L, are these numbers ok?

[kleake]

1993 Mercruiser 7.4L with 135hrs on it. Did a compression test to see what was what, and was surprised to find lowest at 110psi and the highest at 125psi with most at 115-120. These seem pretty low to me, but maybe that is normal for this engine. I was expecting to see around 150 or so.

 

[joewithaboat]

Re: Compression test on 7.4L, are these numbers ok?

110 is just barley out of the 10% acceptance high to low.

You checking just to check or having problems?

 

[makonnen]

Re: Compression test on 7.4L, are these numbers ok?

I can already see the conflict this is going to cause but I'll say it anyways...

Its more important to get even compression across all cylinders than looking at the factory specs.

Its highly unlikely that an engine will wear so evenly that all cylinders will be equally weakened by the wear and tear.

A compression test alone is a pretty poor indicator of engine performance. You're testing the engine cold at a few hundred RPM. Plus compression tests really only look for major malfunctions and stuff that doesn't go wrong all that often.

Question ... how many boat engines get replaced because they genuinely worn out instead of from lack of maintenance, ignorance, cooling / riser / elbow / flapper / impeller problems?

Did you crank open the throttle when you did the test?
Was your battery fully charged?
Did you have a battery charger hooked up to the batteries?
Did you remove all the spark plugs?
Did you remove the accessory belt?

 

[achris]

Re: Compression test on 7.4L, are these numbers ok?

What he ^ said... And 'was the engine warm'?

 

[tpenfield]

Re: Compression test on 7.4L, are these numbers ok?

Here are some compression numbers that I have recorded on my 7.4L (1991) over the years (see below). The first few years I had the throttle closed and the last few I had it opened (after realizing that it made a good difference having it opened)

Your numbers are a bit low, but reasonably consistent. So, it is worth doing some re-tests, etc.

Engine running for 20-30 minutes prior.
All plugs out.
Battery fully charged (I actually leave a charger going during the test so that the last few cylinders have as good a charge as the first few)
For kicks and giggles, you could squirt fogging oil into the cylinder and immediately re-test to see if the number jumps higher.

The spec on my engine is 150 psi . . . So my numbers are close. It is hard to say if my gauge is accurate (cuz it is wicked old). I have 500 hours on the engine and just did a valve job last Fall (getting me to the better numbers listed under "2012").

So, I would think that with a 1993, you should be looking to get similar numbers to my 1991 version. Your engine might be the "Gen V" version, whereas mine is the "Mark IV" version of the 7.4L / 454 ci GM engine, a la Mercruiser.

 

[achris]

Re: Compression test on 7.4L, are these numbers ok?

Here are some compression numbers that I have recorded on my 7.4L (1991) over the years (see below). The first few years I had the throttle closed and the last few I had it opened (after realizing that it made a good difference having it opened)

Your numbers are a bit low, but reasonably consistent. So, it is worth doing some re-tests, etc.

Engine running for 20-30 minutes prior.
All plugs out.
Battery fully charged (I actually leave a charger going during the test so that the last few cylinders have as good a charge as the first few)
For kicks and giggles, you could squirt fogging oil into the cylinder and immediately re-test to see if the number jumps higher.

The spec on my engine is 150 psi . . . So my numbers are close. It is hard to say if my gauge is accurate (cuz it is wicked old). I have 500 hours on the engine and just did a valve job last Fall (getting me to the better numbers listed under "2012").

So, I would think that with a 1993, you should be looking to get similar numbers to my 1991 version. Your engine might be the "Gen V" version, whereas mine is the "Mark IV" version of the 7.4L / 454 ci GM engine, a la Mercruiser.

Somebody's got the same fetish as me.. :facepalm: I do that with everything, my house electricity, water and gas consumption, my car's fuel consumption... Sad isn't it....

 

[kleake]

Re: Compression test on 7.4L, are these numbers ok?

Bought the boat last fall, had fuel issues and only got to test it really once. Did alot of misc. work over the winter and had it out with new plugs and such last weekend. Couldn't get on plane and it was missing on one cylinder. Not sure if I even have the right prop, but I needed to get the engine running right first. Started doing a compression check to find the bad cylinder, but found a plug wire had fallen off to be the culprit. Finished the compression check anyway and after fixing the plug wire it ran MUCH better on the hose so now I am anxious to give it a spin on the water and see what it will do.

The test was just a basic one to find a suspected weak cylinder, so it was cold, throttle closed, and pulled one plug at a time, but I did have a charger on and the batteries are strong so cranking speed was pretty fast. I am sure those numbers would be a tad higher if I did everything per spec, but doing the same method on other engines has produced much higher results. For example I just did a 96 Olds Ciera with 65k miles and it had over 220psi on each cylinder. I was mostly curious if those were even in the ball park or if i should have been expecting over 150 each. From what I see, I may be a tad low, but should be acceptable.

 

[nola mike]

Re: Compression test on 7.4L, are these numbers ok?

You did the compression test wrong, but if anything the way you did it would lead to lower numbers I'd think. I'm not sure if the % variance between the cylinders would change if the testing method changed, but as it is your numbers are OK.

 

[Don S]

Re: Compression test on 7.4L, are these numbers ok?

Redo your compression test and make sure you follow this service bulletin.
A compression test done improperly will not tell you much.

View attachment 97-25 Compression Test.pdf

 

[kleake]

Re: Compression test on 7.4L, are these numbers ok?

Redo your compression test and make sure you follow this service bulletin.
A compression test done improperly will not tell you much.

View attachment 146940

Excellent reference. As is I am satisfied knowing if I adjust my method I can get higher numbers, but where it is looks good. I'll see how it runs on the water and go from there. If I don't get the performance I expect, then I'll re-test using the mentioned methods.

 

[tpenfield]

Re: Compression test on 7.4L, are these numbers ok?

If I don't get the performance I expect, then I'll re-test using the mentioned methods.

. . . just to share a little of my 7.4L mystery over the past year . . . I was dealing with a performance drop-off in the past few years. Lost about 300 RPM and 5 MPH in the top end.

So, I did my compression tests and they came back OK, not great, but OK. (125 - 140 psi) Then I did a cylinder leak test and discovered that #4 and #5 had 50% leak coming through the exhaust valves. . A few other cylinders were in the 20 - 30% leak range, again the valves . . . some exhaust and some intake.

I could have never picked up on the bad valves with just the compression test.

 

[kleake]

Re: Compression test on 7.4L, are these numbers ok?

. . . just to share a little of my 7.4L mystery over the past year . . . I was dealing with a performance drop-off in the past few years. Lost about 300 RPM and 5 MPH in the top end.

So, I did my compression tests and they came back OK, not great, but OK. (125 - 140 psi) Then I did a cylinder leak test and discovered that #4 and #5 had 50% leak coming through the exhaust valves. . A few other cylinders were in the 20 - 30% leak range, again the valves . . . some exhaust and some intake.

I could have never picked up on the bad valves with just the compression test.

That is good info and I may look into it. At this point, I haven't even been able to get her on plane, but feel I should be able to now. If I can, I will see what her performance is because she is very low hours (135), no crankcase blowby that I can see, and seems quite peppy on the hose and I'm really not wanting to tear into her if the performance is at least in the "good" category. I believe my top speed should be somewhere around 40-45 with this engine in this boat. If I can't get to 40, then I'll start looking for problems. If she tops out close to 45, I'll be pretty happy.

 

[Don S]

Re: Compression test on 7.4L, are these numbers ok?

Simply putting oil in the cylinders as stated in the service bulletin will eliminate the rings from low compression. When you put the oil in and redo the compression test, if the pressure comes up, rings are bad. If it doesn't come up it's probably valves.

 

[tpenfield]

Re: Compression test on 7.4L, are these numbers ok?

That is good info and I may look into it. At this point, I haven't even been able to get her on plane, but feel I should be able to now. If I can, I will see what her performance is because she is very low hours (135), no crankcase blowby that I can see, and seems quite peppy on the hose and I'm really not wanting to tear into her if the performance is at least in the "good" category. I believe my top speed should be somewhere around 40-45 with this engine in this boat. If I can't get to 40, then I'll start looking for problems. If she tops out close to 45, I'll be pretty happy.

What kind/size boat are we talking about?

I have a 24 foot deep V that is about 5,000 lbs dry. When the engine was in good shape, I got 48 mph. I was down to 42 mph before the rebuild.

 

[scoflaw]

Re: Compression test on 7.4L, are these numbers ok?

What kind/size boat are we talking about?

I have a 24 foot deep V that is about 5,000 lbs dry. When the engine was in good shape, I got 48 mph. I was down to 42 mph before the rebuild.

So if your boat was 6mph slower, you must have been shy quite a bit of rpm as well.

 

[kleake]

Re: Compression test on 7.4L, are these numbers ok?

agreed, and I may do the compression check again, but first I want to see what the boat does. It's a 27ft Bayliner cabin cruiser that weighs in around 6500 lbs if I remember right. From what I have heard, people with 5.7L in these get about 33mph tops, and the 7.4 gets to the low to mid 40's. So far I made it to 15mph on 7cyl, and the tabs not working as well as my prop may be way off. Gotta see what it does on the water before I want to dig in deeper. I should be able to do that this weekend.

 

[tpenfield]

Re: Compression test on 7.4L, are these numbers ok?

agreed, and I may do the compression check again, but first I want to see what the boat does. It's a 27ft Bayliner cabin cruiser that weighs in around 6500 lbs if I remember right. From what I have heard, people with 5.7L in these get about 33mph tops, and the 7.4 gets to the low to mid 40's. So far I made it to 15mph on 7cyl, and the tabs not working as well as my prop may be way off. Gotta see what it does on the water before I want to dig in deeper. I should be able to do that this weekend.

Yup, fairly big boat for a single engine . . . if you see 40 mph, then I would consider all to be well. The thing about those 454's (7.4L) is that they are fairly 'bullet proof' engines and will still run OK, even if they have issues. I'm thinking that you should have gotten more than 15 mph even on 7 cylinders, but your next sea test will be revealing.

 

[kleake]

Re: Compression test on 7.4L, are these numbers ok?

Well first trial and with 7 people aboard I made it to 36mph at 3900 rpm. I may be over propped but I am going to try a lighter load and see where I stand. It may be a little bit weak though. It seemed to run OK though for the weight I have on here.

 

[tpenfield]

Re: Compression test on 7.4L, are these numbers ok?

I'm thinking you should have no more than a 19" pitch, maybe less. Check the WOT spec on that engine, I think it is around 4500 RPM. So, see if you get near that with a light load.

 

[kleake]

Re: Compression test on 7.4L, are these numbers ok?

With just me onboard I was able to get 40mph GPS speed, but I was only turning 4000rpm. It struggled getting on plane with a full load and still worked pretty hard getting on plane with just me. I think she is running pretty decent, just over propped quite a bit. I think I will drop to a 16 or so and see what that does.