Mercuiser 357 mag low compression on #6 (120 psi)

[PNBOOTH]

Engine seems to run O.K. Head gasket is fine. Is it O.K. to run for a few hours to get to haulout for service or should I have it towed?

 

[alldodge]

Engine seems to run O.K. Head gasket is fine. Is it O.K. to run for a few hours to get to haulout for service or should I have it towed?

If #6 is the lowest I wouldn't worry much about it. While 150 is new motor, 120 for a cold motor is not something to be concerned about right now.

What caused you to run a compression test?

 

[PNBOOTH]

If #6 is the lowest I wouldn't worry much about it. While 150 is new motor, 120 for a cold motor is not something to be concerned about right now.

What caused you to run a compression test?

I had the boat for sale (Bayliner 245) and this surfaced in the inspection. All other cylinders were 180-200. Mechanic recommended head rebuild.

 

[alldodge]

I had the boat for sale (Bayliner 245) and this surfaced in the inspection. All other cylinders were 180-200. Mechanic recommended head rebuild.

180 to 200 is way to high. To be that high the gauge is not accurate

If the test was done cold and not warmed up, this will cause issues also.

Unless the "Mechanic" (so he's called) also did a leak down test I would ask just how does he know the possible issue is the head, the rings or just a bad gauge and improper test procedure

Need to remember, boat Mechanics or may be called surveyors are only as good as their knowledge. Same with home inspectors, they can all hang a shingle out and say "hey I'm a qualified surveyor", or call them something else

Compression above 150 psi means there is liquid in the cylinders or the gauge is bad

 

[Scott Danforth]

Anything over 150 with a good gauge is water or oil in the cylinders

If the gauge is that far off that it's reading 180-200 when it should be reading 150, then your low cylinder is closer to ,80-90 psi

 

[tpenfield]

I'd get a decent gauge and run your own test to see what reading you get for the cylinders, particularly #6.

I assume the inspection was pre-sales? Is the deal still pending?

 

[Scott06]

No problem running it for now. I think you want to look into a leakdown test given your numbers

 

[stresspoint]

h a ha , that bloke that did the inspection needs to learn his job ( or do something else )
sounds as if one of the cylinders was correctly checked and the others were guessed.
even an excessive carbon build up will not give those sort of readings.

grab yourself a cheapo gauge and recheck yourself , numbers within 10% of each other is good (a little more on older motors is fine IMO) , don't be stressing if they are low on a cheapo gauge , that's normal

 

[PNBOOTH]

If #6 is the lowest I wouldn't worry much about it. While 150 is new motor, 120 for a cold motor is not something to be concerned about right now.

What caused you to run a compression test?

Thanks so much for the advice.

 

[JustJason]

Thanks so much for the advice.

Your compression is low. Your engine is 9.4 to 1 compression and you should see compression test results in the 170 to 200 PSI ballpark. I personally would not run the engine. If you have a leaking valve you can end up burning up that valve completely until the point the valve fails, and drops into the cylinder.

 

[Scott Danforth]

Your compression is low. Your engine is 9.4 to 1 compression and you should see compression test results in the 170 to 200 PSI ballpark. I personally would not run the engine. If you have a leaking valve you can end up burning up that valve completely until the point the valve fails, and drops into the cylinder.

uhhhh. spec is 150 psi cranking per GM and Mercruiser and VP....

to get to 200 psi cranking pressure, you need to have well over 14:1 compression

 

[JustJason]

uhhhh. spec is 150 psi cranking per GM and Mercruiser and VP....

to get to 200 psi cranking pressure, you need to have well over 14:1 compression

Can you show me the documentation you are getting your numbers from?

 

[Scott Danforth]

Can you show me the documentation you are getting your numbers from?

the manual

 

[JustJason]

So you are unable to show me the documentation you are getting your numbers from, got it.

 

[alldodge]

Good grief, another show me and manual is not good enough.
Below is a link on how to convert compression ratio to psi.

Formula is X:Y*P= PSI
X:Y = 9.4/1 = 9.4
P = Atmospheric = 14.696

9.4:1 Ratio 9.4*14.696 = 138.1424
14:1 Ratio 14*14.696 = 205.744

https://calculator.academy/compression-ratio-to-psi-calculator/

 

[ScottinAZ]

Good grief, another show me and manual is not good enough.
Below is a link on how to convert compression ratio to psi.

Formula is X:Y*P= PSI
X:Y = 9.4/1 = 9.4
P = Atmospheric = 14.696

9.4:1 Ratio 9.4*14.696 = 138.1424
14:1 Ratio 14*14.696 = 205.744

https://calculator.academy/compression-ratio-to-psi-calculator/

And that’s 14.696 AT SEA LEVEL…. Not all of us live at sea level, so those numbers would be “high” for many folks that are inland and operate on lakes. My elevation is 3500 feet for example (pressure drops about 1inch of mercury per 1000 feet according to my search) sea level is at 29.92 inHg approximately.


In my case it’s about a 10psi difference at an 8.5 compression ratio

 

[JustJason]

Good grief, another show me and manual is not good enough.
Below is a link on how to convert compression ratio to psi.

Formula is X:Y*P= PSI
X:Y = 9.4/1 = 9.4
P = Atmospheric = 14.696

9.4:1 Ratio 9.4*14.696 = 138.1424
14:1 Ratio 14*14.696 = 205.744

https://calculator.academy/compression-ratio-to-psi-calculator/

Good grief....

That's not even remotely correct as it doesn't apply to engines....

For example, where does the formula take into account volumetric efficiency or valve timing?

Here is an example. Honda BF200 engines. The compression ratio listed in the service manual is 9.4 to 1, same as the Merc 357. The expected compression pressure as listed in the service manual is 199 to 228 PSI.

Another example, Suzuki DF300. Compression ratio 9.5 to 1. Compression pressure, 213 PSI.

So why is it a Honda outboard can generate 228 PSI of compression pressure on that same 9.4 to 1 but yet the Suzuki can only generate 213 PSI of compression when the compression ratio on the Suzuki is higher? The answer is valve timing and volumetric efficiency. The Honda and Suzuki outboard engine is more efficient and can cram more air into the cylinders than a GM-based inboard engine can.

You can certainly get more than 200 PSI of compression out of a GM small block with a hotter cam, but you can only go so hot before the engines start ingesting water through reversion.

The expected compression pressure on a new to newish GM-based marine engine is 170 to 200 PSI, and I stand by that. If you are not getting a number in that ballpark on a new engine, then either the compression gauge is faulty or the engine isn't put together right.

I've only been compression-testing marine engines for 20+ years.

Lets do 1 more for fun. A dinky little Suzuki 2.5 horsepower engine. Service manual states a compression ratio of just 9 to 1, but, that same service manual states a compression pressure of up to 199 PSI. How can that be????

 

[alldodge]

More good grief and I don't have the time right now to get into all of it, but the Suzuki df300 shows compression is 10.5:1

The 350 is 12:1

So suggest you show what your talking about this time

 

[JustJason]

More good grief and I don't have the time right now to get into all of it, but the Suzuki df300 shows compression is 10.5:1

The 350 is 12:1

So suggest you show what your talking about this time

Sure thing chicken wing

In all fairness, the DF300 engine I was quoting is the older DF300 without the lean burn technology. The photos below come from that older engine service manual.
However, I do have the manual for the DF350A engine, which does have the lean burn. That engine has a 12.1 compression ratio but only 162 PSI. Hmmmm.... Do you see why your calculator you found doesn't work?

 

[Scott Danforth]

So you are unable to show me the documentation you are getting your numbers from, got it.

Manuals are copyright material so I can not share them in open forums

Manual 17 lists 150 psi cranking compression for the small blocks with 9.4:1 CR

Manual 18 list 180 psi cranking compression for the 4.3 with 9.4:1 CR

The difference is the cam timing

That and 40 years spinning wrenches you get to know the motors.