Compression - can it go back together

[180shabah]

Re: Compression - can it go back together

So what would you all compression readings of 178-185psi???

Rediculously high????

Racecar territory????


WRONG!!!

Mercruiser calls them perfect for a 4.3LX(spec is 180psi)

So which one if you should I believe???
I think I will go with the one that does not have a chip on his shoulder, thin skin and a fragile little ego.

Now... i can state for a fact that a boat motor does not leave the factory with a 12:1 compression ratio.... not for a recreational engine anyways....

OOPS!!!! Wrong again.
According to your chart that is exactly how they leave the factory.

 

[JustJason]

Re: Compression - can it go back together

where do you get that ## from????? page 1b-4 of service manual 32 lists a compression ratio of 9.2/1... when you do the math it works out to be a neat135psi... Even haut medoc agree's most motors run around 140..... so i have no idea where you guys are getting #'s in the 170's 180's and consider them "normal".... you must all shop at the same walmart... name your source for this spec of 180 shabah....
Bondo~ i've seen those numbers to... and after a can of combustion chamber cleaner i get em down into the 130's+ usually....
Even the common complaint of deaselieng... i've found about 75% of the time to be caused by high compression, the numbers are always the same... way above 160. And the decarb works every time.
care to chime in haut medoc?

 

[180shabah]

Re: Compression - can it go back together

where do you get that ## from?????

UHH!!! That would be Mercruise FSM #18 page 1B-7

I will admit that I do not have a copy of #32 (YET) but in all of the other merc manual I have seen if you look the line right after compression ratio, no really, it is the very next one, you will see:

Compression Pressure, and for '93-'97 4.3's that spec is 180psi.

 

[180shabah]

Re: Compression - can it go back together

...Even the common complaint of deaselieng... i've found about 75% of the time to be caused by high compression, the numbers are always the same... way above 160. And the decarb works every time...

Actually, it is the carbon buildup itself, not the high compression. The carbon doesn't accumlate in a nice smooth even surface. It is rough with peaks and valleys. It doesn't dissipate heat, it retains it. It holds enough that it gets red hot (much like the HSI in a gas furnace)and continues to ignite the air fuel charge in the absence of a controlled spark.

That is why decarbing always works. You have removed the ingition source.

 

[Don S]

Re: Compression - can it go back together

Merc and Volvo stopped posting the normal compression numbers, so #32 doesn't have any. I also see compression readings of 160 to 180 psi on the newer V6 and V8 engines, with a 9.3:1 compression ratio, of course I just work on these things for a living, so can only give real world info.

As far as dieseling goes, I find the THREE most common reasons on boats are
1. High engine idle rpm
2. Not allowing the engine to idle down for a minute or two before shutting off.
3. Lean carb adjustment (Especially the 3.0L engines)

Merc Service Bulletin 97-17

 

[JustJason]

Re: Compression - can it go back together

hmmmm... i checked... in manual 7 and also 18 and you are correct, they do list a pressure of 180... however they took number out, and replaced it with a minumun specification of 100psi in the newer manuals of 25 and 32....

in manual 18... they list MCM 4.3L and MCM 4.3LX as having a compression ratio of 9.3/1 and that little number of 180 under it
they also list the 4.3LX gen+ 2bbl, 4bbl and mefi as 9.4/1 with the same number of 180......
Now... even though it is a little... .1 of a difference, when you use the chart i posted as you go up in compression ratio the compression pressure goes up... and according to the chart. a 9.3 should be 140psi, a 9.4 should be 142psi, but 180psi is going to net a ratio of 11.7 (give or take)....
The math never really works out 100% because atomospheric pressure (14.6-14.7ish) is never locked into place... it is dynamic and changing all the time. 14.whatever is just the average. (if it were locked into place then there would be no pressure differentials on this planet... and hence... no wind or weather really)

Here's where i'm going with this... and maybe this is where the confusion is laying.

In manuals 7 and 18 it lists a compression pressure (not ratio) of 180psi. But it does not clearly state if 180psi should be the minumum, the average, or the maximum.
In 25 and 32, they list a compression pressure of 100psi, and it clearly states that that number is a minimum number... but also makes no mention of a maximum.
I'd bet you a jelly donut that the number of 180psi... is a maximum, and anything higher than that is to high.
And i'd also bet that this was a source of confusion at mercury, why else would they take the number out (which was not clearly marked anyways) and replace it with a number that is, well 80psi less than the previous... and clearly stated that it is a minimum number??

you say 180 is good... i say it's to high.... (to high for a boat motor)... you should research race motors... (i have, i've built a few) using my math i have run engines that were 13/1, that were pushing 200psi on my gauges, that were running 105 octane, and sadly... even seen a few blow. (spinning 7k rpms on a pushrod motor and well over 10k on an overhead cam will lead to some not so bueano things... and detecting detonation is 1 of them)

as far as plugs in or out.... what ever floats your boat man (no pun)
It is safer plugs in.... i have seen fuel, water, oil... blow the faces off of compression gauges... i have seen junk getting shot out of those plug holes so hard they hit the ceiling. If you want to sit next to a V8 with 8 little air compressors aiming at your face thats fine by me.... just don't do it next to my tool box....

 

[Don S]

Re: Compression - can it go back together

We are not talking race engine, or airplane engines here. We are talking boat engines and real world outcomes.

you say 180 is good... i say it's to high.... (to high for a boat motor)...

Most here go by what the engine manufacture says, not what you think.

 

[JustJason]

Re: Compression - can it go back together

fair enough don... but what exactly does the manufactorer say????

I know and agree that merc clearly say's 100psi is a minimum.

I know and agree that merc lists 180psi (on the 4.3 in question here) as "compression pressure" in manuals 7 and 18, but not in later manuals.

Can you clearly say this number of 180psi is the minimum, the average, or the maximum???
And if you can answer that... can you back it up with documentation???

if you can i'll totally shut my pie hole and go on my merry way

In your eye's, Don, what would be the maximum on the average boat motor?

 

[Don S]

Re: Compression - can it go back together

I would read it as a TYPICAL reading, not Min, or Max. what it was designed to be, no I don't have documentation for it, the engineers don't ask my permission for anything, and Merc nor Volvo, nor OMC published answers to all those type questions.
I can only give you "REAL WORLD" answers for sure.
When we do warranty work on engines, it's common to do compression checks as part of the BASICS in troubleshooting. Especially on MPI engines.

Meanwhile back to the original post. Notice he had several real high and some in a lower range. Since we have no knowledge of this engine other that what he posted (notice the 195 dropped to 150 something on the second try) that something is creating false compression readings. On boats, it's common for water to get into the engine, rather thru the exhaust, the carb, leaky risers/manifolds or whatever. It's common and is likely a problem
None should have been 195 but if he had said all his compression readings were 160 to 180, it wouldn't have even been a question.
Every boat mechanic really need to use some common sense on a lot of this stuff, he probably could care less about a compression ratio at this point, and your chart seems mostly meaningless to me for the simple fact that I know his engine should have ~180 compression if perfect, and I also know it doesn't have a 12.5:1 compression ration.

 

[JustJason]

Re: Compression - can it go back together

I would read it as a TYPICAL reading, not Min, or Max. what it was designed to be, no I don't have documentation for it, the engineers don't ask my permission for anything, and Merc nor Volvo, nor OMC published answers to all those type questions.

So then... you cannot clearly say what the manufactorer says.. only how YOU read things....

I'm not trying to start a flame war here... and it does make for good discussion.... usually everyone is argueing about the minimums right....

I don't know how your doing your tests.... or what gauge your using.... but on every engine i've done... decent compression psi for me has always been 120-140ish... at cranking rpm with the plugs in (keeping the cranking rpms down...(and probably the overall number too) ) and the throttle wide open. (although.. i have noticed that even throttle closed usually makes a difference of less than 5 or so psi.. and usually only requires an extra turn or 2 of the motor to reach its max psi) It fits the charts i have... and everything i've been taught from both schooling and from working alongside master techs.

I'm not a master... as mercury does it (or did it, i dunno if it has changed) they require 75 credits in 1 of the 3, outboards, inboards, or sterndrives.. through mercury university plus 5 years shop time to become a "master" in 1 of those 3 areas.
I have 25 credits in each, OB, IB, I/O. 150+ clock hours in merc university alone. A degree from MMI (monkey school..... wasn't awefull but can't say i'd recomend it) Certificates from east coast aero tech in powerplants and avionics... Certificates from lycoming. I've been doing this stuff for over 10 years. (not just boats, but everything) and everytime i get a compression check... my numbers always come back just like i've said (unless of course there is a problem) and whenever i've had one up over 160's+.. it's had problems... and decarbing usually brings my numbers down to where they usually go....
Who knows... maybe the air up in washington state is a little cooler.. and a little denser.. and you get ##'s around 180...
But you do have to agree with 1 thing... there is a number that is "just to high" whatever that is for you... is up to you...

 

[chaparral285]

Re: Compression - can it go back together

Hmm. . .wow alot of info to digest. Seems that all agree my engine can go back together. It also seems that it wouldn't hurt for me to use some combustion cleaner when it is back together. As for the information on my original post, sorry if I was lacking detail, I did remove all the plugs and cranked the engine for as long as it seemed to need to clear the cylinders. As I checked the guage and released pressure, in 1 or two cylinders water did come out of the guage. In that case the test was repeated for that cylinder to be as accurate as I could. The numbers I posted were not for consecutive tests but for each cylinder and they were pretty consistant throughout the testing process. Also, I have a volvo fuel injected 5.7. I did disconnect the power to distributor and disconnected the fuel pump - to be safe. What I did not do was open the throttle. Should I repeat the tests with throttle open?

Again, I want to thank all who have responded.

 

[JustJason]

Re: Compression - can it go back together

throttle plates??? well...that's another issue for debate...
if you knowingly got water in your gauge though i would let the gauge dry out for a while... (as even droplets in the gauge is going to through off the readings)
either way... i feel that your readings are on the high side, and a decarb certainly won't hurt.
If your confident that you've found the source of the leak. then by all means put it back together.
run it for a bit, do the decarb, run it for a bit more. When you get back to port pull the plugs and let them sit overnight. check them in the morning for signs of rust. then redo your compression check just to double check. Plugs in or out, throttle plates open or closed... i think you've gotten both sides of the arguement by now....
the most important things are that they have a minimum of 100psi.. that the lowest is no lower than 70% of the highest... and that there are no more signs of water.... and when your all done... double check your oil.

 

[John_S]

Re: Compression - can it go back together

I think the main disagreement here is the psi numbers for a "normal" reading. I found this:

Calculating maximum pressure from the compression ratio:

First you need to determine the compression ratio of your engine do this by reference to the workshop manual. Then you need to calculate what the maximum pressure within the combustion chamber should be, to do this multiply the compression ratio by 1.38 to work in Bar, alternatively multiply the compression ratio by 20 if you want to work in psi.


So, a typical vortec static ratio of 9.4:1 would be 188psi.

But, you can never measure the static ratio. You are measuring the dynamic ratio, which includes the cam timing. So depending on when the intake valve closes, you will have different psi reading for the same cr. I had worked the dynamic compression ratio out, using the stock merc roller cam, and if memory serves me, around 8:1. That would calc to 160psi, which is pretty close to the old merc number of 150psi.

Now, given inacuracies of doing the test, altitudes, many of us using "value" gauges, etc, readings of +- 15% or more would not surprise me.


I also found this, which supports what Jason was saying:

When the compression is found to be too high in one or
more cylinders, this would be an indication of excessive carbon
buildup in the engine. It can only be corrected by performing a
chemical de-carbonizing process on the engine or by removing
the cylinder head(s) and physically removing the carbon that is
attached to the cylinder portion of the head(s) and on top of the
pistons.

http://www.risaa.org/newsletter/0506/0506_12.pdf
http://www.risaa.org/newsletter/0506/0506_40.pdf


Hey, the net is a great place, you can find supporting evidence for whatever side of the arguement you are on.

 

[180shabah]

Re: Compression - can it go back together

WOW, what a ride!!! Started with an issue, turned into a fight(almost), then into some serious education.

I'm exhausted

 

[JustJason]

Re: Compression - can it go back together

hehe... nahhhh... no fights... just heated debate!!!
I do get PO'd though when someone debates me without documentation... i can usually find my documentation for 99% of the crap that spews out of my mouth...

 

[John_S]

Re: Compression - can it go back together

"1. 14.6psi (or 14.7 depending on who you like more, boyle or bernoulli) is going into the cylinder......
2. if 175 is coming out......
3. then do the math... 175/14.6=11.98.... or 12 to 1.... "

Jason, I think this is where you got into trouble. You are dividing by 1 atmosphere, but the 1 atmosphere reads zero on the gauge. 14.7psi is above 1 atmosphere. or look at this way, you did a compression reading and got 14.7psi. Your math says that would be a 1:1 compression ratio. That would have to be incorrect because it had to be compressed to go from zero to 14.7psi reading.

As far as the 20 X CR, the web article did not provide the background math. That is why I applied to an example to see if it was more reasonable.

 

[wire2]

Re: Compression - can it go back together

>>throttle plates??? well...that's another issue for debate... <<

Debate? It's just simple physics.

With throttle plates closed, you'll get a partial vacuum in the intake manifold while cranking. When a piston draws air from it, there's less than 14.7 going into the cylinder. And the compression reading will be low.

In fact, even with the plates fully open, there's still some flow restriction through the carb, intake passage and valve, making 14.7 an erroneous number to work with.

So yes, The plates should be open for a more accurate compression test.

 

[Robj]

Re: Compression - can it go back together

This is great stuff. Jason that table you posted earlier is correct, just simple gas law. However where I think the problem lies may be with the assumption that the cylinder pressure is at atmospheric the moment the intake valve closes and the compression cycle starts. When the intake opens, there is a negative pressure in the cylinder, which causes air to enter it. My guess is that when it closes, there is still negative pressure in the cylinder, something less than 14.7 psia or 1 atmosphere, maybe like 13 psia, or something like that. If you look it at that way, then the compression ratios and pressures make more sense. Because I agree, multiplying the ratio by 14.7 seems to give higher pressures that do not really make sense to me. Now this 20 number I believe takes into account that the cylinder pressure has not yet reached atmospheric when the valve closes. This is where cam geometry comes into play, what is the designed cylinder pressure at the moment the intake valve closes? Another consideration, is how much air leaks past the piston as the air is compressed? I realize that this is a small amount, but would assume there to be some leakage.

Does this sound reasonable??

Have a great day,

Rob.

 

[Robj]

Re: Compression - can it go back together

Another thought, been thinking about this. Very interesting.

When the intake closes, what is the pressure in the cylinder? I think it would be the same as manifold pressure, which we know is negative. When the intake closes, then it is compressed. So I believe that if you measure the absolute manifold pressure, multiply it by the ratio to get compression pressure.

Does that sound reasonable?? Does to me.

Have a great day,

Rob.

 

[John_S]

Re: Compression - can it go back together

I think the way Jason was performing the compression tests lead to lower numbers than what many of the marine mechanics here have stated they have seen on a regular basis. With throttle plate closed, there could be still a partial vacuum before the valve closes. Also, since he was doing it with the plugs in, the speed of the engine turning is slower. Since there is leakage, the speed will have an effect on the reading.