Compression - can it go back together

[chaparral285]

Can someone tell me how much cylinder compression I should have in a 2000 5.7 Volvo GSI?
Thanks,
Harry

 

[rodbolt]

Re: Compression

Re: Compression

while Volvo actually publishs this data I dont know what metcal standards there tester or mine is set to. I have 2 compression testers that read about 8 pounds different at any given time. what I am looking for is less than 15PSI difference from highest to lowest. if there is its on to a leakdown test to determine what is leaking and where is it leaking to.
so dont get hung up on numbers look for the spread, I think Volvo actually says 20%max highest to lowest. I start hunting at 15PSI.

 

[Haut Medoc]

Re: Compression

Re: Compression

Can someone tell me how much cylinder compression I should hace in a 2000 5.7 Volvo GSI?
Thanks,
Harry

Welcome to iboats......
Anything in the 140 + or - range would be good as a general guideline, but as Rodbolt says, the spread is just as important......
Don will probably know the answer off the top of his head....
Anything below 100# is unacceptable, though ....

 

[chaparral285]

Re: Compression

Re: Compression

Thanks for the responses. I did the compression test. Setting - unable to run engine first - starboard side (cylinders 2,4,6,8) of port engine has exhaust manifold removed, port side ( cylinders 1,3,5,7) does not. I did have water in cylinders which is what lead to this process to begine with. All plugs out and crank the engine to remove any water from the 2-4-6-8 cylinders. 4,6,8 seemed to be the only ones with any water. 4,6 had the most.

Results - 1 - 195 / 2 - 152
3 - 178 / 4 - 151
5 - 148 / 6 - 141
7 - 190 / 8 - 195
Any ideas??

 

[chaparral285]

Re: Compression - can it go back together

So. . .if I put this thing back together with new manifolds will it be ok?

 

[Don S]

Re: Compression - can it go back together

If you haven't pulled the old manifolds off yet, remove the raw water pump impeller and start the engine and run it for about 15 seconds without water. Water in the cylinders from bad manifolds can cause high compression readings.
Then make sure your battery is fully charged and do a Compression Test

 

[JustJason]

Re: Compression - can it go back together

Don. That should be a sticky on top of every forum with the amount of times this question gets asked. Can you make it happen?

Chapperal- Your readings are to high. When your getting up over 150-160PSI your getting into what i call "race car" compression ratio's... at 170PSI your ratio is 12:1... way to high for a boat motor... (unless you can find some 105 octane )
If your cylinders are full of water, oil or some other fluid that will boost the readings.
If your all carboned up, that will boost the readings as well.
Cheap (ie~walmart) compression gauges are notorious for just giving bad readings....

I'd pull all the plugs and give it a good spin... to make sure anything in the cylinders is out. Then put all the plugs back (to lower the cranking rpm) and redo the compression test. At this stage in the game i wouldn't worry about a leakdown because you certainly do not have a low compression issue...
If the readings are still high, and you are confident that you've identified the source of the water. Put it all back together and shoot a can of de-carb through it (my personal fave is yamaha's combustion chamber cleaner... about 13 bucks a can i think) Follow the instructions on the can and you might as well run the whole can through.
Let everything cool down. then retest your compression and let us know how you make out.

 

[Robj]

Re: Compression - can it go back together

When I was taught to do a compression test many moons ago, I was told to remove all the plugs. Don's Mercruiser procedure agrees with this. So I would remove all the plugs when doing a test.

Have a great day,

Rob.

 

[newport dave]

Re: Compression - can it go back together

Chapperal- Your readings are to high. When your getting up over 150-160PSI your getting into what i call "race car" compression ratio's... at 170PSI your ratio is 12:1... way to high for a boat motor...

You can't determine an engines compression ratio with a compression gauge. I can tell you from doing hundreds of compression tests, all on stock marine engines, most small block engines will test at between 175 at 195 PSI.

To try to answer the original question, I don't know what Volvo says, but Mercruiser says the lowest cylinder should be no less than 70% of the highest cylinder.

70% of 195 = 136.5

Since the low cylinder is 141, this engine is still servicable.

Dave

 

[rodbolt]

Re: Compression - can it go back together

knowing it had water in a cylinder and with numbers all over the place. I would run a cylinder leakdown and find out what is going on.
water can make a mess.

 

[chaparral285]

Re: Compression - can it go back together

Wow, just saw all the responses and wanted to thank all immediately. Will ponder and go from there. This is a great forum! Thanks again!

 

[JustJason]

Re: Compression - can it go back together

Actually.... !@#$# dave... yes you can.
Just because YOU don't know... doesn't mean that I don't know.
You will soon learn not to contradict me...... If there is a subject that I don't ABSOLUTELY know about... then I won't comment on it.... or I will humbly ask, and not thow my 2 cents out there just try and beat someone down.... but now that you've made me do it i might as well continue...

You can absolutely figure out theoretical compression ratio's... there's a dozen different ways to do it... and 1 is so much easier than you'd think!!
There are other variables, such as cam timing, and variable compression ratio engines, but in a nutshell.... the easiest way to figure it out... is to divide what the gauge says versus what's going into the cylinder.......

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....

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....
so... if your engine is reading 175... its reading that for a reason... and that reason is what you have to find out.... (and it usually comes down to a cheap gauge, something is throwing off your gauge like water or oil... or just plain old carbon buildup)

Most everyday engines run between 7.5/1 to 9.5/1 which should give you psi's in the range of 110 to 140.....
There is always a bit of variance... because this is theoretical compression... and that's why most manufactorers do not publish numbers (as it would just confuse people... people like you dave)

This poor guy, if his gauge is correct, is running a 13.3/1 ratio at 195psi.... which is a one way ticket to denotation city if he keeps running it like that.
HE double hockeystick, my high performance triumph motorcycle is running 12.5/1 at about 180psi with sequential fuel injection, dual knock sensors and a custom mapped ecu and I will tell you... i cannot run 87 or 89 octane... it will throttle up... then knock... and the ecu says no way jose and pulls the timing way back and opens up the injectors.... and my bike doesn't see the 10th of a load that a boat motor sees.....

Educate... don't guestimate.....


oh yeah, i got this back when i was going to east coast aero tech working on lycomings.....

 

[JustJason]

Re: Compression - can it go back together

and haut medoc has it about right.... 140ish is a healthy lil motor

 

[JustJason]

Re: Compression - can it go back together

Imported rob~ pulling all the plugs to do a compression ratio is dangerous... especially in a carbed motor.
First of all... pulling all the plugs will more or less double your cranking rpm.
So... lets say if you pulled the plugs and your cranking rpm is now 1200... (double 600... as the average cranking rpm is 5-600). your carb is pulling all the fuel it normaly would be pulling at 1200 rpm... and with all the plugs out the fuel vapor would be going everywhere....
its a sure way to have a flash fire.... and a 1 way ticket (my term of the day) to the ER. A bilge is a mighty small area... especially when it's full of fuel vapor.

 

[wire2]

Re: Compression - can it go back together

Actually, while it will pull a tiny bit of air/fuel through the carb, most of the displacement volume will be drawn in through the open spark plug holes.

 

[newport dave]

Re: Compression - can it go back together

Imported rob~ pulling all the plugs to do a compression ratio is dangerous... especially in a carbed motor.
First of all... pulling all the plugs will more or less double your cranking rpm.
So... lets say if you pulled the plugs and your cranking rpm is now 1200... (double 600... as the average cranking rpm is 5-600). your carb is pulling all the fuel it normaly would be pulling at 1200 rpm... and with all the plugs out the fuel vapor would be going everywhere....
its a sure way to have a flash fire.... and a 1 way ticket (my term of the day) to the ER. A bilge is a mighty small area... especially when it's full of fuel vapor.

Hey "captain",
You claim to know so much, yet your giving BAD information and your lack of real world experience is painfully obvious.

1) Anybody can tell you that you pull all the plugs for a proper cranking compression test
2) cranking RPM is between 100 to 250 RPM

Have a nice day

Dave

 

[John_S]

Re: Compression - can it go back together

Merc Service Bulliten from 1997: http://www-alt.mercurymarine.com/mnetdata/Service/Cruiser/Bullet/97/mc9725r1.pdf

Yes, all plugs are out.

 

[JustJason]

Re: Compression - can it go back together

What is soooo bad about my advice??????????? that i've even provided documentation for????????
Do you know why mercury says to pull the plugs there bucko????? it's to take the strain off of the starter motor..... it won't help you much if your not building up pressure under normal starting conditions... and plugs out is not normal starting conditions. That... and the fact that fumes will build in the bilge... is why you should do it with the plugs in.
and maybe if you cleaned up your connections... you'd be cranking faster than 100-250rpm... i bet you go through a lot of starters
it's funny how you go out of your way to rebuttle compression testing... but leave alone the fact that i put you in your place when it comes to compression ratio's and PSI....
whose the one giving bad advice here davo~

 

[Don S]

Re: Compression - can it go back together

Do you know why mercury says to pull the plugs there bucko?????

I do,
Because General Motors sent a bulletin to Mercruiser and Volvo stating the way to do the compression test.
If you were to compare boths bulletins on the subject, you will find them almost word for word all the way thru.
They copied what GM said.

You will soon learn not to contradict me......

A little full of yourself there Jason?????

Merc says a lot of things in that service bulletin if you read it.
1 Remove all the spark plugs
2 Open the throttle to wide open
3 Ground the coil wire and/or disable the ignition.

With the plugs out and the throttle wide open, there is not going to be enough vacuum to suck any gas out of the carb.
Sounds to me like that would eliminate the fuel vapors you are speaking of. With the engine hatch open and the blower running, there shouldn't be anything in the bilge to cause an explosion.
If you smell fuel, then find out why.

 

[Bondo]

Re: Compression - can it go back together

Ayuh,.......

I've always done it with the plugs Out,+ Carb Open........
And the #s I've seen mirror the #s Dave reports,.....

Must be I've been working on Racing Engines all these years,.........
I coulda Sworn they were just Boat Motors though,.....................