What is a "good" cylinder pressure?

[smsscuba]

Hi everone,

What should the cylinder PSIG pressure be for a mid 80`s Mariner 2 cylinder 40hp be to be considered acceptable?

Anyone able to shed some light on this?

Cheers

 

[achris]

Re: What is a "good" cylinder pressure?

By the book the cylinder pressures should not vary by more than 10psi. I also like to see pressures, on my gauge, at about 120+. Every gauge will be different, so that's why a specific pressure can't be specified.

Hope this helps.

Chris......

 

[Mark_VTfisherman]

Re: What is a "good" cylinder pressure?

By the book the cylinder pressures should not vary by more than 10psi....

10% is what I have always used.

There are different specs for different motors and what their static PSI should be. Sometimes older motors may not achieve 120psi, but they will be still be in the 90-100psi range and as long as they do not vary over 10%, it is considered ok. That is for example only

 

[sschefer]

Re: What is a "good" cylinder pressure?

PSIG varies widely depending on how they ported the engine. A good rule of thumb is that at 100lbs, it's time to think about a rebuild. This is the point where a two stroke engine will begin to show noticeable signs of poor performance, bad idle and hard starting.

On some engines, compression after breakin should be in the 130-140 lb range and even within 10% on all cylinders. On others with compression relief ports, 110-115 and even within 10% is good.

I suspect that your engine was factory at 135-140 psi but your best bet for confirming this will be to call the Merc tech hotline with the serial number in hand so they can tell you exactly.

 

[Bifflefan]

Re: What is a "good" cylinder pressure?

PSIG varies widely depending on how they ported the engine. A good rule of thumb is that at 100lbs, it's time to think about a rebuild.

If memory serves, 100 is about all that a looper will build when new. Which goes back to the porting.

 

[sschefer]

Re: What is a "good" cylinder pressure?

If memory serves, 100 is about all that a looper will build when new. Which goes back to the porting.

100 before ring seat, 110-115 after break in, (you hope). 10% don't leave you much wiggle room either..

 

[achris]

Re: What is a "good" cylinder pressure?

Guys,

The spec is 10 psi, not 10%....

 

[Texasmark]

Re: What is a "good" cylinder pressure?

Service manual for Merc mid range engines (P/N 90-830234R3, December 1997) Pg, 1C-6 "Compression Check" Para. 5: "Check and record compression of each cylinder. The highest and lowest reading recorded should not differ by more than 15% (see example chart below). A reading below 120 psi might indicate a total engine wear problem."

So what difference does it make? Dunno. May matter to some people, may not to others. May matter to you if your engine is borderline. Meter calibration could very well swamp out any differences between psig and % and so could measurement technique, especially on engines (like the older 4/6 cyl Mercs) where it is hard to get to the top and bottom cylinders. So could worn seals on the instrument. Degree of carbon contamination on rings/grooves also could play a role. Makes for wide interpretation on the part of the owner and the mechanic.

But, I have always used "within 10 psig" until recently when I stumbled upon this. So there you have it.....My 2c and worth every penny you paid for it. Ha.

Mark

 

[sschefer]

Re: What is a "good" cylinder pressure?

Just a personal thing with me, I like doing math.

 

[smsscuba]

Re: What is a "good" cylinder pressure?

Tested both cylinders.I get 138psi for cylinder 1 and 140psi for cylinder 2.Note:Tests were done when engine was cold.Are these results ok?

 

[Texasmark]

Re: What is a "good" cylinder pressure?

Tested both cylinders.I get 138psi for cylinder 1 and 140psi for cylinder 2.Note:Tests were done when engine was cold.Are these results ok?

You bet your bippie...............er ah, yes.

Mark