LOW Compression Readings! Need HELP
 

I have a contract for an older boat (info to be released soon) that I had a local mechanic complete a compression and leak down test on. This is the same mechanic that has completed service work on this boat for last 6 or 7 years.

Before everyone freaks out about the extremely low readings (which surprised him, too) here's his explanation. Cam is an Isky 270/280 mega 114 lobe centers lift .543-.565 duration 270-280. He's installed 100's of these cams, but never did a compression test on an engine with this cam. He called Isky and they said the #'s are legit for "cranking compression" and said with the 114 lobe centers will release compression faster than a typical 108 cam. Being that "cranking compression" is much different that normal "running compression".

Port Engine
1-56# 2-53#
3-58# 4-49#
5-60# 6-62#
7-46# 8-48#
Starboard Engine
1-52# 2-54#
3-55# 4-56#
5-48# 6-52#
7-50# 8-54#

Leak down: 16 to 20 percent on both engines.

My question --- Is this acceptable, or is there something extremely wrong here? I also have concern between #5 & #7 and #6 & #8 on the port engine, nearly 30% difference.

Re: LOW Compression Readings! Need HELP
 

The higher the centerline, the less the overlap. The lower centerline the more overlap would occur & cause the low compression. 114 centerline is a higher centerline which should not cause those low numbers!!! The leak down test if its less then 20% seems ok and doesn't really support your compression readings. Have your mechanic do a wet compression check see what the numbers are, maybe his compression gauge is bad!!!

Re: LOW Compression Readings! Need HELP
 

What's a "wet compression check"?

My hope is that he just read the guage wrong, and that it starts reading at 100psi, which would make more sense. With these low readings, I doubt the things would even run. He claims he used a brand new guage and a backup to verify. More info to come tomorrow. This is just not right. :confused: :confused:

Re: LOW Compression Readings! Need HELP
 

Put a "1" in front of all those numbers and you would be right where I would expect them to be with that leakdown number.

Re: LOW Compression Readings! Need HELP
 

Spoke with the mechanic again just minutes ago. They're claiming (both he and Isky) is that the low readings are due to the "cranking compression" factor, and that it is not the same as "running compression". I've never heard of this before, compression is compression, right?

Even asked the question to double-check his guage that it doesn't start at 100psi. He confirmed it starts at 10psi.

Mech is claiming that these engines run "sweet" and "super smooth". We'll see on Saturday when we sea trial.

Re: LOW Compression Readings! Need HELP
 

You can always stick a Blowby Gauge on it and see what it says.

Re: LOW Compression Readings! Need HELP
 

I don't know what to tell ya. You've gotten some excellent feedback from the OSO folks. The compression numbers look low to me too. 'Cranking' compression would be lower because the very low cranking rpm allows compression to bleed off during the overlap period when both valves are partially open. Overlap helps make big power at high rpms. Be careful with terms like 'sweet running' and 'super smooth'. A motor with consistently low compression cylinder to cylinder will run 'smooth' and 'sweet'. It just won't be making the horsepower it should. Any idea how many hours are on these motors?

Re: LOW Compression Readings! Need HELP
 

No way are those #,s acceptable.
However, the leak down test is Good.
Some thing is not adding up with the static test.
A wet test is squirting oil in cylinder to see if it increases.
Indicating Spent rings.

I would use another gauge.
Im betting its the gauge not seating properly or its faulty.
Both engines are low. This is why im suggesting the
gauge.

Good luck
Gerry

Re: LOW Compression Readings! Need HELP
 

Mechanic is claiming less that 200 total hours. Also, claims these engines were originally installed & rigged at Martin Marine. May be time for call to Tres and the gang to see if they have any record of this boat.

Re: LOW Compression Readings! Need HELP
 

Ya, cavey, I hear ya.

Anyone have a compression guage in the Chicago area that may want to meet me in Winthrop Harbor? I'd even treat whoever to a ride.