Compression Test Results & Issue about Gauge Hose Length

[tpenfield]

I did my annual compression test on my boat. This is the first test that I have done, . . . the marine surveyor did the initial compression test last year. Here are the results below . . Blue line is 2012 pre-purchase survey . . Pink is my recent test. I suspect that there are some differences in the surveyors gauge versus my gauge, but the patterns are consistent year-to-year.



As was the case last year, the PORT engine is a bit 'stronger' than the STARBOARD engine, based on the higher compression numbers for the port engine. I also noticed a slight difference in the appearance of the spark plugs (New AC Delco MR43T) . . . the PORT engine plugs looked perfect. . . the STARBOARD engine plugs had a bit more carbon, but not bad, just different.

The PORT engine looks like it had a valve job somewhere along its life (heads are un-painted), whereas the STARBOARD engine appears to be original. So, that would explain the difference in about 4 psi between the engines.

Compression Gauge Effects

I also did some calculations of the effect that the length of hose for the compression test gauge has on the readings. Since the check valve is up at the gauge (and not at the base of the hose) the hose acts as part of the combustion chamber volume, pressurizing and de-pressurizing with each cycle of the engine. I did some measuring and it looks like the hose is 3/16" ID and 21" long. In total that adds 9 CC (cubic centimeters) to the combustion chamber, which is approx. 118 CC to begin with. . . so the hose makes it act like a 127 CC chamber. The net effect of this particular hose would be to reduce 150 psi in a 118 CC camber to 139 psi in a 127 CC chamber. So 11 psi reduction due to the hose. I am thinking that I could either get a gauge with the check valve near the 'plug' fitting or one that has a smaller dia hose and a shorter hose.

Maybe I can re-configure my existing gauge & hose to move the check valve close to the plug fitting, that way, it will not act as part of the combustion chamber.

 

[mr 88]

Re: Compression Test Results & Issue about Gauge Hose Length

No math genius here, but your compression gauge has a one way check valve and it would only be pressurized,never de-pressurized until you hit the relief button.So in my opinion based on that fact I would say that your gauge is reading the correct compression. I have never seen a mechanic/engine builder or race tech at a track pull out a magic compression gauge that inflates your compression reading,they are all basically made the same way.Are some mote accurate than others ? Yep and unless your numbers are marginal it is not a big deal for 99% of the people. I suspect that is where the majority of your compression loss is,different gauges will give different uniform results .

 

[tpenfield]

Re: Compression Test Results & Issue about Gauge Hose Length

No math genius here, but your compression gauge has a one way check valve and it would only be pressurized,never de-pressurized until you hit the relief button.So in my opinion based on that fact I would say that your gauge is reading the correct compression. I have never seen a mechanic/engine builder or race tech at a track pull out a magic compression gauge that inflates your compression reading,they are all basically made the same way.Are some mote accurate than others ? Yep and unless your numbers are marginal it is not a big deal for 99% of the people. I suspect that is where the majority of your compression loss in,different gauges will give different uniform results .

Boyle's law of chemistry . . .

The problem is that the check valve is up at the gauge, so there is 21" of hose that can back down into the cylinder as the compression stroke passes into the power stroke and then the exhaust stroke. This has the net effect of making the combustion chamber bigger and reducing the compression, psi reading, etc.

To your point, most folks don't worry about it, as it is the consistency among the cylinders that is of greater concern. I did the math (and Boyle's law) just to get an idea of how much the psi is reduced by the hose. Seems like about 10psi or about 7% lower.

I think that I will re-configure the hose apparatus to get the check valve as close to the cylinder as possible. . . Then compare the results. It would be interesting to see if the measured difference agrees with the math predictions.

 

[mr 88]

Re: Compression Test Results & Issue about Gauge Hose Length

Snap On Tools compression gauge has the check valve at the working end of the adapter that screws into the cylinder.Relief valve is by the gauge,not cheap but that is what used by the builders.

 

[tpenfield]

Re: Compression Test Results & Issue about Gauge Hose Length

Yup, probably what I should get . . . the Span-on brand.

I just checked the 'check valve' on my test set, and it seems to be less than perfect. So, I probably should get a new gauge anyway. The tester that I have has 2 pieces with a quick disconnect, so you can easily thread the fitting part into the cylinder head then connect the gauge.

I since I have a couple of different lengths of adapter hose, I did test both the short one and the long one on the lawn mower (since the boat is all buttoned up. With the short adapter hose, I got a compression of 110 psi. . . With the longer hose, I got 75 psi. So, ideally, the closer to the the check valve is to the cylinder, the more true the reading will be.

 

[tpenfield]

Re: Compression Test Results & Issue about Gauge Hose Length

I think I found a compression test gauge that has its check valve near the cylinder fitting and does not cost $200

ATD-5659 - Universal Compression Tester - ATD Tools, Inc.

So, I may try to get this one and see how it compares to my old & tired tester.

 

[Tail_Gunner]

Re: Compression Test Results & Issue about Gauge Hose Length

Why not a leak down test? I am about to evaluate a old 454 and have decided a leak down was the better way to go?

 

[tpenfield]

Re: Compression Test Results & Issue about Gauge Hose Length

Yup, I have the OTC5609 leak tester. I did a couple of the outside facing cylinders just for kicks. A couple of them were in the 5% leak range and I found 1 in the 12% range. (corresponding to the lower compression reading)

The problem I had is that I could not get the hose to thread on the inside facing cylinder rows, as there is only about 2" of space between the engines. I had 2 adapter hoses . . . one was to long. . . so getting it to thread was problematic and the other was too short, so connecting it to the leak tester was impossible :facepalm:

Hopefully, with the new gauge and its assorted attachments, I'll have enough 'stuff' to do both a compression test and a leak test.

Also, since I have a 'full' closed cooling system on the engines, I plan on doing a cooling system pressure test. . . just to see (hear) if there are any issues with the cooling system leaking into places where it shouldn't (like the manifolds/risers, etc). I am still trying to figure out how best to do a cooling system pressure test without buying the pricey "pressure test heat exchanger cap" I saw a YT video a while back, where someone used the valve stem area of a bicycle tube to create a seal around the cap and then pressurize the system. Maybe that sort of thing would work . . . :noidea:

 

[aerobat]

Re: Compression Test Results & Issue about Gauge Hose Length

I think I found a compression test gauge that has its check valve hear the cylinder fitting and does not cost $200

ATD-5659 - Universal Compression Tester - ATD Tools, Inc.

So, I may try to get this one and see how it compares to my old & tired tester.

compression numbers are for findng out all cylinders work in an even way so you have not a gasket or valve or ring nightmare but the engine is ready for the next trip !


enjoy whatever you favour...

 

[Maclin]

Re: Compression Test Results & Issue about Gauge Hose Length

If the hose is soft and expands then the true compression# may be off some. I used to help my Grandpa test engines, he liked old pickups and was always finding one to mess with. This was back in the 60's. He had a comp gauge, and it was old back then. It just had the hard rubber poke in the hole type "connector" and he or I would just jam it in while the other hit the starter. The gauge held the pressure reading until we reset it with the little relief button. One engine was blowing him out of the hole pretty good so he had me get on gauge duty as I was getting older and stronger by this time, and he was going the other direction. I could not hold it in either! So we figured that one was a keeper.

 

[jerryjerry05]

Re: Compression Test Results & Issue about Gauge Hose Length

The results are what your looking for.
Any difference in #s shows there is a problem??
1-4# between gauges is probably the gauge.
Just make sure the gauge is a GOOD one.
I paid $160 back in 88 for my set.

 

[tpenfield]

Re: Compression Test Results & Issue about Gauge Hose Length

I got the new gauge yesterday, So I ran another test today. 2-3 psi higher across the board, so those are the numbers.

I also did a cylinder Leak test and there are a few cylinders with some higher amounts of leak. 800 hours on the engine, so probably not surprising.

Cylinder Leak

1 . . . . 10 %
3 . . . .. 6 %
5 . . . . 13 %
7 . . . . 23 %

2 . . . . 24 %
4 . . . .. 8 %
6 . . . . 31 %
8 . . . .. 2 %

So, note to self . . . maybe a valve job on this engine over the next few years.