compression loss due to hose on gauge

[The red boat]

I’ve been fascinated reading on here about how the hose length and location of the check valve on a compression gauge can affect the compression readings.
The smaller the motor the more effect it will have due to the size of the combustion chamber. Some very informative posts about it here.

Has anyone come up with a rule of thumb for various motor sizes that one could use to adjust the readings they receive from a standard gauge with a hose and valve near the gauge?
For instance, I have a compression gauge just like this with the valve near the gauge. It is newish and works well on other larger motors.

If I use it on my RD-17 25hp Johnson, is there a number I would add to my readings to get a more accurate psi number?
I’d rather avoid the cost of buying a $50 Mercury gauge that screws directly into the head.

[Mumbles]

The problem has more to do with where the check valve is located, not the size or length of the hose.

A poorly designed compression tester has the check valve near the gauge while it should be right at the fitting which screws into the motor. A better quality tester with the a long hose and check valve near the fitting will give an accurate reading but it will take a few more pulls than using a gauge with a shorter hose. A poor quality gauge with the check valve near the gauge will never give an accurate reading no matter what length of hose is being used as all the compressed volume of air in the hose is lost with each cycle. A smaller bore size has nothing to do with the readings except for taking more pulls to get the final reading as it has to bring all of the air in the hose up to pressure a little bit at a time

[The red boat]

I just meant that the smaller combustion chambers are more sensitive to the air in the hose as compared to something like a Ford 427. The amount of volume in the hose is miniscule compared to the combustion chamber of a much larger engine but plays a bigger role in a smaller chamber like my outboard.

I understand and agree with the check valve location and its role in the process. I was just wondering if there was a rule-of-thumb factor when using the gauge with a valve near the dial that I could apply to my readings to get a more accurate understanding of my psi.
i.e. – if my gauge with improper check valve is reading 90 psi, can I add 5 or 10 to that to get a reading that more closely resembles a gauge that screws directly into the head (like the $50 mercury one).

thanks for the reply as always!

[Mumbles]

You would need a physics scientist to figure that one out! All related to the volume of air in the hose between the check valve and the screw in adapter.

[billw]

I’m just curious…Who made/sold this poorly designed gauge? If it’s from the Chinese Tool Store, maybe it can serve as a warning to others who would buy one.

Long live American manufacturing!

[crosbyman]

gents, a defective meter I understand ….but would a few pounds variance be true cause for concern ?

Joining AOMCI has priviledges 🙂

[modhydro]

Even with the hose discrepancy, a multi cylinder engine can still be diagnosed with it being a comparitor and not absolute measurement. So long as all cylinders measure roughly the same, you can be confident in the condition. You will have to have a basic understanding of the ballpark that engine should pull though. Is it a 140psi engine or 80? If you know a rough target for a single cylinder, the same thing holds true.

With the testers with the check valve at the cylinder, how accurate is the gauge itself? Lots of variables and things that could easily get a guy sidetracked for sure.

[Tubs]

crosbyman wrote:

gents, a defective meter I understand ….but would a few pounds variance be true cause for concern ?

A "Boathouse Repair" is one that done without having tools or the skills to do it properly.

[The red boat]

my gauge works just fine for 99% of my applications.
that wasn’t the problem.
I’m not experiencing unusual or low compression either.
both cylinders are showing almost identical compression at close to 100 psi.

I was just curious about the air in the hose and how much of an effect it would have when measuring compression on a small outboard.
this was based on many, many other posts on AOMCI discussing the very same thing.
I wondered if there was a factor or rule-of-thumb that would apply to a gauge that had the valve next to the gauge vs. one that had the valve at the other end of the hose.

doesn’t sound like there is or would be a consensus on such a factor.
the only way to get such a number would be to buy the type of gauge that screws directly into the block and compare the numbers with the other gauge.
i was trying to avoid buying another gauge if someone had a ballpark number that i could apply to my original readings.

seems that answer is nope.

thanks for all the replies.

[frankr]

I’d say your conclusion is correct. Compare your gauge with one you trust.

You say that you have read many posts on the subject. You probably have read one where I testified that in an earlier life I would sit down and test the opening pressures of 100’s of those Schrader-type valves at a time. I can testify that they vary all over the place. So we can assume there are going to be variances between compression gauges even within same models and types. What really matters is what to expect from YOUR gauge. Other than that, as earlier said, it would take a physics graduate to come up with the answer you are seeking.

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