Compression tester ~ does hose length matter?

[colbyt]

I've read opinions both ways and let's keep this thread family friendly even though I started it with the tile.


When one is checking compression on a small twin cylinder engine ( 4 HP) would the length of the connecting hose make a lot of difference? The check valve is located near the gauge, any pressure in the hose is lost as the piston cycles. It seems to make sense to me that it would.

What say you with more experience?

 

[Frank Acampora]

Re: Compression tester ~ does hose length matter?

Well, Let's say that the inside diameter of the hose is 3/16 inch. The total volume for a one foot length would be .034 cubic inches. let's say that the 4 horse engine is a 2 cylinder 8 cubic inch engine, leaving 4 cubic inches per cylinder. That makes hose volume about 1% of total cylinder volume. If the four horse engine is conservatively compressed at 50 PSI, one percent would be about 1/2 PSI --or less than the accuracy of the gauge.

 

[tpenfield]

Re: Compression tester ~ does hose length matter?

Less is better . . . particularly on a small displacement cylinder. Since the check valve is near the guage, the volume inside the hose will become part of the cylinder volume, effectively reducing the compression.

 

[Hoosmatroos]

Re: Compression tester ~ does hose length matter?

Well, Let's say that the inside diameter of the hose is 3/16 inch. The total volume for a one foot length would be .034 cubic inches. let's say that the 4 horse engine is a 2 cylinder 8 cubic inch engine, leaving 4 cubic inches per cylinder. That makes hose volume about 1% of total cylinder volume. If the four horse engine is conservatively compressed at 50 PSI, one percent would be about 1/2 PSI --or less than the accuracy of the gauge.

Seems right but....in compressed state, lets say compression rate is 1:10, the actual volume of the hose is much more in respect to the volume of the cylinder. So accuracy is much less with small engines and large hoses.

 

[tpenfield]

Re: Compression tester ~ does hose length matter?

Seems right but....in compressed state, lets say compression rate is 1:10, the actual volume of the hose is much more in respect to the volume of the cylinder. So accuracy is much less with small engines and large hoses.

Yes, in the example given, the 4 cubic inches is the change in volume of the cylinder. So, if the compression were 10:1, then the volume at TDC would be 0.44444 Cu. in.

I get 0.33 cubic inches for 1 foot of 3/16" dia. tube (not 0.034) :noidea:

So, the effective compression/combustion chamber with the tube attached is 0.4444 + 0.33 = 0.77444 Cu inches and the resulting compression would be:

0.77444/4.44444 = 1/5.7 or 5.7:1 ratio, whereas the engine is designed for a 10:1 ratio.

It is early on a Sunday . . . so feel free to check my math . . .

 

[tpenfield]

Re: Compression tester ~ does hose length matter?

For a larger engine, like my 454's, it does not make much difference . . .

Assuming 10:1 ratio. . .

cylinder volume BDC = 63.05 cu in
cylinder volume TDC = 6.305 cu in

Adding the "0.34 cu in" from the 12" of hose to the volume at TDC does not make much difference . . .

6.305 + 0.33 = 6.635

6.635 / 63.05 = 1/9.5 or a 9.5:1 compression ratio

 

[gm280]

Re: Compression tester ~ does hose length matter?

I understand all the math and issues with this question, however, since most compression gauges are not that finely graduated/marked and the needle width could easily cover up a few pounds of compression itself, these compression tests are mere generalistic figures then absolutes. The sole reason for doing any compression test is to see IF each cylinder/piston/rings are compressing the volume enough to support proper ignition and fall into a range of acceptability. There is absolutely no way to get finite figures doing such a test. I?d even bet that using three different compression gauges will post three different, but very close, readings as well... You also are looking to see if the multiple cylinder readings are close to each other to balance the power output. So to answer this question...?yes? the length of the hose does play into the final reading but not so much to warrant any true substantial error in the final results...

 

[colbyt]

Re: Compression tester ~ does hose length matter?

I am going to accept that as "not enough to matter" As the compression does not really begin until the intake port is closed I was't sure how to even begin to do the math. Piston size and stroke are a known.

I think I will make a home made leak down tester and see what happens. I currently suspect some stuck rings, have one cylinder soaking now and later today I will soak the other cylinder for 24 hours. Currently rain with freezing rain sleet and snow to look forward to don't think I need to get it started today anyway.

Thanks.

 

[Frank Acampora]

Re: Compression tester ~ does hose length matter?

You guys are making one major error: 10 -1 compression would be 14.7 psi (atmspheric0 plus 10 X 14.7 or 147 plus 14.7=152. NO small engine has this compression ratio because it would be very difficult to pull start it. Most of the smaller horsepower engines use less than 90 PSI.

And, for my figure of .034, I used 3.1416 for pi and then rounded up from 0338 to 034.

But embellishing what I said in my original reply: The gauges we buy to test are accurate probably in the 2-5% range and are usually graduated in 2 PSI increments. Any difference the hose length would make is well below the level of inaccuracy we would achieve between reading errors and gauge error.

And, to test any engine for compression ratio. we think we have a problem: Thus, Compression ratio will either be high or low. We don't care about splitting hairs over a pound.

 

[tpenfield]

Re: Compression tester ~ does hose length matter?

I think the thing would be if you are testing a small engine with a gauge that has a fairly long lead hose, then you should expect your PSI readings to be a bit lower than you might expect. The bigger thing with a compression test is to see consistency in the readings versus any particular reading for all of the reasons mentioned.

It also sounds like the OP is trying to diagnose an issue with the engine, and perhaps can rule out the gauge at this point.

 

[colbyt]

Re: Compression tester ~ does hose length matter?

I think the thing would be if you are testing a small engine with a gauge that has a fairly long lead hose, then you should expect your PSI readings to be a bit lower than you might expect. The bigger thing with a compression test is to see consistency in the readings versus any particular reading for all of the reasons mentioned.

It also sounds like the OP is trying to diagnose an issue with the engine, and perhaps can rule out the gauge at this point.

Right. After replacing the head gasket which was visibly blown between the cylinders, I am getting consistent but low readings on both cylinders.

 

[Texasmark]

Re: Compression tester ~ does hose length matter?

Tell you how it looks from here.

The hose is significantly filled before the needle ever moves. The pressure fitting at the spark plug keeps the hose/meter filled with air during the recycling of the piston so that volume doesn't have to be replaced on each stroke.

The way I was taught and directions I read on using a CT was to cycle the piston till the meter stops moving. Usually that is 3 or 4 piston strokes.

Since the CT hose is reinforced rubber it tolerates pressure and doesn't expand.

The diaphragm in the meter however is designed to expand (uncoil) with an increase in pressure/volume thus moving the needle to the appropriate position.

The initial pulse is the most dramatic as the whole monitoring system has to get charged and then it's just the incremental input variances coming from the cylinder on each cycle until the diaphragm has expanded to correspond to the pressure. When it stops moving, the system is in equilibrium and you have your reading.

So, based upon that, the length of the hose would merely determine the length of time (number of strokes) the piston would have to move up and down till equilibrium was established.

Now, if you were required to do the test with a finite number of strokes of the piston, then yes the total volume would have to be considered.

My 2c,
Mark

 

[minuteman62-64]

Re: Compression tester ~ does hose length matter?

Tell you how it looks from here.

The hose is significantly filled before the needle ever moves. The pressure fitting at the spark plug keeps the hose/meter filled with air during the recycling of the piston so that volume doesn't have to be replaced on each stroke.

The way I was taught and directions I read on using a CT was to cycle the piston till the meter stops moving. Usually that is 3 or 4 piston strokes.

Since the CT hose is reinforced rubber it tolerates pressure and doesn't expand.

The diaphragm in the meter however is designed to expand (uncoil) with an increase in pressure/volume thus moving the needle to the appropriate position.

The initial pulse is the most dramatic as the whole monitoring system has to get charged and then it's just the incremental input variances coming from the cylinder on each cycle until the diaphragm has expanded to correspond to the pressure. When it stops moving, the system is in equilibrium and you have your reading.

So, based upon that, the length of the hose would merely determine the length of time (number of strokes) the piston would have to move up and down till equilibrium was established.

Now, if you were required to do the test with a finite number of strokes of the piston, then yes the total volume would have to be considered.

My 2c,
Mark

That's the way I remember it (based on Physics 101, about 50 years ago).

More important would be having a length of hose that lets you get the gauge in where you want to use it (in some cases, maybe longer would be essential).

 

[tpenfield]

Re: Compression tester ~ does hose length matter?

Tell you how it looks from here.

The hose is significantly filled before the needle ever moves. The pressure fitting at the spark plug keeps the hose/meter filled with air during the recycling of the piston so that volume doesn't have to be replaced on each stroke.

The way I was taught and directions I read on using a CT was to cycle the piston till the meter stops moving. Usually that is 3 or 4 piston strokes.

Since the CT hose is reinforced rubber it tolerates pressure and doesn't expand.

The diaphragm in the meter however is designed to expand (uncoil) with an increase in pressure/volume thus moving the needle to the appropriate position.

The initial pulse is the most dramatic as the whole monitoring system has to get charged and then it's just the incremental input variances coming from the cylinder on each cycle until the diaphragm has expanded to correspond to the pressure. When it stops moving, the system is in equilibrium and you have your reading.

So, based upon that, the length of the hose would merely determine the length of time (number of strokes) the piston would have to move up and down till equilibrium was established.

Now, if you were required to do the test with a finite number of strokes of the piston, then yes the total volume would have to be considered.

My 2c,
Mark

Here is a link that talks about the size (diameter, length, etc.) of the hose on the test gauge impacting the results . . .

Engine FAQ

Perhaps there is a link to a video or something that puts 2 different hose lengths to the test. (I'll keep looking)

Here is another link from an engineering company that talks about the impact of the hose volume and the importance to have the check valve as close to the cylinder fitting as possible. Most gauges have the check valve up at the gauge . . .

http://mullengineering.com/rt/files/2_tech_docs/THE TRUTH ABOUT COMPRESSION TESTING.pdf

Here is another link as well:
http://www.totallyamaha.com/snowmob...e doesnt tell the whole story about rings.htm

 

[WIMUSKY]

Re: Compression tester ~ does hose length matter?

I am going to accept that as "not enough to matter"

Thanks.

That's right. You also won't get a brain cramp with that logic......

 

[gm280]

Re: Compression tester ~ does hose length matter?

I see a few more comments on this over-worked subject now and each displaying their version of their past educational experiences as well. Nothing wrong with that, we all like to try to impress others with our vast knowledge and experiences and I do likewise myself. But that bottom line is a simple (well use to be simple anyway before all this) compression test is nothing more than a mere simple evaluation tool to gauge if your engine is doing a good enough job of supporting ignition and nothing so grandioso other than that... If your readings are within that 10% to 15% range of all the other readings and those readings are enough to support proper ignition, then that’s all you can expect from the test. How close or accurate those readings truly are is of very little concern. I mean you could read a reading of 120 PSI and it could truly be a little more or less and nothing will truly matter. It is when you get huge differences that will show your problem. True compression will differ depending on a lot of things. The temperature, the port timings, ring condition, piston slope, cylinder shape, gasket(s) condition, carbon buildup and so many other things too will affect it as well as how fast you can spin the engine over for the test. Try doing a compression test by pulling a rope, and then use the starter and see the results... Also do a dry cylinder test and then a wet cylinder test. So all a compression tests does is gives you a warm fuzzy result of how the engine is in general. It is worth doing by any means just not an absolute accuracy test...too many variables to consider for absolute dead-on accuracy...

 

[Texasmark]

Re: Compression tester ~ does hose length matter?

First of all I was wrong about the hose remaining filled with air due to the Schrader Valve being at the spark plug. As Mr. Enfield mentioned it is at the manifold where it, the gauge, and the input hose connect. So that pretty much invalidates my primary assumption.

Secondly I am not a professional mariner, nor have I ever been in the business. I am a life long boater who came up through the ranks learning as I went. I have had a ton of boats and have maintained all them myself. My experiences date back into the mid 50's. One can forget a lot in that period of time.

If I am bringing something up from memory I say so. If from the manual I say so. If quoting from how my equipment operates which may be different from yours, I say so. Being a retired professional, crediting your sources is just part of wearing the badge.

The initial intent is to, attempt to, apply one's resources in assisting a poster in solving his problem. Lot of variable there on both sides which make for some interesting conversations.

TV is a total bore. If it's not reruns on the movie channels, it's 5 minutes of programming and 15 minutes of commercials. Whoopie!!

I read books in the winter. Current book is titled: "Wheels for the World", by Mr. Douglas Brinkley. It is a biography of Henry Ford, his accomplishments, the time line in the formation of the industrial revolution, and their impact on the world. Very interesting reading.

I have a small farm I maintain but this is the off season due to the weather. I have all my problems fixed so there is nothing else to do there till spring. Same with boating. Also, it's cold and the lakes are low due to the drought like conditions. Not much fun trying to launch and load in the mud, much less encounter all the obstacles you can run your prop into.

What about the internet.

Watching the news is a total waste of time for the most part. Lies, misrepresentations, half truths, biased opinions, ads that lead you no where and on and on. Whoopee! Rather than talk about things that really matter to us, they ramble on about absolutely nothing that has any significance. I guess reporters get paid for the word count.

So that leaves Forums.

With forums you can keep your typing skills, maintain some kind of mental intelligence, attempt to maintain some of the knowledge you gained over your life, and do a little mingling with folks with some sense.....unlike the coffee shop, and along the way maybe help someone with a problem.

So, here I am and I imagine I will be staying, right or wrong. But one thing is for sure, if I'm wrong, not an opinion, but give out the wrong advice, I'm man enough to acknowledge it.

Happy boating,
Mark

 

[tpenfield]

Re: Compression tester ~ does hose length matter?

I never gave much thought to the compression testers that I have used over the years, knowing that there is probably some inaccuracy in the gauge. However, the question posed by the OP has made me realize that the length of the connecting hose makes a difference; one that I never considered. It is barely noticeable on larger engines, but if it is a smaller engine, then the difference would be more significant.

For the most part, the actual psi reading does not matter, as stated several times, because we are looking for the relative variation among the cylinders. However, each engine is specified with a compression test result that should be achieved in order to be 'in good health'. If your compression test results fall too far short of that number, then it raises some questions. The cylinders could all be within a certain percent of each other that is deemed acceptable, but are they all equally worn out?

I know that the engine in my boat is spec'ed to test at 150 psi. If it the cylinders were to come in at 110-115 psi, then that might be cause for concern and further evaluation, even though the variation among them is quite acceptable. So, the absolute number achieved is important in determining the general health of an engine, just as is the variation among the cylinders.

After considering the question that the OP raises, I think that I will look for a tester with the least amount of connecting hose as possible. If the check valve were located near/at the cylinder thread fitting, that would be even better.

 

[boobie]

Re: Compression tester ~ does hose length matter?

I have four different compression gauges and can test the same cyl on the same mtr and get four different readings. All you use the gauge for is to compare the difference between the cyls.

 

[nwcove]

Re: Compression tester ~ does hose length matter?

....this thread should be in the top secret file....." how to over complicate a simple procedure".