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Compression tester

Started by Guydreads, August 26, 2022, 11:24:09 AM

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Guydreads

Any recommendations on a good compression tester that isn't 500 bucks lol? Thanks

DHansen

If price is the main concern, look for a clean, good condition used tester.  Plenty of them setting idle in tool boxes and garages.  I picked up a MAC brand set for $50.00. It looked like new and in a plastic case that kept it clean.  Cal-Van also makes a good one.  Snap-on is good, but people tend to think there worth more, and they are not!  Napa had a Belkamp brand or New Britain that were good also.  All better than a new one made over seas.

Southside

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Guydreads

Lol that's funny. lol Thanks, probably the best idea to get a used one. I'll keep an eye out. I've had a Chinese tester, but it has failed (Why am I surprised?) I'm taking as big a stand as possible to get away from Made in China.... I got a Farmertec 52mm kit for my Jonsered. It runs great, but I really don't want to support practical (if not) slave labor. I'll be picking up something made elsewhere. 

Real1shepherd

I read a thread once where they said having a compression release close to the gauge or further down towards the spark plug adapter makes a difference in 2 cycles readings. I can't find the thread now or understand the logic in that.

Kevin


rusticretreater

It has to do with accumulated pressure.  As the piston comes up and pushes the compressed air out of the cylinder through the spark plug hole, it then has to push that air through the hose, the release valve and into the gauge pressure area.  It may seem minor, but the length of the compression tester hose makes a big difference on how this happens.

The piston can't travel along the hose to keep the pressure at its highest level.  So as the compressed air/gas travels along the hose, it loses some pressure due to expansion.  The longer the hose, the more pressure loss.  This means less pressure to push past the relief valve near a gauge.  When the engine cycles to exhaust/intake, the hose loses its pressure as the vacuum pulls the air/gas back into the cylinder. So the pressure in the entire hose must be reestablished each time the engine cycles and the pressure at the relief valve near the gauge is never the highest pressure produced.

When the valve is nearer the spark plug, a higher pressure exists in the short section of hose and is pushed past the relief valve.  Yes, it loses pressure on the other side but it is not evacuated when the engine cycles to intake.  Over several rotations of the engine, the pressure continues to build in the area past the relief valve to give you the highest reading.

All of this is further affected by the way a two cycle engine operates in comparison to a four cycle engine.  the power and exhaust strokes occur together in a two cycle engine, so any pressure that remains in the compression tester hose when the engine cycles to the power stroke is lost when the exhaust port is exposed near the end of the stroke.

The effect is pronounced in smaller two stroke engines than in gasoline four stroke engines as the latter are usually much larger in size and can more easily fill the tester hose without a larger drop in pressure.
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DHansen

Excellent response rusticretreater.  

Guydreads

Very educational response, glad to learn about that. Thanks for taking the time. Unfortunately, I don't think that it's going to cause a disparity as great as a perfectly running saw being at 25 psi. lol

DHansen

I have been mislead by inaccurate compression test results.  I would prefer no information vs. the wrong information.  The smaller the displacement, the more critical the proper tool and procedure becomes.  Problems I have seen with compression testers has been hard brittle hose, leaking schrader core, missing schrader core and needle rubbing on gauge face from being dropped.  The more you use the tool, the more faith you put into the results.

Spike60

Wow, rustic gets extra points for that in depth post!

Easy for me to scource, so I use the Husky gauges. Gotta have one for the small plug hole on the 550's and 562's. Both have fairly short hoses as they are intended for chainsaw work. Not sure how the cost compares to other stuff.
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Tacotodd

@Spike60 can you give us a selling price on the gauge that you speak of?
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Al_Smith

An example with two sets .The small dial gauge is not very good because of the dial size and the fact the  

 Schrader valve is in the end of the hose .The one in the case is accurate .It's an import of course and not too costly .

Al_Smith

I might interject a thought on this .A comp test is under static conditions and is only an indicator and not a rule of thumb .Under dynamic or running conditions the actual pressure will be higher .The only two I'm certain about is Stihl and Partner which give a trapped comp ratio at 9.5 to 1.While some would like to see 150-160 PSI Stihl says anything above 100 is good to go .It' s right in the book .

Guydreads

@Al_Smith That's exactly what I have, but in blue. It reads 25 psi no matter what saw I put it on. I don't know what's going on with it.

YellowHammer

You should do a quick calibration on it.  You ought to connect it to a known pressure source, such as your air compressor or similar and see what it reads.  If your air compressor is at 100psi and your gauge reads 25psi, then it's time to do some detective work and see if the hose is plugged, there is an obstruction you are not seeing (Schrader or check valve) or if the pressure gauge itself is faulty.

If you don't have trust in your measuring devices, then everything you measure with it is just a guess.  
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Al_Smith

Could be the check valve below the dial or the dial itself. Mine so far works fine .I've got what I think is a stock Husqvarna 2100 that blows 165 PSI,two Partner P-100s of recent rebuilds are at 135-140 and are improving as they get more run time .A modified Stihl 038 magnum ,my super saw is right at 195 PSI
Although I've never had no reason to do it I'd imagine by bypassing the check valve and using an air compressor  the dial could be verified . .

DHansen

I always thought that a running compression test reading was normally lower than static compression test readings.  And not sure how one could perform a running compression test on a one cylinder engine.

Al_Smith

Actually I don't either so it might be theoretical .It was either MacDizzy or A Graham Bell who mentioned it .According to one or both they mentioned ring expansion played into the situation .Then again with ignition it could be very high and I've heard as much as 800 PSI .Weather that's true or not really don't know .
Thinking out loud it might be able to be done through the decomp hole but not with a 300 PSI gauge .On that I've got a DIY high pressure hand pump using an oxygen gauge from a torch set I used to set the pop off pressure on diesel fuel injectors .The cracking pressure on my old D4 Cats is 1550 PSI .

Guydreads

That is true. The dial works perfect. I checked it on my compressor, reads 100 psi no leaks. 

Guydreads

What I've found is that sometimes it reads well, then seems to leak out as I continue to pull it over. Something along those lines. When I first got it, it read like 110PSI. Then I got one of my big saws to read 150. Now it's reading 25. I think I need a new hose???

DHansen

When I have done static and running compression testing on Gasoline 3,4,6 and 8 cylinders running compression test results were always less than static pressure readings.   And a good way to find problems with flow through the reeds or valves.  And static was/is always with throttle open and choke open to promote the best air flow.  Exhaust back pressure can also affect the results. 

DHansen

Guydreads, is there a valve core in the hose that may be sticking at times or may have a bent stem on the core?  Look at the end of the gauge and at end of the hose to see where the valve is placed.  Easy to change, and inexpensive,  and see if the results are affected by a new valve core.

Guydreads

I'll try that DHansen... The valve core is at the cylinder end, I have suspected it. I'll see what I can do.

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