Compression measurement

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WI-790R

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As others have said before, any air volume below the check valve (in a compression tester), beyond what a spark plug has, will lower the compression reading. I’ve been playing in the shop, using spark plug bases to make my own with the schrader valve positioned near the end of the plug base (like others have done before me). And like has been said before, the spring tension in the valve introduces a measurement error also. The special low pressure versions are used in compression testers to minimize that error contribution.

While waiting for some new valves to arrive, I experimented with a pressure transducer. I had a few saved from a project at work years ago, Honeywell 40C250G. These are signal conditioned/temperature compensated to output 16mV/psi, offset 0.5v at 0psi. I epoxied a small rubber tube in a plug base, connecting to the port on the pressure sensor. Used an oscilloscope to capture the voltage waveform as I pulled the recoil. This approach eliminates the need for a check valve, since the air volume being “added” to the combustion chamber is no bigger than what the spark plug does. In this case it’s a bit less than the spark plug, so my reading is a little higher than what it should be for that reason.

I believe the voltage peaks shown on the oscilloscope (representing pressure) are higher as my arm accelerates the crankshaft. The highest reading I got equated to 188PSI. The sensor’s datasheet shows about a +/- 4psi possible error at that operating region. I have only about 1 1/2 tanks fuel on these new Caber rings at this point. Do you think this effect will lessen as I get more time on the saw?

I need to find some .050“ solder to better measure squish. I have electronic type solder too small, and plumbing solder way big (and hard). I did a base gasket delete but did no machining on the cylinder. The piston is stock (no pop-up other than the slight dome). What do other 076 Super’s measure for compression?68D1279C-3B88-4E7C-9F24-398532291863.jpegD1AD7F89-53CF-4A43-B0EC-DC7CEBDF793F.jpegBA658715-B332-4619-AEC5-371858DB431D.jpeg

Thanks for reading, and tell me what you think.
 
The highest reading I got equated to 188PSI. The sensor’s datasheet shows about a +/- 4psi possible error at that operating region. I have only about 1 1/2 tanks fuel on these new Caber rings at this point.
Do you think this effect will lessen as I get more time on the saw?

I think that the millivolt/pressure spikes will increase as the rings break-in, by running 7-8 tanks of fuel through it. Check it again after that, just to compare.

That’s the same static pressure method as used by an automotive in-cylinder pressure tester, like the PICO scopes.

It’s not really linear in comparison to a pressure gauge, but it works great to compare to other cylinders in a multiple cylinder engine.

An average 076 gauge type compression test is 150 psig, so your highest 180+ psia at sea level on a base gasket deleted saw makes sense.
 
hotshot,
Thanks for your reply. The low pressure schrader valves arrived today so I tested using the plug base I made connected to this 200psi tire pressure gage purchased from Amazon:

JACO ElitePro Digital Tire Pressure Gauge - Professional Accuracy - 200 PSI​




It took multiple pulls to pressurize all the volume in that hose, but it reached an indicated 155psi. The new valve seems to seal quite well.

I twisted up a bunch of small diameter solder and used that to measure squish at 0.027”.

Like you mentioned, will be fun to remeasure again after more hrs on the saw.
 
Really interesting way to check compression. I wonder what the operating pressure of the crankcase is in these type of saws. Just because you pressure test them less than 15 psi and vacuum so it has to run very low pressure. I always wondered what happens to blowby of the rings.
 
also you gotta be careful: sensor could have inertia, reading might be false peak. only easy way to check taht would be hook a compressor line up to the guage with a ball valve, something you can open super fast. set the source to the highest it goes, 174psi is best. slap the valve open and see if the pressure shoots to 175 or if it overshoots, which is clearly impossible, and adjust your interpretation of the trace based on that.

also i might copy your setup, depending on price tags lol. I hate my gauge currently.
 
also you gotta be careful: sensor could have inertia, reading might be false peak.
At one time I had a more complete datasheet for the pressure sensor, that gave some information regarding the response of the output. But searching now I only find this: https://www.mouser.com/datasheet/2/187/c15_31-1149922.pdf
which has no such specifications.

I was hopeful that given the relatively slow sinusoid (with me pulling the recoil), that the response would be void of overshoot. But your idea to test that is good. I’m mostly interested to see if compression measures higher once I use the saw for a few more tanks of fuel.

It’s fun playing with this stuff!
 

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