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?
Thanks for reading, and tell me what you think.
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?
Thanks for reading, and tell me what you think.
