DIY digital compression tester < $200
05-05-05, 05:48 PM
#26
Originally Posted by FDjunkie
Pele - As you may have guessed, it’s the transducer combined with an inexpensive analog-to-digital converter that makes this really easy. I used a transducer from Grainger that unfortunately is no longer available, but I found a few similar units on the Internet that I think would work, too. [/size][/font]http://www.msiusa.com/download/pdf/english/msp/msp_300.pdf[/color]
http://dkc3.digikey.com/PDF/T051/1454-1455.pdf
(Assuming the 1-5V output, 250 psi one is appropriate, the one without the Packard connector.)
Last edited by moconnor; 05-05-05 at 05:53 PM.
05-05-05, 07:40 PM
#27
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Here's a closeup of the orginal transducer.
I think the ones moconnor has found are very similar in specifications. Digi-Key's part number MSP3251P4A1-ND looks real close except for having a 0~250 PSIG range compared to 0~200 on the Grainger. Might loose just a little bit of resolution but for $85...
I think the ones moconnor has found are very similar in specifications. Digi-Key's part number MSP3251P4A1-ND looks real close except for having a 0~250 PSIG range compared to 0~200 on the Grainger. Might loose just a little bit of resolution but for $85...
05-06-05, 09:56 PM
#28
I have also found a 1/4" NPT to spark plug thread adapter that will make it possible to attach the transducer to the block without using a hose. It is KD Tools Air Hose Fitting Set (part # 901) and costs about $4. The air tools section of an auto store may have an equivalent part. The 1/4" NPT transducer in the DigiKey catalogue will be needed if this is used (part #MSP3251P3-ND, $85). I am not sure if there will be clearance isssues if this adapter is used. I will try myself in the next few weeks.
05-07-05, 08:35 AM
#29
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i have been measuring pre turbo exhaust back pressure w a honeywell pressure sensor thru my datalogit.
the thread got me to thinking that w another sensor ( the honeywell is 0 to 100 psi) i could just as easily do a compression test.
unlike the old style mazda compression tester the digital sensors sample every X per second so there is a need for either a high sample rate or lots of cranking to make sure you capture the high tick compressionwise.
i see that on my backpressure logs... big diff between max reading, min reading and the average. since the motor is spinning around 250 300 rpm during a compression check v much higher as i monitor backpressure maybe that's not too much of an issue.
i will find the correct sensor and do my compression checks thru my datalogit.
howard coleman
the thread got me to thinking that w another sensor ( the honeywell is 0 to 100 psi) i could just as easily do a compression test.
unlike the old style mazda compression tester the digital sensors sample every X per second so there is a need for either a high sample rate or lots of cranking to make sure you capture the high tick compressionwise.
i see that on my backpressure logs... big diff between max reading, min reading and the average. since the motor is spinning around 250 300 rpm during a compression check v much higher as i monitor backpressure maybe that's not too much of an issue.
i will find the correct sensor and do my compression checks thru my datalogit.
howard coleman
05-07-05, 10:04 AM
#30
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I think this is due to the increased crank speed from the engine trying to start. If you look at the FD shop manual you will see a compression chart for altitude, and crank speed.
Originally Posted by FDjunkie
During one of my tests, and included in my write-up, I saw the minimum pressure within the rotor rise when the engine tried to fire. I thought this was due to backpressure from exhaust restriction, but could it also be a result of port timing? I was measuring compression from the leading plug at the time and apparently there was just enough fuel left in the injector to let the engine cough a little. As this happened the minimum pressure went up to about 20-25 PSI, and the maximum was around 155 PSI. What do you think? Backpressure or port timing effect, or???
Attached is the pressure trace.
Attached is the pressure trace.
08-17-09, 07:16 PM
#31
The attached picture shows some test results using a digital compression tester that I assembled from readily available commercial parts. The total cost is way less than $200 USD. Almost anyone can do the same.
I have a write up but no way to post so if your interested send me a PM or e-mail and I'll send you a copy. The Word 2000 document is a little over one meg because of the pictures it contains. I'll post some other pictures here, too.
I have a write up but no way to post so if your interested send me a PM or e-mail and I'll send you a copy. The Word 2000 document is a little over one meg because of the pictures it contains. I'll post some other pictures here, too.
:-) neil
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