DIY digital compression tester < $200
Joined: Jul 2003
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From: Bay Area, CA
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; May 5, 2005 at 05:53 PM.
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From: Bartlesville, OK.
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...
Joined: Jul 2003
Posts: 4,678
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From: Bay Area, CA
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.
Joined: Oct 2001
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From: Florence, Alabama
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
There and back again
Joined: Feb 2001
Posts: 967
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From: Camarillo, Ca
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.
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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