DIY digital compression tester < $200
 

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.

The assembled tester minus computer.

Good job, home made is always the way to go.

Picture from below engine with tester screwed in to the front leading plughole. The engine was at normal temperature, too, so be careful.

Hit the wrong button...

Shopping list for this DIY tester:

1. Pressure transducer – www.grainger.com part number 1X742 @ $103.05 plus tax & shipping; total $117.04

2. Data acquisition unit – www.dataq.com part number DI-194RS @ $24.95 plus tax & shipping; total $30.02

3. Misc. – Radio Shack – 16-gauge 2-conductor wire with one red and one black jacket, battery clips, fuse holder & fuse, and shrink tubing assortment for about $30 +/-

4. Stolen flex hose from previous compression tester – free

Last one. Picture of computer in the passenger's seat while testing. And for the computer savvy, yes that's a wireless NIC card and I could have been surfing the net at the same time.

Very neat... good job.

This is what I was thinking of doing, but I was having a hard time finding a pressure transducer for under 150 that I thought would work. I ended up getting a steal of deal on a Mazda tester new in the box for 200 bucks so I just bought that :)

Brian

Very good! I won't be surprised if your readings are a little low, because of the length of the hose between the plug hole and the transducer. If you estimate that the volume of the hose will add about 0.5cc to the compressed volume, then your readings would be about 3.5% lower than actual. Not much, but visible.

FormerPorscheGuy - Thanks for the positive feedback. I like doing things like this myself, too.

Wargasm - You're right that the key was in finding this transducer; and it has all its voltage supply electronics built-in.

Orange!FD - I hadn't even thought about the effects of that flex line. My decidedly un-calibrated numbers are looking way too low right now at 85-87 PSI, so a 3-4 PSI increase is important! At least they're balanced, though.

do you happen to know how much a new Mazda tester usually runs for? thanks

There are at least two "official" testers that I know Mazda used.

The original one was set up with a spool of calculator-type thermal paper with little grids on it indicating the different PSI. When the hose received pressure waves, it moves an arm with a heated needle and draws a line.

The original is the one I have and I think it's better than the new one because it's CHEAP and it WORKS.

You can see some sample outputs from mine:

http://www.zeroglabs.com/rx7/compression.htm
http://www.zeroglabs.com/rx7/compression2.htm

The newer one is digital and MUCH more expensive. I've seen them on websites for 1800-2400 dollars... hahaha can you say, "Markup"?

Digital is WAY more expensive and what the heck are you getting?

Anyhow, I got mine from some sort of Rotary Recyler place and I think I paid around 200 + shipping.

Brian

You are bad ass! We need more people like you in this community.

Chuck Huang

:bigthumb: good job!

I'm really please with all the positive feedback. Here's a new question:

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.

Well, Grainger now says they can't get the 1X742 anymore. I haven't called my "regional rep" for a substitute, although I did find this page elsewhere on the web that seems to be the same transducer. Of course it's $190 instead of $103.50:

http://www.omnicontrols.net/index.html?item1015.html

Errr how is it firing? When you do the test, you're supposed to disconnect the ignitor connector (driver's side near firewall/fender under the cruise control if you have cruise).

Disconnect that, and hold the throttle all the way down. My testing shows about 0.5 bar decrease in the pressure if you don't hold the pedal down.

Brian

I pulled fuel pump relay and ran the fuel system dry, but did not disconnect igniter. And I did run my tests with the throttle held open as you correctly point out.

Incidentally, I also ran tests with the engine both hot and cold to see those differences, which were significant, too (I don't have the results with me here at work). However, in my write-up I suggest that having results from a cold test might be valuable for comparison at a later date if your engine wouldn’t start.

Yo. Stumbled on this thread after someone in the Other Rotary section proposed ideas for trying to build one outta a A/C pressure sensor and light bulb. (No go for me. I need actual PSI, not on/off... I get that for free with my ears.)

How bout ordering replacement sensors from these guys:

http://www.japanicity.com/~mak/

Looks to be slightly above $100 for a replacement sensor. And it's DESIGNED to test engine compression, so we know it'll handle gasoline vapors and fit the pressure range.

I'd rather use the DAQ unit above or build and program my own controller/readout than to pay $900 for a prebuilt...

EDIT:
WOAH... Way to read man... Sorry for bringing up a dead thread... And on foreign territory, no less.

Great post.

How good is the data logging software and manual that comes with the DI-194RS?

Do you need to calibrate the transducer in some way? Or does the transducer have a fixed linear mapping function, i.e.,

transducer output volts = measured pressure * someNumber?

Interesting. Thanks for bringing it back, Pele :). I think I need one of these bastards :bigthumb:

Go to other rotorys and check out something less then 15$ to build

I'd assume some kinda spec sheet comes with it...

But it's a moot point. That specific transducer isn't available anymore... I'm looking for a suitable replacement for around $100 as original... All replacement seem to be around $200...

Great thread, thanks for bringing this back.



This one's $150, though I'm not sure if it's exactly compatible with the system that FDjunkie built:
http://oeiwcs.omron.com/webapp/comme...6&prrfnbr=2611


This one looks good also, but I haven't got any price info on it yet.
http://www.druck.com/usa/products/pdsa118jan02.pdf


If we know more about the connector, it might be possible to find a small pressure sensor that attaches directly to the spark plug hole, which would eliminate the cost and error of the adapters that are being used. According to NGK, our plugs are 14mm , but I haven't found the thread specs.

-s-