Possible stupid question on a situation I've never encountered, could use help!
 

Ok, so one of my rotary customers calls me today and says that he was out of state and had something or other done, but the shop also checked his compression (non-mazda checker i assume) and he had 75psi on each "bounce" of the needle on both rotors. Then, he tells me they checked it from the leading plug holes. I mean, I've always checked from the trails, but could this explain the low comp? '88 Turbo II, 109k miles btw.

Simply asking for knowledge's sake.

hey man im new to the club and ive never done a comp test myself but i know on my 91 nonturbo my comp was 82 psi and the t2 sould be lower and my good friend with a t2 said thats perfect!

82 PSI is BELOW the number that the FSM considers acceptable... according to mazda, anything under 85 PSI on a rotor or more than 20 PSI of difference between front and rear is a case of rebuild...

t2monster : always check compression on the TRAILING side plug hole with the accelerator pedal fully depressed

I thought that anything over 90psi is good. My Na vert put out something like 79psi. <---Not good...btw...187k miles on original engine.

From Rotary Resurrection:

115+ is like new
100-115 is healthy
90-100 is getting weak
below 90 could blow at any moment.

Pedal all the way down???? I didnt know that. Would that change my compression PSI???

duh


-Ted

TII: I know this. I was merely asking a question on whether it COULD be checked from the leading side.

On an off-note, i've always tested w/ pedal down. How much of a difference does the throttle open and breathing easier make?

so the mazda spec of minimum being 85psi for both 9.4:1 engines and 8.5:1 engines doesn't sound strange to any of you? Compression test results are directly affected by static compression, at least on every IC engine I've ever worked on.

it is a little strange but it is just a guideline spec. compression is directly related to cranking speed as well so be sure he has a full charge on the battery and that the starter is turning strong. the faster the engine cranks the higher the static compression will be.

This is from some testing I've done on my old, s4 TII engine. You'll notice the cold #s vary a great deal and this is due to the slow cranking speed when cold. Maybe my battery is a little weak.

Wow, how did you datalog that?

James

Pressure transducer, cheap datalogger, and my old laptop. ± 0.25% full scale accuracy and samples @ 240/sec., pretty slick. But the really cool part is having all the raw data to play with in excel.

I think you missed the coolest part man....

Not paying $1K for a Mazda tester:)

James

Hear hear! I've got less than two hundred bucks in it including the laptop.:) I like being able to see what's going on in there to such a finite degree.

That's why I swear by my digital Mazda compression tester, it also accounts cranking speeds and recalulates it to 250rpm wich is the rpm where the mazda specs are given.

More air entering the engine means more pressure when it's compressed.

Mazda's specs are for full throttle so that is the way you must test. Otherwise your numbers are meaningless.

It's a minimum requirement, not what a new engine should give. It's just the point at which Mazda says you need a rebuild.

Thanks for that beautiful graph ... good stuff.

How much did you shell out for the pressure transducer, and which model is it and where did you get it ?

hugues-

agreed. Beautiful datalogging.

They couldn't be meaningless, just out of our grasp. As you can see on that sweet datalogged chart, the numbers are related. The difference b/w open and closed on the chart is similar on both cold and hot testing.

Again, impressive chart.

How accurate are the pressure readings on the graph ???
I assume you took some readings for various known pressures to calibrate or were they provided with the transducer ?
Can you tell us on how you did the calibration process ?

thanks,
hugues-

Now that's the way to do a compression test!

I'm surprised the cold pressures are higher then the hot pressures.

it is supposed to be that way but that's a nice confirmation and we even get the differential.

Too bad there's no logging for the rpms, that would have been gold.

hugues-

Very, 1/100 psi.Yes.I ran it up against several different gauges.
There's no box displaying the number or anything, but it's right there on the graph waiting for your calculation.;)

How does that thing work anyway? I've never seen one. Do you punch buttons to start and end a session, or does it just give you the average of the last activity?

As stated in the title, that graph is adjusted for RPM. I did a manual calc. before graphing. From a table I created based on the one in the FSM. That's why the valleys don't go to zero.

I'll put my tester up against a Mazda one any day. Bring it on.

Cheers

There's your display of the RPM.