Hi fellows,

I had the opportunity today to use a genuine Mazda's compression tester.
Bad news beside the fact my 13BT runs like clockwork and never refused to start or idle smooth!
We did it right... until I've checked the FSM and found I forgot the WOT position and the CAS unplugged.

So as the results are bad I wanna know if the WOT may have given better results. I hope so as the chamber may have picked up less air than with WOT condition.

Also is the cranking speed of my 13BT is 250RPM? The used compression tester is with a paper roll, not digital like on FSM with RPM.
Was cranking as usual... but wich speed... and that mays change a lot the results aslo.

At least we also found that the Cosmo's FSM (yes a real one but that's normal to set up a ... ) so with this FSM they give a chart having no like with the paper. I think that sadly the paper is the truth....

Will show pics tomorrow.

Thanks!
Matthieu

 

WOT will make the pressure numbers much higher (and is the actual test condition), and higher speed will make them slightly higher

 

Is there any rules about notWOT/WOT results? Anyone did both and can tell how much more it was?
Maybe that is the Cosmo Sport's FSM difference, I don't remeber they said you have to WOT but they scale up my result from 6 to "between 7 and 8"!
And I have no chance to redo it :'(

 

Fully closed throttle :'(

 

As indicated by others here, there is a significant difference between depressing the gas pedal or not during a compression test. I have conducted some tests to determine the difference, but the exact numbers have faded from my memory. I seem to recall only that it was significant--like 10% or so, but I will search out the original data and add it to this post later. SEE EDIT.

As far as engine cranking speed during the test goes, it too can make a difference in the readings. I have no experience with the tester you mention, but I would expect that it would record the engine cranking speed during the test or at least allow for calculation of the speed using the data displayed on the chart. If there are time tics at the pulses, you can calculate the engine speed using a simple ratio calculation. In other words, so many eccentric shaft revolutions per so many seconds converted to so many revolutions per minute using a ratio computation. [Remember, each pulse equates to one eccentric shaft revolution. That is, one complete rotor revolution=3 eccentric shaft revolutions.]

Depending on several factors like battery/starter condition, number of spark plugs removed for the test, etc., engine cranking speed can vary considerably. In order to compare the recorded data with the Mazda specification, the numbers need to be "normalized" to 250 rpm. And then we have the effect of altitude/elevation on the recorded data, which I won't get into here except to say that if the test was conducted at altitudes significantly greater than sea level, the data needs to be adjusted accordingly. The Mazda workshop manual has graphs that can be used for normalizing the recorded data.

EDIT: Here's the data from the tests I conducted. Same engine, same rotor, same tester, same day. Bottom line, as you can see, the difference between gas pedal depressed or not, can be as much as 16% if testing a warm engine or about half that when testing a cold engine. It's interesting to note that the engine temperature does not make a significant difference in the readings as long as the gas pedal is depressed during the test. Specifically, the peak numbers in both the cold and warm tests with the gas pedal depressed were around 116 PSI. BTW, these are recorded numbers that have not been normalized by engine cranking speed or altitude conditions during the test.

 

Thank you very much!
That is the info I needed plus a simple way to correct the cranking speed.
Here I go for a full corrected result and will post it here.

 

Here are the final results, my engine was warm the only problem was non-WOT.

I've compared with other papers from previous owner of the compression tester ( his stuff here : http://rx7.ifrance.com/R1.htm ) and seen my cranking speed was like him and many other results found on the Web.
So I've just applied the ratio found with your tests, 16%:

worst 6.7 and best 7.1.

Around 7, I'm happy for I'll be able to sleep tonight.

Thanks all!

 

C'est toujours un grand plaisir mon ami!

 

May I send you Roquefort to rewrad such a good french?
http://www.roquefort-papillon.com/
Oh my I have to leave and go right now to the fridge!
Gooooooooone!

( tu parles français ? la communauté des RX-7 http://www.rx7france.com ! )

Matthieu from les châteaux de la Loire between les Selles sur Cher and le Sainte Maure de Touraine

 

Sad to say, my French is very limited, but thank you for the links anyway.

 

Things cleaned:

 

In order to completely normalize the readings it is important to correct for engine cranking speed during the test. That can be significant if it was much less or greater than 250 rpm. If less than 250 rpm the recorded readings are lower than "normal", if higher than 250 rpm they are higher than "normal".

 

Yes I've checked other pulses frenquencies on the papers avalables ont the Net and the one on the website where the guy used the tester used for my test, it's the same speed of cranking (and he has done the test @ the good speed).

edit: the 0 axis was not that good on rear rotor...

 

Steps are 0.2 not 0.1...

 

That tester look anything like the attached jpgs?

 

Yes it was the same!
But with Mazda stickers on it.

 

The one shown was made to check Suzukin rotary motorcycle engines. Just FYI.

 

Heh, heh! Mine are prettier than that.

 

OK, this is the one Mazda France used for their 7.

 

http://rotarydiagnostics.com/

Go48 has one of those in the url above. No response required. Just FYI.