compression test mazda rx8

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compression test mazda rx8

Postby goatblower » Wed Sep 10, 2014 4:12 pm

is there any chance picoscope could be used to do a relative compression test on a rx8 as it being its a wankle engine, as I don't want to take it into mazda if it something we can do here

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jeff
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Re: compression test mazda rx8

Postby Steve Smith » Thu Sep 11, 2014 11:27 am

Hello Jeff and thanks for the post.

This is an interesting question and one we should try to simulate.

I see no reason why we cannot carry out a relative compression test using the starter motor current in PicoScope. We have 3 compressions per revolution of the rotor.

My understanding is we have 2 rotors (forgive my lack of product knowledge) generating 6 compression events which should impact upon the current draw of the starter motor.

We can measure this draw whilst using another channel as a sync to identify an offending rotor housing.

Just thinking ahead whilst I write this post, would it be possible to identify the offending rotor apex seal using rotor shaft speed and position sensor if applicable?

Do you have access to such a vehicle Jeff to carry out the above tests?
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Re: compression test mazda rx8

Postby goatblower » Thu Sep 11, 2014 9:20 pm

Thanks for the response
Today I carried a relative compression test (I will post a screen shot tomorrow ) I set for a 6 cylinder and every other cylinder was on average 56% so me deduction of that is one rotor is low on compression not sure of which one but either either I need to pull the engine and rebuild though would be nice to work out Which rotor is the duff one

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Re: compression test mazda rx8

Postby Steve Smith » Fri Sep 12, 2014 1:30 pm

Thanks for the feedback Jeff.

Before you strip the engine it would be of great value if you could carry out a compression test, especially if you have the WPS 500 pressure transducer?

If not, using a conventional gauge would help and once we know the offending rotor we could then see how this relates to your test results. (Using the scope with an ignition sync as well as Pico diagnostics)

Below is a link to compression testing such engines with a conventional gauge and does contain some great pointers.

https://www.youtube.com/watch?v=_jOLN-AGQO0

No problem Jeff if you have the vehicle in bits by now, I understand, take care......Steve
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Re: compression test mazda rx8

Postby goatblower » Fri Sep 12, 2014 7:45 pm

I would love a wps 500 pressure transducer but the main dealer I work for has to have a picoscope but refused to buy one so I bought it myself and only I use it so a wps 500 will come
Anyway tried to post screen shot but couldn't (give me your email and I will send it for you to post) so I carried out a standard compression test which was the same as the video link and as I thought number 2 rotor was low so engine nearly out sorry ,but it's my apprentice's own car so when it's rebuilt I can test as much as I like which will be handy

Jeff
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Re: compression test mazda rx8

Postby Steve Smith » Mon Sep 15, 2014 7:48 am

Hi Jeff, thanks for the feedback and any information you have before or after fix will be valuable.

Can you send any images or attachments to support@picotech for the attention of Steve Smith

I will re post it here.

Thanks again, take care......Steve
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Re: compression test mazda rx8

Postby Steve Smith » Tue Aug 18, 2015 3:04 pm

I know its been a long time since this post has been active but compression testing these vehicles has been on my mind for a while now.

A few weeks ago I had the privilege to play with one to my hearts content and wanted to share the results below.

Applying the pressure transducer was paramount during the tests and I have to say I was hoping for something special.

Whilst the results revealed activity inside the rotor chamber I guess what we have was fairly predictable but never the less incredibly valuable.

TEST 14 CYL INTAKE CRANK EX .psdata
(4.6 MiB) Downloaded 50 times


The RPM formula for the RX8 rotary engine was quite intriguing as RPM values are essential when carrying out compression tests. (Mazda include a sliding scale for compression values depending on cranking speed)

The eccentric shaft (crankshaft in a conventional engine) will rotate 3 times per 1 full rotor revolution, so generating 3 compression events.

The formula to denote RPM utilises:
9 eccentric shaft rotations which equals 3 x rotor revolutions generating 9 compressions
Measure and record the elapsed time between 9 compression peaks (2.889 seconds below)
RPM = 9 compression X 60 / Time taken to generate 9 compression events.
RPM = 9 X 60 = 540 / 2.889 = 186 rpm.

MAZDA RX8 ROTARY ENGINE RPM CALCULATION.jpg


Below is a psdata file monitoring front chamber pressure during Crank-idle-and engine stop (with combustion)



Here we have the RPM calculation for the above psdata file.

MAZDA RX8 ROTARY ENGINE RPM CALCULATION 2.jpg


Take care......Steve
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