Using stock E30 Tacho with standalone ECU

[SIR E30 68]

Hi All,

Was trying to refrain from posting until car is finished but I'm stuck..

Long story short - Facelift E30 320i with series 5 13b Turbo rotary, Microtech LTX-8 ECU.

I connected the tacho wire from the ECU to the tacho signal pin on the C101 so the tacho reads but its about 500 rpm out at idle (have had it plugged into laptop and seen the digital reading from the ECU) on microtech software.

My theory: If i changed the instrument cluster/tacho/whatever to a 4 cyl unit will that solve my problem? The 13b has 4 coils, so technically the tach would think it was 4 cyl? right or wrong?
Would be helpful as the odometer gears in my current dash are flogged anyway..

I would love to keep the stock tacho so as to keep the interior as original in appearance as possible, I really dont want to mount a big ugly tacho on the dash etc..

Thanks in advance..

-Jason

[westy 614]

Trying to remember how many times the rotary fires per rev. 3 times each rotor? Anyway, number of coils doesn't matter as each rotors pair fire in sequence.

Is it 500 high or low?

[polley 916]

Run a ecu that's not stuck in the 1980's. Then you can configure how many tacho pulses per rev required to drive the tacho correctly.

Anyway, the 4 cylinder gauge cluster is expecting 2 pulses / rev.

6 cylinder = 3 pulses / rev.

It's expecting a 12v square wave signal.

Not sure how many pulses you would get from the rotor, you also have leading and trailing so would that not double it?

[SIR E30 68]

Trying to remember how many times the rotary fires per rev. 3 times each rotor? Anyway, number of coils doesn't matter as each rotors pair fire in sequence.

Is it 500 high or low?

According to this it fires twice http://www.racingbeat.com/mazda/performance/rotary-tech-tips/tachometer-plugs.html as for high or low, I cant remember - but going on the dyno next week hopefully so will be able to check it out.

Run a ecu that's not stuck in the 1980's. Then you can configure how many tacho pulses per rev required to drive the tacho correctly.

Anyway, the 4 cylinder gauge cluster is expecting 2 pulses / rev.

6 cylinder = 3 pulses / rev.

It's expecting a 12v square wave signal.

Not sure how many pulses you would get from the rotor, you also have leading and trailing so would that not double it?

Yea now that I know more I do slightly regret the microtech ECU - but it works for now, and Im not doing the wiring again if I can avoid it. Will upgrade one day if I actually like owning a rotary :S

Check the link above.. I know its talking about getting the signal from the coils/distributor instead of the ECU - but it does reinforce that the firing pattern is similar to 4 cyl.

Bearing in mind that for one rotor revolution the eccentric shaft (crank) turns three times..

Edited July 7, 2015 by SIR E30

[polley 916]

If the car was originally 6 cylinder, would expect it to be reading low.

Change to 4 cylinder coding plug, or change whole gauge cluster.

[SIR E30 68]

If the car was originally 6 cylinder, would expect it to be reading low.

Pretty sure it was reading 700 when the computer said 1200.. and i guess it would get exponentially worse as revs increase, but I dont know as it hasnt been on the dyno yet.

Sorry to be dense, but could you explain "coding plug" to me??

Edit: Google answered my questions. Are these easy to acquire?? Will it affect the fuel gauge as some threads suggest..?

Edited July 8, 2015 by SIR E30

[westy 614]

According to this it fires twice http://www.racingbeat.com/mazda/performance/rotary-tech-tips/tachometer-plugs.html as for high or low, I cant remember - but going on the dyno next week hopefully so will be able to check it out.

Yea now that I know more I do slightly regret the microtech ECU - but it works for now, and Im not doing the wiring again if I can avoid it. Will upgrade one day if I actually like owning a rotary :S

Check the link above.. I know its talking about getting the signal from the coils/distributor instead of the ECU - but it does reinforce that the firing pattern is similar to 4 cyl.

Bearing in mind that for one rotor revolution the eccentric shaft (crank) turns three times..

Ah yup, each rotor fires once per rev, so engine fires twice per rev like a four. Leading and trailing plugs fire together (not at the same time, just cause it's a long chamber needs 2 plugs for complete burn) for each combustion though . Kinda a like a twin spark engine thingy. But you probably know this. Anyway, 4cyl plug sounds like it's worth a crack.

ps, I love a good rotor, till it eats itself.

[SIR E30 68]

Ah yup, each rotor fires once per rev, so engine fires twice per rev like a four. Leading and trailing plugs fire together (not at the same time, just cause it's a long chamber needs 2 plugs for complete burn) for each combustion though . Kinda a like a twin spark engine thingy. But you probably know this. Anyway, 4cyl plug sounds like it's worth a crack.

ps, I love a good rotor, till it eats itself.

All correct, the leading plug fires at 5atdc and the trailing fires just after at 15-20atdc. I reckon the 4cyl plug sounds good - just hope it doesnt mess with the fuel gauge.

As do i, they are great fun - especially in a cool chassis such as an E30! Mine is a standard port series 5 turbo, no major mods just front mount and exhaust with all unnecessary things deleted. Hoping for a nice safe tune and just see what happens.. Nothing beats the sound and smell though!

Itll upset a few people on here but at the end of the day its my car - will post a build thread once its done.

cheers for your input Troy and Westy

[westy 614]

Mmmmm the smell arrrgh... Had a Jport 13b in an RX4 coupe. A big bugger with 51mm IDA. Was a hell of a laugh till it did it's water seals.

Sold it for peanuts about 15 years ago. Stupid given current prices, but house deposits etc.

[_ethrty-Andy_ 2136]

wouldnt the M40 coding plug be looking for a reletively even signal though?

[polley 916]

Pretty sure it was reading 700 when the computer said 1200.. and i guess it would get exponentially worse as revs increase, but I dont know as it hasnt been on the dyno yet.

Sorry to be dense, but could you explain "coding plug" to me??

Edit: Google answered my questions. Are these easy to acquire?? Will it affect the fuel gauge as some threads suggest..?

The fuel gauge should be fine, I believe it will affect the economy gauge which will never be correct now anyway.

[polley 916]

wouldnt the M40 coding plug be looking for a reletively even signal though?

It will be fine, its only counting ignition events in a certain amount of time to calculate RPM.

[SIR E30 68]

The fuel gauge should be fine, I believe it will affect the economy gauge which will never be correct now anyway.

Economy gauge gon' be sitting on the redline haha.. So long as the fuel level works.

I got a coding plug off Kerry tonight, see what happens when Im back at home on saturday!