1987 3.2 Carrera Backdate project - Sladeys hotrod

[Nige]

I wouldn't normally be taking it over 6K revs

Really!

We all know that last couple 100rpm equates to an extra surge of 200 bhp!!!

[sladey]

I wouldn't normally be taking it over 6K revs

Really!

We all know that last couple 100rpm equates to an extra surge of 200 bhp!!!

True dat

[964RS]

You do realise this is 270 on the infamous 'ninemeister' dyno.....

[hot66]

You do realise this is 270 on the infamous 'ninemeister' dyno.....

Is that the 400+ bhp N/a 993 motor dyno ?

[sladey]

You do realise this is 270 on the infamous 'ninemeister' dyno.....

Works for me, funsuckerrr

[964RS]

You do realise this is 270 on the infamous 'ninemeister' dyno.....

Works for me, funsuckerrr

[sladey]

OK slight update after some testing on the car at the week-end and driving it for a week.

On the rev-limiting at 6200, Ian came round on Sunday and we ran a cable from the engine bay to the ECU plug to check continuity on the live, Earth and shielding. This was where we were hoping to find the 'smoking gun' but no joy. All three wires showed continuity.

We also looked at the resistor extension that Wayne had wired in. The resistor had been added into the earth wire instead of the live wire and we wondered if it would make a difference if it was swapped over to the live. I took my time and resoldered it onto the live.

No change. If I remove Wayne's resistor extension the rev limiter cuts in at 4800 - with it plugged in then it happens at 6200. I got the same result with the resistor before and after I moved it from the earth wire to the live wire. So, plug it in, 6200, unplug it, 4800.

Next thing to try is to run a completely fresh shielded cable from the engine bay to the ECU - to eliminate the possibility of some sort of noise at higher revs causing the ECU to get scared. I've got suitable cable and a plug on order so will be trying that this week-end. If that doesn't work then I'm wondering about adding in a higher resistor. But let's see if that works.

I've also been driving it every day. Overall it's driving absolutely brilliantly - really pulling strongly in every gear and right through the rev range. It feels F A S T. If I'm being picky I'd say the idle is variable - generally OK but sometime still hunting around 1200. Also after cold starting (which it does OK) it hunts around a bit before settling down to an idle. I'm happy to have a bit of a play with that going forward.

I obviously don't know what I'm talking about in relation to electrics (amongst many other subjects) but I wonder if the cable carrying the signal has issues whether this would also affect the idle? I'm just thinking if it's a 'noisy' signal coming through then maybe that won't help.

And although we got continuity on each wire, that's just passing a small voltage down it. I found this online (so it must be true) written by someone that runs a site dealing in engine management stuff - trigger wheels.com

"It is also worth correcting the misnomer that "it's just a couple of volts" - the voltage increases pretty much linearly with RPM, so on cranking 2 volts may be all you see but in use 50 volts isn't unusual and we've seen well over 150 volts on our test bench"

I took that as a pointer that there may be an issue following my grinder/wire interaction that means it's OK at lower revs but when the higher voltage kicks in it gets messy and the ECU doesn't like it.

Comments from the more knowledgeable (Tim?) very gratefully received.

We'll see how it goes with this week-end's testing.....

[KS]

On mine, when I went to the rolling road last time, it wouldn't rev over 4800rpm, or so. Changed the coil pack and it was OK. Random misfire and stumble - rerouted lead from crank sensor away from plug leads. Cured 100 per cent. Fussy little gits, but worth the hassle.

[sladey]

Interesting - thanks for that

[Jonny Hart]

Unwanted electrical noise is the number one concern for many electronic systems where the real world is connected to a microprocessor. It is often the most difficult part of any design to get a reasonable amount of filtering/isolation without degrading the signal.

In an engine bay you've got 30kV on the HT leads and other sources of electrical noise like the alternator, various motors etc.

There have been many words written on the subject but generally speaking, you can help matters by:

1) Making sure the signal source is as large as possible (moving the crank sensor closer to the trigger wheel may help)
2) Keeping signal wires away from sources of electrical noise ( HT leads and anything with a motor in it)
3) Using a shielded signal wire. The shield should be grounded at the ECU end only.
4) Making sure the ECU itself is properly grounded.

[sladey]

Thanks for all that Jonny

[impmad2000]

Let me know if you run out of ideas, I have an oscilloscope and you are not too far away.

[sladey]

Thanks Tim - much appreciated - currently got fingers crossed for tomorrow

[sladey]

OK so yesterday was D-Day, as in 'Didntwork' day.

I made up the new lead and ran it from the plug in the engine directly to the ECU. I removed Wayne's resistor lead and plugged my new lead in. I took my time to get it all right.

Then took it for a test drive - first time I was able to rev it up the limiter kicked in around 4800 I tried it a few time and identical result. There was another symptom that had happened during last week's testing but as this was a longer drive it happened more - while between 3 and 4K revs and on a trailing throttle it just suddenly cut revs - dropped straight down to 1000 on the rev counter. However the engine itself was slowly falling (like you'd taken your foot off the accelerator - which I hadn't) - I was concerned it was cutting out so moved my throttle up and down a bit and eventually it caught with a bit of a bang from the exhaust. This happened 5 or 6 times. If I stayed below 3000 revs it behaved perfectly normally.

Like I say this did happen last week-end when I was testing it without the resistor in place but it only happened once - and I switch over to the resistor cable straight away.

Got home and popped the resistor cable in series again and took it out. Drove exactly as it has all week - rev limiter cuts in at 6200 and no other problems.

There is a facility on the ECU for data logging so I might do a bit of that today to see if it throws up something else happening when it cuts in.

I still can't help thinking that something cable related is interfering but this test seems to show it's not related to the speed sensor cable. I only think that because that's the only 'unusual ' event that has happened in relation to the engine or it's management systems. I don't know if one of the other cables hasn't been joined correctly and that causes an issue at higher revs. However it's odd that it's partially cured by adding the resistor in series to the speed sensor cable.

Other actions

Going to PM Tim
Going to email Canems
Maybe buy and try a bigger resistor - like adding a fault to cure a fault but if it works....

So, overall a bit of a pisser. However once again need to get it in perspective as with the resistor in place it is driving better than ever before. Still want to solve it though.......

[Nige]

I reckon some sexual favours thrown Tim's way for his time and mind would have it sorted.