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Slow RPM Drop/Hang

SLIXK

By SLIXK,
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SLIXK

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So I’ve finally manually converted my stagea (r33 gtr box) with an Exedy heavy duty clutch. When up shifting gears there’s a hang and slow rpm drop which doesn’t feel right (it’s not that bothersome) but I’m just curious if this is right.

*For example, starting from 1st gear, I accelerate to about 3200 RPM, gas off and clutch in, shift from 1 to 2. At this point RPM needs to be around 2000 to match. But even after I finished shifting, RPM usually still much higher than 2000, I have to wait a little to let it drop before I let clutch out, or I may hear clunk drive train noise due to sudden torque shock from higher than needed RPM*

I copied the above paragraph from a different forum regarding a completely different platform but that best explains my experience.

I’m currently thinking it could have something to do with the Auto ECU being retained in the conversion or plainly because of the Heavy Duty clutch.

Also, would haltech be able to control this issue as that will be my standalone soon enough.

Edited by SLIXK
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GTSBoy Grand Master

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That's not how a clutch works. It's not a switch. Clutch goes from disengaged to engaged smoothly and takes up speed difference. Throttle applied at the same time to satisfy load change.

If you truly have a long rev hang, have you considered that the auto tranny will mask a lot of problems with things like the idle speed being wrong/too high from the throttle plate being stopped too open or a vacuum leak/IACV problem? The manual makes you have to take care of that, via the first paragraph^.

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Duncan Grand Master

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It won't be the auto ecu causing the issue, and it's not normal. I ran my manual swapped Stagea on the auto ECU for a while before I moved to PowerPC, the reason I moved was to run more boost.

Like GTSBoy said, for revs to stay up (or fall slowly) there has to be some additional air in the system somehow. Even if the ECU is adding too much fuel you need 14x the air to keep the engine running.

Does the same issue happen if you rev it when stationary, out of gear?

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SLIXK Collaborator

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On 7/27/2022 at 1:34 AM, GTSBoy said:

That's not how a clutch works. It's not a switch. Clutch goes from disengaged to engaged smoothly and takes up speed difference. Throttle applied at the same time to satisfy load change.

If you truly have a long rev hang, have you considered that the auto tranny will mask a lot of problems with things like the idle speed being wrong/too high from the throttle plate being stopped too open or a vacuum leak/IACV problem? The manual makes you have to take care of that, via the first paragraph^.

Yep hangs longer from 3rd or 4th to neutral and releasing clutch sometimes it stays at a certain RPM like 1200 until the car comes to a complete stop.

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SLIXK Collaborator

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On 7/27/2022 at 8:46 AM, Duncan said:

It won't be the auto ecu causing the issue, and it's not normal. I ran my manual swapped Stagea on the auto ECU for a while before I moved to PowerPC, the reason I moved was to run more boost.

Like GTSBoy said, for revs to stay up (or fall slowly) there has to be some additional air in the system somehow. Even if the ECU is adding too much fuel you need 14x the air to keep the engine running.

Does the same issue happen if you rev it when stationary, out of gear?

 

This is out of gear and stationary, it was the like this when it was auto too.

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Duncan Grand Master

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Got it, so I would say that is normal returning to about 1500 then something is adding air for a little until it returns normally to 800.  I'd disconnect the IACV and AAC wiring and see if it still happens, might be ECU sending a signal to add air, or might be a faulty air valve if it still happens

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Chi-Town Explorer

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I'm curious to see what you find on this. I had this same thing happen in a S13 that I had converted to manual years ago. I replaced the IACV but never got it to behave properly.

I always wondered if the ECU is looking for an "in gear" signal opposed to just being in neutral all the time (signal wise). In the Suby ECUs it would not use the correct mapping until it saw it was in gear. It still ran/drove fine but was never quite 100%.

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GTSBoy Grand Master

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On 7/28/2022 at 12:04 AM, Chi-Town said:

I always wondered if the ECU is looking for an "in gear" signal opposed to just being in neutral all the time (signal wise)

There is no "in gear" signal. If you correctly wire up the original auto neutral switch input to the manual's neutral switch, the ECU works properly.

Incorrectly (ie, not) wiring the neutral switch is not assumed (by me) to be something that would happen.....because it is so bloody easy to do it right.

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SLIXK Collaborator

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On 7/28/2022 at 11:37 AM, GTSBoy said:

There is no "in gear" signal. If you correctly wire up the original auto neutral switch input to the manual's neutral switch, the ECU works properly.

Incorrectly (ie, not) wiring the neutral switch is not assumed (by me) to be something that would happen.....because it is so bloody easy to do it right.

Would the neutral switch not being connected cause these symptoms I’ve described?

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SLIXK Collaborator

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On 7/27/2022 at 1:10 PM, Duncan said:

Got it, so I would say that is normal returning to about 1500 then something is adding air for a little until it returns normally to 800.  I'd disconnect the IACV and AAC wiring and see if it still happens, might be ECU sending a signal to add air, or might be a faulty air valve if it still happens

Will try this a good day, maybe today lol.

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Chi-Town Explorer

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On 7/27/2022 at 4:37 PM, GTSBoy said:

There is no "in gear" signal. If you correctly wire up the original auto neutral switch input to the manual's neutral switch, the ECU works properly.

Incorrectly (ie, not) wiring the neutral switch is not assumed (by me) to be something that would happen.....because it is so bloody easy to do it right.

I was thinking the auto ECU sees drive/reverse, the neutral/park is only for the starter circuit correct?

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GTSBoy Grand Master

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On 8/1/2022 at 3:16 AM, Chi-Town said:

the neutral/park is only for the starter circuit correct?

No. Absolutely not. The ECU needs to know if the auto trans is trying to stall the engine, which it is, when it is not in N (or P, which is the same thing). The idle control relies on it.

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SLIXK Collaborator

SLIXK

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On 7/29/2022 at 4:41 PM, GTSBoy said:

Absolutely. Clutch switch too.

Back onto this thread, how can I check to confirm that the neutral switch and clutch switch is hooked up?? Maybe this problem is also linked to my new throttle body problems ?

Edited by SLIXK
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