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931 S2 cuts off @ 5800 rpm
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Dutch924-racer  



Joined: 23 Jul 2007
Posts: 1076
Location: The Netherlands

PostPosted: Sun Jul 09, 2017 4:22 am    Post subject: Reply with quote

Carrera RSR wrote:
Is the rev limit a hard cut off or just that the motor feels strangled at the top end? I hardly ever get to 5800 as my motor starts to feel less effective. All the boost/torque is at 3000-5000 rpm. All starts to tail off over 5000.
You havent mentioned what boost you are running?
Forge bov are known to leak boost if the spring is too soft
Is the rev limit electrical or fuel related?
I had a bad fuel filter once which wouldn't allow my motor to rev over 5000rpm. Flow was too restrictive
An advanced cam has great low end pick up but the motor struggles at the top end. A retarded cam has slow pick up but great top end ability. One tooth out does make a difference

Is your fast idle and rev limitation connected?


by change I flipped the Forge recirculation valve today and put a heavier spring in it, the car now revs up to 6300/6400 rpm and doesn't cut abrubtly off like before and no bouncing tacho anymore (still don't understand that).

6300/6400 rpm is fine but I have a even more heavier spring (red/strongest available). Obviously the values for the springs on the Forge website are not suitable for a 931 setup

The picture in my older topic of the way the Forge was mounted, now the boost is on the other port, but don't know if that made a difference. http://www.924board.org/viewtopic.php?t=43093&highlight=

I did some other minor thing, like cleaning the contacts of the coil and tacho, but do'n't think that made the difference.

Hopefully the problem stay's away

The idle is still to high, will go further on that in te other topic : http://www.924board.org/viewtopic.php?t=43184
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937 trackday car
Series 2 engine
951S turbo and intercooler
Fast road camshaft
Forge recirculation valve + boost controller
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Dutch924-racer  



Joined: 23 Jul 2007
Posts: 1076
Location: The Netherlands

PostPosted: Sat Jul 22, 2017 6:04 am    Post subject: Reply with quote

Update, the cut of problem seems to be in the rev counter.

I swapped back the rev counter of a early 1976 924 but the cut off came back.

How is the possible?

I wanted to use the early 924 rev counter because the red part is on top and better visible on track driving.


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937 trackday car
Series 2 engine
951S turbo and intercooler
Fast road camshaft
Forge recirculation valve + boost controller


Last edited by Dutch924-racer on Sat Jul 22, 2017 3:31 pm; edited 1 time in total
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ideola  



Joined: 01 Oct 2004
Posts: 15548
Location: Spring Lake MI

PostPosted: Sat Jul 22, 2017 6:40 am    Post subject: Reply with quote

Wow, baffled! In4TheUpdates!
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Dutch924-racer  



Joined: 23 Jul 2007
Posts: 1076
Location: The Netherlands

PostPosted: Mon Mar 25, 2019 5:23 am    Post subject: Reply with quote

I got the 931 out of the winter sleep and back on the road again. Great fun

Last year I thought the rev cut out problem was gone, but it is not.

The engine cut's of at around 5800 rpm. At the same moment the tacho goes al the way to zero at the same moment'. If I shift up at the same moment there is no problem in the next gear until 5800 again....

What I already dit:
- swapped the DITC
- swapped the DITC sensor
- temperature sensor
- timing control unit
- swapped the tacho, it seems that the old 76 style tacho made the problem wors ???
- New Bosch coil
- Other fuel relay

Personally I think it is something electric as the tacho goes al the way down at the same moment. The rotor is the non rev limiter version.

Any ideas ?
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937 trackday car
Series 2 engine
951S turbo and intercooler
Fast road camshaft
Forge recirculation valve + boost controller
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Cedric  



Joined: 27 Aug 2004
Posts: 2600
Location: Sweden

PostPosted: Mon Mar 25, 2019 10:04 pm    Post subject: Reply with quote

If i remember correctly the bouncing of the tach is what i get when i go into the std revlimiter at 6700 or whatever it is. I know you have run 2 different tachs, but have you tried some more reliable way of measuring the RPM so that you are sure that the RPM is correct ?
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 2309
Location: Somerset, UK

PostPosted: Mon Mar 25, 2019 11:23 pm    Post subject: Reply with quote

I had an issue where the relatively new rotor arm started to hit the dizzy cap at high rpm. Rotor was wobbling on the shaft a mm or two. Felt like a top end misfire/limiter and would not go beyond. Just a thought. New rotor and cap fixed it.
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
Now www.924board.org/viewtopic.php?t=34690
Then www.924board.org/viewtopic.php?t=31252
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Dutch924-racer  



Joined: 23 Jul 2007
Posts: 1076
Location: The Netherlands

PostPosted: Tue Mar 26, 2019 5:54 am    Post subject: Reply with quote

Thanks for the replies, I haven't checked without the tacho yet, good idee to exclude that.

I just checked the rotor, there is play, mainly axial. I

Can anybody check if that is normal ?

Video:

https://youtu.be/jygq_d7Y8q8
_________________
937 trackday car
Series 2 engine
951S turbo and intercooler
Fast road camshaft
Forge recirculation valve + boost controller
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Dutch924-racer  



Joined: 23 Jul 2007
Posts: 1076
Location: The Netherlands

PostPosted: Thu Mar 28, 2019 12:36 am    Post subject: Reply with quote

I checked the distributor of the 924 na and it feels the same
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937 trackday car
Series 2 engine
951S turbo and intercooler
Fast road camshaft
Forge recirculation valve + boost controller
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 2309
Location: Somerset, UK

PostPosted: Thu Mar 28, 2019 1:53 am    Post subject: Reply with quote

Is the tach and rev limit driven by DITC box? Is there an intermediate box?
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
Now www.924board.org/viewtopic.php?t=34690
Then www.924board.org/viewtopic.php?t=31252
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