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daniel · Joined: 18 Jun 2009 Posts: 670 Location: Sydney, Australia

Hi All
My car has just returned from the dyno (117RW HP!) and we have the following problem:
The mixture is rich in the mid RPM's and leans off as RPM increases above ~4500.
I have already replaced the WUR with an adjustable fuel pressure regulator.

Is it possible to trigger the cold start injector in time with the ignition pulses above say 4000rpm for enrichment, running the circuit through a WOT switch ofcourse. I am concerned that the cold start injector in the side of the intake manifold will not deliver fuel evenly to all cylinders.

Another thought is the later turbo cars with the Lambda control. Who do these cars achieve enrichment?

Cheers
_________________
Over the top of skyline, total brake failure.... hit the wall at over 200 kp/h at the dipper, so anyone who has to brake for the esses is a pussy.
1977.5 Race Car, CAMS Group S Spec
1989 944 Cabriolet

Kenodog · Joined: 25 Jun 2003 Posts: 2651 Location: Vancouver,B.C.

IIRC the enrichment is through a switch under the throttle body which in turn changes the control pressure in the WUR. Been a while though since I applied brain power to the 931 so I may be wrong....

Me
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1979 Euro 931, Olive
1981 931, Sabine
1991 Ford Ranger XLT 4x4, Ricky
1996 Ford E-350 ex-FedEx Van
2014 Mazda CX-5 (Kinderwagon)
2019 KTM 790 Adventure
2024 KLX300
2024 KLX140

72rallye · Joined: 08 Apr 2023 Posts: 16 Location: San Diego, CA, USA

Daniel, have you verified that there is sufficient fuel pressure at higher loads? I had so much voltage drop in the various connections that the pump would only see 8V. Fuel pressure at idle was ok, but under full load it completely fell apart.
To make matters worse, once the fuel pressure falls below the range of the pressure regulator, it shuts off the return flow of the WUR and the control pressure increases to the full system pressure.

I agree that activating the CSV is a bad idea, as it would create a much richer mixture on cylinder #4 than on #1.

The 81 931 has two means of enrichment:
- The WUR samples the manifold pressure and lowers the control pressure as soon as boost pressure kicks in.
- The control unit increases the duty cycle of the frequency valve to 65% at full throttle and above 3500rpm.

Thomas
_________________
81 931 currently being resurrected after a 22yr hiatus
86 951, 86 928 5spd, 75 911S coupe

924RACR · Posted: Wed Jun 07, 2023 9:12 pm Post subject:

Good call on the fuel pump wiring; we had previously had good fueling (on our NA racecar), but then I found recently that the wiring had gotten worse and I too was only seeing 8V at the pump.

Still have a few more things to touch on the fuel system, then need to get it on the dyno to check the rework; last time we were on track we were seeing 15-16:1 AFR and had to park the car to avoid blowing it up...
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

daniel · Joined: 18 Jun 2009 Posts: 670 Location: Sydney, Australia

No problems with fuel pressure - we have a gauge on it for tuning purposes.
How does the frequency valve work?
_________________
Over the top of skyline, total brake failure.... hit the wall at over 200 kp/h at the dipper, so anyone who has to brake for the esses is a pussy.
1977.5 Race Car, CAMS Group S Spec
1989 944 Cabriolet

kondzi · Joined: 02 Jul 2018 Posts: 485 Location: Poland/EU

72rallye · Joined: 08 Apr 2023 Posts: 16 Location: San Diego, CA, USA

or the Bosch manual:
https://e28goodies.com/wp-content/uploads/2016/11/Bosch-Kjetronic-Fuel-Injection-Manual.pdf
_________________
81 931 currently being resurrected after a 22yr hiatus
86 951, 86 928 5spd, 75 911S coupe

daniel · Joined: 18 Jun 2009 Posts: 670 Location: Sydney, Australia

That Bosch manual is exactly what I was looking for.
From what I can gather: When the frequency valve is triggered it increases pressure to the underside of the plunger's hence increasing fuel volume. This I can work with. Very easy to create a circuit that triggers the valve with ignition pulses above say 4500rpm at WOT. The question is: How do I plumb this into the fuel distributor to pressurizes No. 5 on the diagram is this a turbo only option?

_________________
Over the top of skyline, total brake failure.... hit the wall at over 200 kp/h at the dipper, so anyone who has to brake for the esses is a pussy.
1977.5 Race Car, CAMS Group S Spec
1989 944 Cabriolet

72rallye · Joined: 08 Apr 2023 Posts: 16 Location: San Diego, CA, USA

It works the other way around. When the duty cycle of the frequency valve is increased it bleeds off pressure in the lower chambers. Via the 4 individual pressure regulators this also decreases the pressure in the upper chambers. As a result, the pressure across the metering slits (difference between 8 and 10) is larger and increases the flow.
None of that will help you, as I believe that the FDs of non-oxygen sensor cars are missing the port for the FV and probably also the orifice to the lower chamber.
_________________
81 931 currently being resurrected after a 22yr hiatus
86 951, 86 928 5spd, 75 911S coupe

daniel · Joined: 18 Jun 2009 Posts: 670 Location: Sydney, Australia

Looks like I'm back to trigger the cold start injector...
_________________
Over the top of skyline, total brake failure.... hit the wall at over 200 kp/h at the dipper, so anyone who has to brake for the esses is a pussy.
1977.5 Race Car, CAMS Group S Spec
1989 944 Cabriolet

kondzi · Joined: 02 Jul 2018 Posts: 485 Location: Poland/EU

Hmmm... Let me think. The most important thing in CIS is the WUR. This is the gremlin that changes the fuel pressure during the engine run (other than idle). Either cold or warm or part warm or whatever. There are WURs with different characteristics. You can even convert the WUR to a kind of adjustable one (well it's kind of adjustable anyway, but adding one tap and screw makes life easier). Then there are WURs with vacuum input - obviously there is vacuum at idle and part throttle and no vacuum at WOT (atmospheric pressure). Maybe I'm wrong, but as this is a WOT scenario, there should be a way to adjust the WUR at WOT (the one that has vacuum port) to go more rich?
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Konrad
'89 951 US
'88 Mustang 5.0 LX Convertible (factory specs)
'84 911 Carrera 3.2 RoW (factory specs)
'81 931 RoW (TBD)
'81 Ford Capri 2.8i (factory specs)
'79 Ford Capri 2.9 (heavily modded)

daniel · Joined: 18 Jun 2009 Posts: 670 Location: Sydney, Australia

Hi Konrad
I have already replaced the WUR with a adjustable fuel pressure regulator. Using a fuel pressure reg with a vacuum input won't help in K-Jet cars ad they are designed to increase fuel pressure when vacuum drops. In K-Jet cars this will lead to a leaner mixture.
_________________
Over the top of skyline, total brake failure.... hit the wall at over 200 kp/h at the dipper, so anyone who has to brake for the esses is a pussy.
1977.5 Race Car, CAMS Group S Spec
1989 944 Cabriolet

kondzi · Joined: 02 Jul 2018 Posts: 485 Location: Poland/EU

Not sure how this works with N/A WUR with vacuum, but in the 931 WUR there is a screw, that can limit the enrichment (or allow it more). It basically allows diaphragm to move more or less, therefore limits the change in the pressure related to vacuum/boost. If such exists in the N/A WUR, maybe this could be a way to limit the leaning at WOT? That's my way of thinking. Just an idea.
_________________
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Konrad
'89 951 US
'88 Mustang 5.0 LX Convertible (factory specs)
'84 911 Carrera 3.2 RoW (factory specs)
'81 931 RoW (TBD)
'81 Ford Capri 2.8i (factory specs)
'79 Ford Capri 2.9 (heavily modded)