924Board.org

[Rasta Monsta]

The answer I am looking for will come from someone who is:

1. Running a functional factory lambda system
2. Running an AFR gauge
3. Using the frequency valve duty cycle to set idle mixture

So, the question is, when the FV duty cycle is at 50%, what AFR are you getting? In other words, what is the "target" idle AFR for the lambda system?

TIA for any responses. I will add my own observations when I get more time to fine tune Shaggy.
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[Nein37]

I'm not sure if this will help but I'm running my lambda in open loop... I guess that means the FV is at 50% duty cycle???

I have my warm idle set at 13.5 AFR. I've tried it at 14.7 AFR and at 12.5 AFR neither was optimal as far as my butt dyno and ears were concerned.

At cruise it'll sneak up to 14 AFR and I get perfect enrichment at around 11.3 AFR at WOT. However the WOT enrichment changes very little with idle adjustment changes. At 12.5 AFR the WOT enrichment will sneak down to around 11 AFR sometimes hit the high 10s.

John
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1981 931 CGT replica, OEM CGT intercooler, .8 BAR WG spring, GTS Headlights, Innovative Wide band A/F, A/C delete, 16" Fuchs, Weltmeister 200lb lowering springs, Bilstein HDs front, Koni Sport rears.

[Rasta Monsta]

Thanks, man, any inp;ut is helpful. I plan on fine tuning next weekend, and will post my results.

CAN'T.

WAIT.

DRIVE.

CAR,
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[ideola]

Rasta, did you ever find an answer to this question???

I have wideband on both the 941 and the Club Sport, but I'm getting wildly different AFRs at idle.

The Club Sport has factory Euro-spec DITC with no Lambda system, and is equipped with an AEM analog wideband AFR.

The 941 has factory S1 ignition and Lambda, but the Lambda is not currently connected; it is equipped with a PLX Devices digital wideband AFR.

I am trying to determine what the optimum idle AFR reading should be.

I'm also trying to sort out a slight stumble / hesitation on the Club Sport when accelerating at part throttle. It appeared to be related to a too-rich mixture (which I have enleaned), but I'm concerned about enleaning any further, as idle AFR is pegged at 18:1. It still goes to full rich (10:1) at WOT, but it's the in between I'm trying to figure out.

The 941 wideband is not currently wired up to emulate narrow band...something I plan to do which should allow me to reconnect the Lambda system. But that won't help with the CS, which doesn't have Lambda. Consequently, I can't presently use the dwell method to attempt to set mixture.
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erstwhile owner of just about every 924 variant ever made

[Rasta Monsta]

Answered in THIS POST as follows:

If you have a 50% FV duty cycle at idle (45° dwell on the 4 cylinder scale of a dwell meter, attached to the diagnostic port), you should be sitting at stoich with the car warm. On my car, you can watch the gauge swing back and forth a few tenths above and below 14.7 as lambda corrects mixture on the fly. Cruising under zero or light boost, you should also be right around stoich. In my case, both of these parameters are verified through wide band O2 sensor. My wideband 02 likewise verifies enrichment into the low 12s under boost, courtesy of the control pressure regulator.

It should also be noted that I have TWO O2 sensors running in parallel, a wide band for the cockpit gauge, and the factory narrow band for the lambda signal.

Surging under light acceleration is usually an air leak. I would smoke your intake and verify everything is sealed up. I would also verify throttle stop position per FSM.
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[ideola]

Thanks Rasta. So it seems like I should be aiming to set the idle AFR at 14.7 once the car is fully warmed up. I'll give that a go and see what happens. I had some weird behavior from the AEM gauge when I first installed it, and I'm not sure I trust it just yet.

[Rasta Monsta]

[ideola]

Good thought...I'll do all the CIS tests too to see what that tells me.
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erstwhile owner of just about every 924 variant ever made

[Rasta Monsta]

Oh, and I guess I would also stick with my recommendation for a smoke test. While your vacuum numbers certainly point to a healthy engine, they are no guarantee you don't have an air leak between the AFM and throttle plate.
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[flosho]

Megasquirt.
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[This Space For Rent]