John Speake

Hi Jim,
Yes you are correct. In fact the idle mixture pot on the MAF is not wired into the MAF circuits at all. It is a seperate component and it is only connected to the LH ECU, no where else.

So the MAF electronics are identical, for all LH 928s.

85/86 32v and Euro LH 84-86 use the pot on the MAF. But for the non-cat early S4's it is impossible to get at the pot on the MAF, so for those cars the MAF pot is not wired into the cableform, ,and there is a pot located on the plate holding the LH/EZK ECUs. Or on some cars, it is mounted above the fuse panel.

I guess it's a feature rarely seen on cars delivered to the USA, as most of these were cat equipped ?

Regards

Fastest928

It is highly likely that the 85/86 also has a 5v ish limit on closed loop. The reason the control goes to map based pw is due to end of range inaccuracy of the maf.

John Speake

Hi Marc,
I don't think the problem is the MAF, which is still very accurate at much higher flow rates. The problem with trying to get more power is that the hardware system was set up expecting not much more than 5v at max power/torque. So the A/D convertors are near their upper limit. Any more input volts, and then can't give a correspondingly higher output value.

Also, with stock injectors, they are working at almost 100% duty cycle at max power/torque.

Does this match your experience ?

Regards

mspiegle

what's the easiest way to grab my duty cycle and log it? Tony seems to be able to do this easilly with his innovate motorsports unit. Duty Cycle is a special type of logging, right? It isn't just a 0-5 voltage.

MarkRobinson

[QUOTE=John Speake]
Also, with stock injectors, they are working at almost 100% duty cycle at max power/torque.

This is not necessarily the case: stock '85-86 cars have 24# injectors @ 30psi (w/vacuum). With our old supercharger kit, we ran 30# (25% larger), but can easily support mere 500hp @ 28psi (w/vacuum) with the MAF providing the injector pulse width changes on it's own (stock chips). That stated, the 24# units could support .75 of 500hp w/o issue, or 375hp, eventhough the cars are rated at a 292hp.

if the 24# injectors were at 100% duty cycle at 292hp, then you could ONLY provide more fuel with more fuel pressure, and we repeatedly did the opposite of w/o any signs of leaning out. I normally run 28psi on 24# injectors for my '85-86 (past) cars, making between 330-350hp flywheel.

Mark

mspiegle

You should get some logging and check the duty cycle. Then you'll know for sure exactly what is happening. I like to log things as much as possible.. doesn't leave room for guessing.

Fastest928

I use a fluke 88 to check duty cycle and have never seen over 90%. I have a male female that goes between an injector and the clip with a take off to the fluke.

All maf have a decending/flattening slope, particularly after 5v, meaning it is more difficult to measure change in air flow, as the change yeilds less voltage delta...just like a o2 sensor. The Bosch site shows the curves. This curve is one of the reasons that high perf engine use two mafs rather than one big one....better sensitivity at sub 5 volts. MKax sensitivity at half the max volts.

I have modified mafs by decreasing the hot wire "venturi", experiment failed, and by increasing the id of the tube .. successful.

Hope this helps

John Speake

Hi Marc,
Good to hear your experiences. Yes, the MAF has a log type law of the type you describe. I can recalibrate them to lift the curve as it were, and give better definition at the top end.

Regards

MarkRobinson

I'll get my new AFC-R installed this Friday & I'll have data logged for my car: est. at 330fwhp, before I dial in my corrections. Factory literature states that 1.5v will be measured with the key on but motor not running, so I'll start from there.

Thanks for all the input guys.

Mark

mspiegle

I just looked at my 1 meg pure textfile log of some of my dynoruns @ devek. I have well over 6000 log lines (each line containing rpm, maf, ignition, temps, etc).

I did a sort on the MAF voltage, then took all MAF voltages greater than 5.7 and sorted those by RPM. The lowest RPM my car would generate a 5.7 voltage was at 3800RPMs. I then took all RPM readings from 5.7 to 6.0, then did an average and received 4370rpm. SO, I can infer that if 5.7 - 6.0 is the voltage at which injetors goto 100% duty, it will happen at around 4300rpm. I will figure out how to check my duty cycle and get some more concrete data.

Fastest928

Michael,
check duty cycle again ... I do not think that it is 100% .. I ahv enever seen it, and there is too much manufacturer liability at that duty cycle since bosch does not recommend any cycle over 85%

Let me know.

Jim_H

Hopefully since you stated this it will be taken seriously. I ran into a brick wall with the 85% max duty cycle discussion.

mspiegle

didn't Tony determine what the duty cycle was with his setup?

MarkRobinson

Ok, did some data logging on the way to work this am:

For a reference, my 928 is making in the 280-ish RWHP range with chips & exhaust: here's what I found for MAF voltage:
Ignition on: 1.7v (literature states about 1.5, but we're close).
Cranking: 2.4v
Idle: 2.6/2.7v
Most low-throttle, low RPM crusing: 3.4v
Semi-easy acceleration: 3.8-4v
WOT: @ 5k, about 4.2v
WOT: @6k, about 5.4v
WOT: @6500, 5.8v

I'll set my range from 1.7 (as it'll see this when the key's turned) to 6.2 volts (give me a cushion)

Installing my AFC-R tonight, dyno in the AM

MarkRobinson

As suspected, AFC dropped the maps at 100% MAF signal, and I didn't get my programming done in time for the dyno as I installed the AFC the night before.

Run #1: 3rd gear, AFC engaged, preparing to lift when MAF reached 100% (& did), but, corrections programmed into the AFC (rich), did register & show on the graph.

Run 2 & 3: AFC disconnected, two 4th gear runs back/back. Exactly as I thought: too rich at first (torque reduction), ideal through mid-range, way too rich up top. If you lean to maximize HP, you loose your mid-range torque.

Mark