Hi there all-

I've been working with someone that is digitizing the AFM output. It's a cool project and will leave the car looking stock and getting the expected signals based on opening position, but hopefully stave off old AFM-itis.

Anyway, as part of the work there has been some information that I've learned about that I'd like your opinion on. Here comes the background information:

This graph shows that there is a region, right at the beginning of the opening of the flap where there is no signal sent to the DME:


This graph comes courtesy of the following link: https://www.mye28.com/tech/rods_page...by_fr_wilk.pdf

If you look at the very beginning of that graph, between about 0 - 5 degrees of opening, there is no output signal being sent, even though the flap has started opening and, presumably, the throttle cable has begun sending signals to dispense fuel.

Measurements on my AFM confirm this initial dead zone.

I can't help but think that this was an intentional choice by Porsche but I'm having trouble understanding what that does for the car/engine.

Any insight would be greatly appreciated.

Thanks

 

How thick is the barn door flap? Unless it's a knife edge the first few degrees are actually going to close the opening slightly (because the diagonal of the rectangle is longer than the side). Just brainstorming though....

 

Wild guess- low rpm air pulses in the intake tract are violent and may make the door flutter.

There's a narrow bypass passage for idle/low airflow that goes around the flap so it can still breathe.

Either way, better option is to modify the AFM housing to accept a hot wire maf sensor if you're able to do ECU stuff.

 

Is it possible that the flap angle between 0 and 5 degrees is the resting range of the flap at idle? You note there is no signal to the DME below 5 degrees. The graph indicates .05v but either way, there is likely a base idle fueling parameter. The TPS off (above) idle indication may also be impacting how the DME transitions from any base idle fuel settings.

 

Would there be any reason to build a little pressure in the manifold prior to ignition? Maybe thinking about this the other way, once you get off throttle, is there significant fuel in the system that needs a little more air so that the engine doesn't run too rich before you get back on throttle?

Those ideas seem like a stretch to me. I do like the suggestion that flutter, and maybe overall crappy air flow at low opening angles makes it more driveable if you sort of ignore that area of the curve.

I REALLY like Spencer's idea of going to a MAF and have some thoughts in my head about how to use the (currently unused) part from my 968. I have found MAF to AFM signal converters and it's tempting. If I ever go to a full ECU change, which I feel much more comfortable with now that I've made an engine harness and have a better handle on what's required for wiring, I'd love to go to ITBs. That may be a future project on a different car.

Cheers

 

when you close the throttle above ~1600rpm or so, the fuel injectors turn off so there isnt any fuel to burn off.
then once rpm drops below 1600 or so (somewhere around there) the injectors turn back on with throttle still closed so the engine can catch itself, expecting to fall back to idle speed.

 

Hi Zirconocene.
Any pressure build up in the intake manifold on startup depends on intake vs exhaust overlap. The barn door AFM is not air tight at closure, so on intake stroke there is airflow in. If it was air tight, the intake stroke would produce higher vacuum than if not air tight. With valve overlap, the question is on the exhaust stroke does the back pressure flow fast enough to pressurize the intake for
milliseconds, also considering the airflow into the intake, under vacuum, after the intake valve closes.

My understanding is Joe, F9Tech, is engineering a MAF to be integrated into the F9Tech Sport DME.

 

@T&T Racing : I've been super jealous of the F9Tech development that's been going on for all the other 944s; I like everything that I read about the path he's taken. I'll keep waiting for something to work on the 3L engines, goodness knows that there are enough other things to fix and improve before I get to that point. I'm subscribed to his development thread and try to keep tabs on all the new things that get rolled out.

Cheers

 

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