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Tuning AFM
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TOPIC: Tuning AFM

Re: Tuning AFM 10 years, 3 months ago #14358

OK, I took a look at at A/F ratios over several events. The most interesting was and enduro. I looked at A/F ratios at the start of the race, before the pitstop, and right after the pitstop (master power off). All looked stable, whithin a few tenths.

Interestingly the air fuel stayed flat through 125 MPH, when the ram air is really working, too.
Eric Kuhns

National Director Emeritus

2007, & 2008 National Champion
2011, 2012 2nd
Last Edit: 10 years, 2 months ago by Sterling Doc.

Re: Tuning AFM 10 years, 2 months ago #14713

  • tcomeau
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Guys, this LTFT is news to me. I've never seen or heard anything about it for our 944's in the factory DME pubs. I'm not convinced it applies.

Re: how turning the spring loaded black wheel in the AFM affects mixture at WOT. Here's my best guess, and it's simple.
Think about the system. WOT is determined ONLY by the thottle position switch. That gives A/F ratio control from AFM back to DME and its set maps.
But what did we do by adjusting the AFM black wheel? That is the spring force that determines how easily the door opens in the AFM per x amount of airflow in. If we tightened that spring (richer?), the door won't open as far per the same amount of airflow in. In effect, we're richening the mixture at WOT by slightly restricting the possible airflow in. Think about if we tripled the spring tension and the door was only able to open 1/3 of the way at WOT. This is the only connection I can make between the black spring wheel and WOT performance.
IF THIS IS CORRECT, We're adding an air restriction to the throat of the engine to richen A/F ratios... which makes me believe that we should be adjusting A/F ratios at the 5mm set screw (CO) on the AFM. A better A/F ratio makes more power, yes, but a better A/F ratio PLUS more air flow should mean more power sooner. I already know from testing that the AFM is the biggest limiting factor for power in our engines. We make considerable mixture adjustments using that set screw during smog tests.
Does that make sense? Any nay sayers? Other ideas?
Tim Comeau
SoCal 944 Spec #22 since Feb 2003.
Let's keep building it!

Re: Tuning AFM 10 years, 2 months ago #14714

  • tcomeau
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Also, wouldn't adjusting the FQS as well as the CO set screw yield cumulative results?
For example, go to FQS setting #1 for making fuel 4% richer, then also adjust Set screw clockwise (lessen air bypass) on AFM for even richer setting?
Or open bypass to let even more air into the engine easier, and go to FQS #3 for 6% fuel increase?
......or just switch to carbs
Tim Comeau
SoCal 944 Spec #22 since Feb 2003.
Let's keep building it!

Re: Tuning AFM 10 years, 2 months ago #14716

tcomeau wrote:
Guys, this LTFT is news to me. I've never seen or heard anything about it for our 944's in the factory DME pubs. I'm not convinced it applies.

Re: how turning the spring loaded black wheel in the AFM affects mixture at WOT. Here's my best guess, and it's simple.
Think about the system. WOT is determined ONLY by the thottle position switch. That gives A/F ratio control from AFM back to DME and its set maps.
But what did we do by adjusting the AFM black wheel? That is the spring force that determines how easily the door opens in the AFM per x amount of airflow in. If we tightened that spring (richer?), the door won't open as far per the same amount of airflow in. In effect, we're richening the mixture at WOT by slightly restricting the possible airflow in. Think about if we tripled the spring tension and the door was only able to open 1/3 of the way at WOT. This is the only connection I can make between the black spring wheel and WOT performance.
IF THIS IS CORRECT, We're adding an air restriction to the throat of the engine to richen A/F ratios... which makes me believe that we should be adjusting A/F ratios at the 5mm set screw (CO) on the AFM. A better A/F ratio makes more power, yes, but a better A/F ratio PLUS more air flow should mean more power sooner. I already know from testing that the AFM is the biggest limiting factor for power in our engines. We make considerable mixture adjustments using that set screw during smog tests.
Does that make sense? Any nay sayers? Other ideas?


I have never adjusted the set screw on the AFM so can't comment. I've read a number of references that said "don't screw with it" and, uncharacteristically, actually did restrain myself from screwing with it.

If there is no LTFT than I agree with your analysis of the effect of adjusting the AFM spring. As you implied tho, we're really talking about mid-rpm operation because because once the AFM door is fully open (~4500rpm?) it doesn't matter what it's spring rate is.

Re. "do 944's have LTFT." Maybe it syncs with version of AFM. That is to say since e30's have LTFT with the late AFM, if you are running the late AFM it's plausible that you have LTFT too. Wilk said that '86 was the AFM change-over year. That would mean that within your series there could be meaningfully different versions of engine management. Is that possible?
www.Gress.org
"All that is necessary for the triumph of evil, is for good men to do nothing. -E. Burke

Re: Tuning AFM 10 years, 2 months ago #14722

In practice, at least the 88 ECU's seem to respond the same to tuning as the earlier versions, and the A/F's don't seem to wander over time, which you'd expect to see if LTFT were an issue.

Dave Dirks did some testing on the AFM, and I don't believe he found the AFM door ever wound up fully open/against the stops. If it did, adjusting spring tension would have no effect on AF ratios at WOT & high RPM, which it clearly does. It seems for our application the AFM has enough headroom to not max out with the airflow our motors can generate.

Much to my chagrin, I know the E30's make significantly more HP out of their 2.5 liters, so maybe they flow enough air to max the AFM (presuming E30's use the same AFM).
Eric Kuhns

National Director Emeritus

2007, & 2008 National Champion
2011, 2012 2nd
Last Edit: 10 years, 2 months ago by Sterling Doc.

Re: Tuning AFM 10 years, 2 months ago #14727

Sterling Doc wrote:
In practice, at least the 88 ECU's seem to respond the same to tuning as the earlier versions, and the A/F's don't seem to wander over time, which you'd expect to see if LTFT were an issue.

Dave Dirks did some testing on the AFM, and I don't believe he found the AFM door ever wound up fully open/against the stops. If it did, adjusting spring tension would have no effect on AF ratios at WOT & high RPM, which it clearly does. It seems for our application the AFM has enough headroom to not max out with the airflow our motors can generate.

Much to my chagrin, I know the E30's make significantly more HP out of their 2.5 liters, so maybe they flow enough air to max the AFM (presuming E30's use the same AFM).

It would be interesting to pop the top of the AFM and put a video camera on it while on a dyno. The camera would have to be synched with the data logger so you could later overlay rpm on to the video. Then one could see sweeper hand deflection as a function of rpm. Would be even cooler to do it a couple clicks in each direction so one might actually see the change in door movement vs. rpm.

Or is your AFM upside down like a BMW M30 motor (80's 533i & 535i)?

We may put out a bit more hp, but we'll never be a Porsche. I've had 3 911's, one of which was my DE car. I'll always love Porsches.
www.Gress.org
"All that is necessary for the triumph of evil, is for good men to do nothing. -E. Burke
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