Tuning AFM

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TOPIC: Tuning AFM

Re: Tuning AFM 12 years, 1 month ago #14325

RangerGress
OFFLINE
Drivers Ed
Posts: 11

Sterling Doc wrote:

We can definitely change the A/F mixture a lot by messing with the AFM - we've seen this with my car, Dan Pina's, Neal Agran's, David Dirks', and it's not subtle. When I was chasing a what turned out to be a bad AFM wiper track earlier this year, I got my car so rich it made my eyes water, and shot flames out the back on over-run.

The question is does the O2 sensor screw these adjustments up over the long term. I can look at the Traqmate logs over the course of a several weekends when I get home, to see if it stays the same over time.

It is interesting that Dan's car was running so lean - the O2 sensor wasn't helping there, or at least didn't have enough adjustment to fix his problem, while adjusting the AFM does.

It's very important to discount dyno results that did not allow LTFT time to adapt, unless the purpose of the test was to do precisely that. The first time my AFM was messed with was by the guy that built that particular engine, #7 of 15, IIRC. The mixture seemed to richen up a little and I gained a couple hp. It wasn't until the DME had time to adapt to the that the lean problems started.

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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Re: Tuning AFM 12 years, 1 month ago #14327

Sterling Doc OFFLINE Endurance Racer Posts: 2102	But the lean condition happened with an uncracked/virgin AFM, on the first dyno run. The car had run all season in that configuration. Whatever adaptation was going to happen, would have by then. I'd think.
	Eric Kuhns National Director Emeritus 2007, & 2008 National Champion 2011, 2012 2nd Reply Quote

Re: Tuning AFM 12 years, 1 month ago #14328

RangerGress
OFFLINE
Drivers Ed
Posts: 11

Sterling Doc wrote:

But the lean condition happened with an uncracked/virgin AFM, on the first dyno run. The car had run all season in that configuration. Whatever adaptation was going to happen, would have by then. I'd think.

You changed input variables in a system designed to be adaptable. Maybe you drew conclusions based on #'s prior to the new adaptation.

Also have to control for reset of LTFT. Every time we turn our kill switch LTFT is reset.

Is hard to know what a virgin AFM is these days. Can we really be assured that the remanufacturered AFMs are properly calibrated?

Don't get me wrong, I don't have any perfect answers. I've gotten to the point where I 100% trust almost nothing when it comes to testing and engine management.

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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Re: Tuning AFM 12 years, 1 month ago #14332

Sterling Doc
OFFLINE
Endurance Racer
Posts: 2102

Here's some deep reading on this if anyone is interested. It would seem to imply the maps are static and the O2 sensor adjusts the output after it gets looked up on the fuel table.

Link is here:http://www.the944.com/lambda.htm, excepts below

--------------------------------------------------
"Lambda Inside" by FR Wilk ©2002

How the Motronic DME monitors Oxygen levels in exhaust gases

This page is dedicated to how the Motronic functions by taking the raw Lambda input and converting this signal so that the computer can understand it. Some computer programming and flow charting will be discussed. 'Oxygen sensor' or 'O2 sensor' are other commonly used names for the Lambda sensor.

The Lambda is a minor system in the Motronic. It is an option, an add-on. As such, it is not a critical system. It is a smog device and in no way does it enhance an engines power. Just the opposite. All that it is doing is fine tuning the fuel to reach an ideal air/fuel ratio. This original value of fuel is read from maps just like it is without the Lambda system installed. It is only capable of making minor changes to the fuel level

The basis ML1.1 system, the early Euro 944, has no Lambda system. The air flow sensor and speed sensor are used to determine fuel flow from a map that is stored in system memory.

Next to it is the ML1.2 Lambda system flow chart, the early USA 944, which has a Lambda sensor (O2 sensor). The fuel level is a two level control process, map and Lambda control. The Lambda system also uses the air flow sensor and speed sensor to determine fuel flow from a map. The Lambda control makes positive or negative changes to the fuel level in order to fine tune the fuel output. The range of the Lambda control is quite limited which requires the fuel map to contain values very close to Lambda=1 (air/fuel ratio 14.7). Comparing the maps of a non-Lambda (Euro 944) and Lambda system, the Lambda maps will appear flat and unimpressive. The power is in the non-Lambda map.

The fuel map is always used with a Lambda system. One common but false saying is that the fuel map is no longer used when the car is equipped with an Lambda sensor. This is far from the truth. Where does the original fuel value come from? Is it a guess? No. Any change in load or engine rpm, the DME must go to the fuel map to get its initial value. Then the fine tuning begins. The Lambda sensor subroutines take charge and the fuel is continuously toggled back and forth. If the Lambda sensor is disconnected, the Motronic gets it fuel levels from the map only. It never just "goes rich to protect the engine" which is another famous and incorrect saying. When the Lambda is disconnected, it uses the fuel map.

Eric Kuhns

National Director Emeritus

2007, & 2008 National Champion
2011, 2012 2nd

Last Edit: 12 years, 1 month ago by Sterling Doc.

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Re: Tuning AFM 12 years, 1 month ago #14333

944Racer72 OFFLINE Endurance Racer Posts: 748	The kill switch is a very interesting point. It didn't occur to me that we "unhook" the DME every session (at least I do). I use the kill switch to stop battery drain after shutting down the car every session.
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Re: Tuning AFM 12 years, 1 month ago #14334

Bottoz OFFLINE Seasoned Racer Posts: 205	So... LongTerm FuelTrim isn't an issue, right?
	C.J. Botts 1984 944 #37 2007 RM HPDE Driver of the Year 2010 RM Instructor of the Year 2013 RM 944SPEC Director Reply Quote