ECU maps & AFM voltage and MAF conversions (long)

[Jeff N.]

OK - here's a summary of some stuff that I've been trying to muse through as I ponder the effect of the MAF conversion on my setup. I think these are trueisms....challenge as you wish. Posted for interest and to stimulate thought.

All related to the M30 setup.

1 - the AFM has a voltage range of basically 0 to 5 volts. The output voltage is based on the angle of the flapper door.

2 - the AFM flapper door goes to a maximum angle before before the engine reaches maximum RPM. Exact point is not 100% clear - I've heard suggestions that this occurs at about 3000 to 3500 RPM at WOT. Never tested it though. I have no clue at what point, if ever, the AFM reaches max flapper angle at anything less than WOT.

3 - The ECU selects the correct fuel map (mid map vs WOT map) based on the TPS. If the TPS stays WOT, then the ECU ignores the AFM flapper and bases fuel and advance on the engine RPM only.

4 - If the TPS has not signaled WOT, then the mid map is used and the fuel and advance settings are a function of the engine RPM and the flapper door angle. The flapper door angle is basically synonomous with the "load" on engine. This is sent to the ECU as voltage between 0 and 5 volts.

5 - A MAF sensor replacing the AFM has the ability to "process" more air flow than the AFM. This means that a MAF can measure more air passing thru the unit before it is maxed out.

6 - When either a MAF or AFM is maxed out, it will send 5 volts to the ECU.

7 - For a MAF kit to correctly "work" in the 535 the following must be true. For a given amount of airflow, the output voltage of the MAF must be the same as the output voltage of the AFM. If the MAF sends less voltage for a given amount of air, the ECU assumes less load and the mixture will tend to be lean. If the MAF sends more voltage, the ECU assumes more load and the mixture will tend to be rich. Timing will also be effected.

8 - The MAFs ability to spoof the ECU via changing the "load" (voltage) gives you a basic ability to "tune" your mixture. However, this "tune" only changes the relative lookup in the WOT and Mid throttle maps. The map profile contained in the chip is not modified or effected.


Sooo...there seem to be a couple of interesting thoughts or questions - assuming this is more or less true.

a - tuning your mixture by modifying the voltage output of the MAF relative to airflow can have unintended consequences on the timing. The more you spoof the ECU on the airflow, the more likely you are to have an unintended timing change.

b - it's likely better to modify the fuel and ignition maps to be correct for your (modified) engine and do only very nominal fine tuning via the MAF.

c - The best MAF setting is as close to a 1 to 1 relationship as you can get with the AFM. The only way to calibrate your MAF voltage to the AFM voltage is to compare the two on a flowbench. (BTW, this is done for you with a base 535 voltage curve with the Racing King MAF).

d - I think - At less than WOT on the TPS, the AFM flapper must not ever reach max angle. At not WOT on the TPS, you are running the mid range map and you need two inputs to locate the correct fuel and timing value - load and RPM. If the flapper reaches max angle while the engine conditions are still changing, there is no way for the ECU to measure and adjust mixture and timing. The purpose of the WOT map, which determines values ONLY based on RPM, is to account for the flapper door going to max angle.

BTW - I really like the MAF kit on my car. There's no question the car runs better. I'm just trying to get my arms around what tinkering with the voltage table in the control box really does.

Hmmm...ok. That's enough for now. Thoughts?

Jeff

[Robin-535im]

Great thoughts Jeff. Been thinkin' some myself...
1 and 2 sound like I expected.

3 - The ECU selects the correct fuel map (mid map vs WOT map) based on the TPS. If the TPS stays WOT, then the ECU ignores the AFM flapper and bases fuel and advance on the engine RPM only.

So then WOT map should be more customized to your particular setup than mid-map because there's only one free variable, RPM. Mid map can be more generic and tweaked by MAF settings.

4 - If the TPS has not signaled WOT, then the mid map is used and the fuel and advance settings are a function of the engine RPM and the flapper door angle. The flapper door angle is basically synonomous with the "load" on engine. This is sent to the ECU as voltage between 0 and 5 volts.

So - you want to tune the mid map to match fuel and timing assuming correct MAF readings, then tweak MAF at the dyno to optimize...

5 - A MAF sensor replacing the AFM has the ability to "process" more air flow than the AFM. This means that a MAF can measure more air passing thru the unit before it is maxed out.

So you get a finer, more accurate measurement of the air flow, meaning that your fuel/timing maps can have more detail to them... i.e., be tuned more tightly to your car's dynamics...

7 - For a MAF kit to correctly "work" in the 535 the following must be true. For a given amount of airflow, the output voltage of the MAF must be the same as the output voltage of the AFM. If the MAF sends less voltage for a given amount of air, the ECU assumes less load and the mixture will tend to be lean. If the MAF sends more voltage, the ECU assumes more load and the mixture will tend to be rich. Timing will also be effected.

Conversely, you can tweak sub-optimal settings in the mid map by spoofing the ECU - if the map is too lean at a certain flow, you can tweak the MAF reading the give you a better mixture. - wait, that's what you say in 8:

8 - The MAFs ability to spoof the ECU via changing the "load" (voltage) gives you a basic ability to "tune" your mixture. However, this "tune" only changes the relative lookup in the WOT and Mid throttle maps. The map profile contained in the chip is not modified or effected.

Sooo...there seem to be a couple of interesting thoughts or questions - assuming this is more or less true.

a - tuning your mixture by modifying the voltage output of the MAF relative to airflow can have unintended consequences on the timing. The more you spoof the ECU on the airflow, the more likely you are to have an intended timing change.

b - it's likely better to modify the fuel and ignition maps to be correct for your (modified) engine and do only very nominal fine tuning via the MAF.

c - The best MAF setting is as close to a 1 to 1 relationship as you can get with the AFM. The only way to calibrate your MAF voltage to the AFM voltage is to compare the two on a flowbench. (BTW, this is done for you with a base 535 voltage curve with the Racing King MAF).

d - I think - At less than WOT on the TPS, the AFM flapper must not ever reach max angle. At not WOT on the TPS, you are running the mid range map and you need two inputs to locate the correct fuel and timing value - load and RPM. If the flapper reaches max angle while the engine conditions are still changing, there is no way for the ECU to measure and adjust mixture and timing. The purpose of the WOT map, which determines values ONLY based on RPM, is to account for the flapper door going to max angle.

I'm with you, except for c should be "best starting setting" so you can tune it later. The biggest question I still have is how does the O2 sensor change it? I believe the O2 sensor operates closed loop, controlling the exhaust gas to the stoiciometric balance best for emissions by tweaking either the timing or fuel or both. This tells me you'll negate the changes your chip made if the chip was trying to put you at a mixture tuned for power, not emissions.

Then again, the O2 circuit runs a 1Hz (one update per second) right? Maybe the ECU works faster, hence the O2 circuit can't catch up to it when the mixture is changing, i.e., when you're climbing through the RPMs. Once you stabilize, the O2 system tunes your engine back to better emissions.

What if the O2 sensor was disabled or spoofed too? Well, then you could tune the engine more, but probably dump too much fuel into the cat and burn it up. Maybe it's okay for WOT range, but you wouldn't want to run it off stoic all the time...

BTW - I really like the MAF kit on my car. There's no question the car runs better. I'm just trying to get my arms around what tinkering with the voltage table in the control box really does.

Hate to hear you say that, now I want one too...

[Robin-535im]

Then again, the O2 circuit runs a 1Hz (one update per second) right? Maybe the ECU works faster, hence the O2 circuit can't catch up to it when the mixture is changing, i.e., when you're climbing through the RPMs. Once you stabilize, the O2 system tunes your engine back to better emissions.

In fact, that might explain the pulsing I get when climing a gradual hill. 2nd gear, slowly accelerating up a hill, my car pulses, as if the mixture is being tweaked once a second... Maybe this is the lambda circuit fighting the EAT chip? Every one second, the mixture is tweaked back to the "better emissions" setting but the O2 system, but in between the chip dumps in a little more power.

[Martin in Bellevue]

I've been playing with the minute changes available with the maf software lately. The sharply rising curve from origin is intended to mimic the stock air box flapper resistance. It does seem to smooth things out in the mornings. I guess the whole curve sits a bit left of things due to the 21 pound injectors? The existing curve was taken primarily from the settings retained from the dyno session tune. We'll see how far things have deviated with another dyno session in a few weeks.



I'm not too interested in engaging in the motronic discussion as I seem to understand conflicting theories of its operation. I'd hope the O2 sensor controls things quicker than once per second? I'd also like to know the finite range of the stock air meter, as 5v may be off, I dunno.The stock air meter doesn't work off 12v as expected, but the 4.something, if memory serves. I suspect our MAF settings may be peaking higher than needed.

[Jeff N.]

I wouldn't be surprised that it's the lamda circuit trying to make adjustments. Without a good mixture meature, hard to know if the lamba circuit is trying to lean or richen the mixture.

Try it open loop and see what happens. Just unscrew the O2 plug underneath the starter.

Jeff

[Jeff N.]

I think that bundle curve is from when Derek sent in that AFM a while ago - before the next gen setup. Is that the curve from the dyno session?

I'm beginning to think 8 degrees is a bit too much advance on the cam. Top end is suffering a bit. Thinking we try either the 5 or the 7 setting on the Korman gear.

Set a schedule for your cam yet?

Jeff

[Jeff N.]

So then WOT map should be more customized to your particular setup than mid-map because there's only one free variable, RPM. Mid map can be more generic and tweaked by MAF settings.

Hadn't considered that. Interesting thought.

So - you want to tune the mid map to match fuel and timing assuming correct MAF readings, then tweak MAF at the dyno to optimize...

Yup. That's what I'm thinking. Only tweek via the MAF.

So you get a finer, more accurate measurement of the air flow, meaning that your fuel/timing maps can have more detail to them... i.e., be tuned more tightly to your car's dynamics...

I'm not sure the measurement is better. The map detail is fixed - there are a set number of data points in the map. I think the trick is to have the correct map values for a given lookup point.

I'm with you, except for c should be "best starting setting" so you can tune it later. The biggest question I still have is how does the O2 sensor change it? I believe the O2 sensor operates closed loop, controlling the exhaust gas to the stoiciometric balance best for emissions by tweaking either the timing or fuel or both. This tells me you'll negate the changes your chip made if the chip was trying to put you at a mixture tuned for power, not emissions.

Yes...best starting point is better phrasing. I think Martin's MAF curve looks pretty good.

I think the Lamda circuit tries to "control" the mixture by sniffing the exhaust output then skewing the injector pulse such that a particular stoic setting is reached. I believe the target for the lamda is based on what's best for the cat, not what's best for HP

Then again, the O2 circuit runs a 1Hz (one update per second) right? Maybe the ECU works faster, hence the O2 circuit can't catch up to it when the mixture is changing, i.e., when you're climbing through the RPMs. Once you stabilize, the O2 system tunes your engine back to better emissions.

Yeah...I wonder too. Never quite understood the learning aspects of the lambda system. Mark's explained it once or twice if I recall but I still haven't quite grocked it.

Jeff

[Martin in Bellevue]

The dyno curve isn't too linear & the bottom starts off high, as there isn't a way to change the initial measures manually with the pots. The previous posted curve is taken from that dyno session, modifying the initial measures near 0 volts to mimic the stock air meter.

Extended family has put the cam stuff off schedule for a few days.

[George M]

...on balance, I agree with what you wrote Jeff. I worked with Bruno on setting up his MAF and the few kits he sold. Your observation about setting the MAF up to replicate the AFM in terms of telling the DME what voltage pertains to volumetric air flow rate is a good one. The reason is the DME uses the AFM as only a reference point for air-flow into the motor and not a device for adjusting air-flow in accordance with ideal air/fuel mixture. That is the O2 sensor's job...to sniff the exhaust and determine combustion temperature and adjust injector duty cycle in accordance with how much air is being ingested per information provided to the DME from the AFM/MAF. There is one consideration however....and that is...the calibration of the AFM from the factory may not be idealized in terms of how much actual air is being drawn into the engine. For all intents however it is quite close and when Bruno was having the prototype MAF flow tested...the curve was derived based upon the stock AFM and not tweaked or offset accordingly. Fooling the DME with a different Voltage versus Airflow curve pays little dividend. Best to provide accurate air volumetric flowrate information to the DME and if desired to optimize air/fuel, best to do so with an EPROM change adjusting injection firing duration (duty cycle) in accordance amount of air reported by the AFM/MAF. In summary, the AFM/MAF is simply a sensor...best to leave its calibration alone which the DME relies upon to crunch air/fuel per the air fuel map encoded into the EPROM be it performance or stock chip.
George

[Jeff N.]

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