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Discussion Starter #1
I contacted Haltech to ask about changing the AFM to a Lexus one and was told that I could just toss it and use a 1k resistor to fool the stock ECU. The F3 unit on my car uses MAP from the manifold pressure sensor and Throttle Position Sensing to drive fuel and the AFM is not even in the loop.
I would love to just dump the AFM and just plumb a cold air pipe from the filter under the headlight. Please tell me if anyone has done this, It may save me a few dozen dyno runs to sort this out.
 

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well you can't just use a 1k resistor because the afm in these cars are actually square wave generators. i don't know what happens if you don't connect the afm, but it certainly uses it for timing. i can't really think of anything right now, but if you will have a bit of time and money soon, i'd try to come up with something very temporary and get an ignition controller, see if you can score a haltech ig5 to accompany your f3, or preferably sell the f3 and go full standalone.

hm maybe you could somehow try to get a pulse to the ecu that varies with RPM as a bandaid solution. at idle karmann freq is what 18hz or so? so like 18hz at 750rpm engine speed? that's like 1100rpm at 750rpm. hm. if you could ground the ECU's AFM line for every time any coil was fired, you would get a 3:1 karmann freq to engine speed ratio which might be a reasonable estimate. you might want to actually have the karmann freq have that higher ratio to 'simulate' boost.

it really won't be a meaningful respresentation of flow on a boosted car but hey it might be a start.

i don't know much about electronics, but maybe you could have the Ks wire from the ECU to the AFM cut from the AFM, and hook it up to all three of the ignitor's coil outputs through a resistor or something. the ignitor will "fire" similarly to how the AFM 'fires.' the only thing that would be different is the pulse width, but it might not matter if the frequency is the same. don't hold me responsible if you try it... just an idea.

shiva
 

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Discussion Starter #3
It was my impression that the afm is there to tell the ecu how much air is coming in so it can know how much fuel to mix with it. Hence the change in injectors to 25% larger ones to correspond with the 25% more air the Lexus AFM flows. Since the haltech handles the injectors according to the fuel maps you provide based on the Manifold Absolute Pressure it detects, the main goal of fooling the ecu regarding the AFM is to keep the check engine light from coming on. The Haltech actually has its own injector harness and the ones from the ECU harness are not used. I am not aware of any influence this has on ignition timing, and have not seen it mentioned in this context before.
 

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i'm pretty sure the ECU uses the afm as a basis for its ignition timing maps as well, it does not use it solely for fueling. most importantly for your engine, it might use it to pull timing under high load. at other load conditions the way it controls timing will likely be very important for how responsive your engine is and how much power its making.

hey if all it is is a check engine light go ahead and try it but be VERY careful listening for knock. i do not know exactly how the computer will control timing in the abscence of an afm but you know what there's always a chance it will do it by fixing timing to like 10deg which will kind of suck pre-boost (unless perhaps you add a lot of fuel with your haltech and try to do an anti-lag or something...)
and who knows what else it might do-- i'd be worried about its just being totally off in the map and advancing timing waay too much under high load.

just be careful

shiva
 

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damn i didnt know much about karmann freqs but judging from a post i just read it will be waaay higher than 3 x engine speed under load so hooking it up to the ignitor probly wont work

if you run it with no afm, just be careful with knock.

can the engine run ok with the ecu diagnostic check connector thingy jumpered? jumpering it does hold timing at 10deg.

one thing you might want to do if it is the case that the ecu will run way too much ignition advance under load with no afm is maybe jumper that connector under certain conditions.

the haltech has an output for nitrous or something, right? something that comes on at a user-defined MAP/TPS/RPM threshold? if so maybe you could hook that up to a relay across the check connector so on boost timing's at 10deg and you dont have to worry about knock.

that said, i still think you're better off getting a lexus AFM. proper ignition timing's going to be worth $150 and a drop in power from an added intake restriction.

gl

shiva
 
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