Removing AFM from FC TII

[pdtaylor18]

So having looked at how tiny an aperture the stock air flow meter is then has anyone tried to replace it with some a bit more unrestrictive.  I have a 3" bore AFM from my Nissan which uses elecronics compared to a mechanical/elect as in the FC and would be totally free flowing.   Surely apart from a switch for the fuel pump the rest is just a case of variable resistance to send a signal to the ECU.  Just how different can they all vary from car to car.  Apart from the shape they all pretty much do the same job.  Now before we start on Apexi ECU\'s which do away with a AFM, I will have a piggy backed stock ECU with a HKS F-con and GCC. Just thinking about all those that want to stay stock....ish but something as simple as this can give huge dividends, especially if its straight through from the turbo.
How does the FD get its signal of airflow?
I just want to get rid of the AFM and replace it with something larger or get rid of it totally, just need someone to tell me how it can be done.

[Prof]

Some good points raised there Pete !
 
I\'ve looked into this quite a bit, as the eManage offers the facility to both replace the AFM with another model or eliminate it all together.
 
When it\'s eliminated, the eManage provides a map that uses MAP and RPM as it\'s x and y factors. This is how the FD works in any case.
 
The AFM or MAF or whatever you want to call it has 2 versions on the FC. They both have a variable resistor about 1K that\'s referenced to both a 5V ref from the ECU and ground. The variable tap of the resistor is the input to the ECU. However, there is also a thermistor to measure the temperature so that corrections can be made to the air mass based on the temperature of the air at the inlet.
 
The earlier FC\'s had a paddle type AFM which could only be mounted upright, as there was a cut out switch for the fuel pump built in, and the flapper needed to operate in the horizontal plane. The new FC\'s (Turbo II\'s etc) had a cone type AFM and flow better and also can be mounted in any orientation.
 
If you were to replace the AFM, the Nissan hot wire type is probably the best alternative with regards to air flow, as it\'s just a pipe with a bit of wire in it. The air flowing past the heated wire cools it and changes it\'s resistance - which is what\'s measured. The beauty of this is that colder air will cool it more, so air temp correction is factored in automatically. This can\'t be substituted directly as it\'s only a 2 terminal device, and the resistance of the wire will be much lower than the resistance of the stock AFM\'s. Also, I don\'t know how much current the stock ECU can provide on the 5V line. Some form of adaptor would have to be made to use this type.
 
The other type of AFM provides a frequency output - and that won\'t work directly either. It\'s probably no better flowing that the stock ones either - so not worth the bother of looking into.
 
The one problem I\'ve had with the eManage is that it doesn\'t seem to provide the \'sensor present\' signal or resistance to the stock ECU - even though the AFM output voltage matches and tracks the AFM signal. This means the stock ECU throws an error code 8 and runs pig rich
 
Once I figure out what the stock ECU is looking to see on it\'s input terminal, I should be able to work up a circuit to replace both the thermistor input - although that could still be fed in using a suitable thermistor - and the variable resistor.
 
Suitable replacements would be :
 
Nissan 300ZX AFM
Nissan Skyline or 200SX AFM\'s (smaller than the 300zx version)
Nissan Q45 - that\'s a big beast.
 
There are also AUDI and VW AFM\'s that use the hot wire system.

[pdtaylor18]

My one I have came from a Nissan Largo but its the same one for the Terrano.  I agreee that the hot wire one is best for a lot of reasons..  I might try the outputs on the largo and see what they are and compare with a stock FC AFM.  If I can get some data then maybe a simple resistor can be wired in to fool the ECU in seeing what it thinks it should see.  This could also be used to fool the emissions bit at idle as with  a lot of the hot wire AFM\'s they have no mixture screw like the TII has on the AFM.  We will need some way of getting the mixture right.  At the end of the day they sell the aftermarket resistors to wire into the AFM wires to get some more BHP so plenty out there if we know the right one.

[pdtaylor18]

Wow, this is worth a read, got really baffled by some of it and I got taught fluid dynamics and propulsion science by the RAF !! Not sure if it proves too much as the AFM of a 929 actually ended up with the same internal diameter or at least volume of air coming from it.
http://www.rx7club.com/showthread.php?t=509627&highlight=replacing+AFM
If this bit about replacing the AFM being pointless is true and the only way is to go standalone to get the bigger gains then i would still want to investigate the idea of doing away with the AFM without an expensive standalone.

[Prof]

I\'m guessing on a stock or mildly modded engine this won\'t make any or hardly any difference, but with larger power engines, it could be worth investigating the change.
 
It\'s only going to be useful to people retaining the stock ECU and going with a piggy back fuel controller or all rounder like the eManage, as if you want serious power and a huge turbo then you\'ve probably gone down the Power FC or Microtech route anyway.

[pdtaylor18]

yep you are right, think they said the stock AFM is good to 300 maybe 400 BHP anyway, plenty of other restriction to bog things down. I am only looking for 300 maybe 350 so for me it will stay until someone comes up with something. By the way I am on nights so hence the 0245am posting on the previous thread.

[Prof]

found this useful site - good explanation of the AFM operation:
 
http://www.picoauto.com/tutorials/fuel-injection3.html

[pdtaylor18]

Got some figures for you, when the AFM is closed it pushes 3.9v and when fully open it drops to 0.26v. Not sure if it will help your side of things. These were only what someone found on their AFM and not necessary the correct ones but its a guide.
your connector was posted this morning by the way Dave.

[Prof]

Cheers for that Pete.
 
That thread on Rx7 club takes a lot of reading  Still some good info on it.  Seems there are quite a few threads out there on doing this.  I might try and figure out a circuit to drive a hot wire, and see if a sensible output can be munged or processed into something the stock ECU would understand.  I think the other half still has a spare 300zx MAF to test with, but I might just get some resistance wire and make my own