Its super obvious once you give it a go, but I couldnt find a writeup so here it is.
All na/nb chassis cars are a little leaner than preferred up until 4000rpm, where the ecu goes open loop and runs like an aftermarket ecu, ignoring emissions controls etc and then it gets far richer than perfect.
This next extremely accurate and highly technical image shows where the car is too lean and too rich at wide open throttle (WOT). The graph reads left to right from 1000rpm to 8000rpm.
Why is this important? Well if you dont baby the car the fuel economy will suck, cos past 4k the car is about 30% too rich, yep, 30%. And of course this costs horsepower also. A decent amount, this is a big part of why an ecu alone is worth 10hp and another 4 with the AFM restriction removed all together.
You can see the graph also happens to look a lot like what wide duration cams will do for your horsepower, a bit less before 4-5000rpm and then a fair bit more. So a set of decent sized cams happens to run quite nicely on a stock ecu, because the fuel map is basically tuned for them.
What I'm going to explain will affect the WHOLE rpm range. So if you want the top end leaner, the bottom end which is already on the leanish side will get worse, and if you want the bottom end richer, the top end will also get richer, super duper rich infact.
This next picture is the stock AFM with the black 'D' shaped cover off it, it simply pries off with a blade or flathead.
The green circled area shows a gear, to which the clockspring is attached, and the factory self tensioning ratchet mechanism. The red circled bit is the potentiometer, also known as a variable resistor, this is the same as the throttle on kids slot cars, and also the volume knob on an analog stereo (one that doesnt click up and down volume but is smooth).
When the engine runs the flappy door inside the AFM turns the large black thing in the centre and this in turn changes the 'volume' on the potentiometer, and the ecu uses this to determine how much fuel goes in the engine. With the engine idling you can play with this with your fingers and make the car run rich and lean and stall the engine.
What we want to do is adjust the gear with the ratchet mechanism. It affects the spring pressure holding the potentiometer in the closed low fuel area (where its sitting in the picture).
CLOCKWISE WILL LEAN THE CAR OUT.
COUNTER CLOCKWISE RICHENS IT.
Simply push the ratchet spring away from the gear and move the gear one tooth at a time with a small screwdriver, and test, move again and retest until you are satisfied.
We adjusted PaulF's AFM using an Innovate motorsports LC1 wideband to monitor actual air fuel ratios, and turned the gear a further 8 clicks more than it had already self adjusted over time. This was a changed of about 2 points. IE: it was 10:1 AFR at WOT but now is 11.8-12:1.
DO NOT DO THIS WITHOUT A WIDEBAND ATTACHED AND SOME TUNING KNOWLEDGE.
This made an excellent difference to the top end you could feel immediately.
We were aiming for around 12.8:1 but the bottom end was just too lean to go any further.
So the moral of the story is that you can do it, you can improve it, but you will end up with a bottom end a little too lean and a top end a bit rich and a proper ECU can of course fix both.
Dann
