Look at it this way remember the gas law?

PV = nRT

MAP sensed systems guess n which is the amount of air going in. It's n that affect the mixture ratio. MAP systems guess n from knowing P and T - they assume V - the volume So any change in V from those that where known when the ECU was burnt in cannot be accounted for.

Looking at AFM systems:

They know V, they know T, and P = atmospheric at the AFM so they can calculate n correctly every time. If the ECU is mapped right you shouldn't need to change the ECU at all until the AFM reading max's out. The ONLY let downs of an AFM system is sensor restriction and patents on the AFM design. That's why (correct me if I'm wrong) Supra TT's in the US can push 500+RWHP on the stock ECU while say an RX-7 -TT (MAP sensed) needs an ECU after about 250RWHP.

LOL I'm such a nerd.

Short answer:
Stock for stock MAP sensed = best / AFM sensed = 4RWHP less
Modded vs modded: AFM sensed = best / MAP sensed = BANG!

In saying AFM I am assuming you mean a MAF, aka Mass Airflow Meter, which is a hot wire anemometer. If so, your theory that pressure is consistent at the inlet of the AFM sensor is incorrect. There are fixed restrictions like the filter, airbox and plumbing ahead of the MAF sensor that produce pressure drops. The pressure drops vary by the mass of air passing thru them. At idle the pressure drop can be nearly zero. At the power peak the air drop will be significant.In addition, atmospheric pressure varies day to day and much more significantly by altitude.

The reason a MAF system is so good is that they measure the actual 'mass' of air and therefore know the mass of fuel required.

Not quite. Most people in australia use AFM. Thats because it refers to all types of air-flow sensing. Hotwire is the most common but some companies (eg toyota) use Karmann Vortex and other use (shudder) variations on the old trap-door method.

As for atmospheric variations - yes you're right. Most ECU's (Bosch, Haltech, MoTeC) even my EMS use a seperate pressure sensor to compensate for those atmospheric variations. You can't escape the fact that AFM (or MAF if you like) systems can compensate for an increase in the flow characteristics of the engine.

I am not sure of GM's hotwire sytem, but I do know that Toyota's KV system compensates for the pressure drop of the upstream intake (eg filter) at the sensor.

-pete

Pete,

The original MX-5's used the old trap door setup. My '99 MX-5 uses a hot wire AFM to improve accuracy and I imagine the RX-8 will use a hot wire AFM.

Originally posted by babylou
...I imagine the RX-8 will use a hot wire AFM.

Mazda has used MAP on the rx-7's (Turbo I excluded) and hotwire on the Cosmo's so it's anyone's guess , but I hope you're right.

-pete