I am in lockstep with everything Bob said, Mike. I would add a couple of additional thoughts. The Megasquirt ECU will allow you to run either a Speed Density style system or a Mas Air Flow, MAF based system.
The Speed Density style system is tuned by modifying the VE (volumetric efficiency) table essentially until the ECU is providing you the smallest fuel trim compensations in closed loop operation.The MAF based system is sort of cool because the MAF actually reports back to the ECU the lbs of air being drawn into the engine. Then based on your commanded AFR the ECU calculates an injector pulse width to deliver the proper amount of fuel to the cylinders to produce the AFR you commanded in the tune.
The MAF based system sounds like its the plan. The challenge is most MAF suppliers either do not provide or provide a MAF transfer function that both you and the ECU soon discover is somewhere out in left field. That's a frustrating clean up that you are left to do in your tune.
The best MAF supplier I have found is a place called Pro-M Racing. It was founded by one of the original fuel system calibration engineers at Ford that designed the Mass Air Flow systems that Ford still uses today. The reason for the MAF based system preference at Ford is the need to meet emissions standards anywhere in the US without the need to retune. High altitude, low altitude, Death Valley in the summer or Nome Alaska in the winter — the MAF based system works correctly everywhere, always, all the time.
So where is the rub? Why doesn't everyone use a MAF based system? The MAF based system challenge turns out to be a composite problem made up of the squirrely MAF suppliers and the actual meter geometry along with a poorly calibrated mass air meter.
The Pro-M guys do it the same way that Ford does it because the founder, as luck would have it, is one of a hand full of guys that actually noodled it out at Ford for Ford. Sadly he has passed on but his original partner and current President, Chris Richards, carries the tradition on for him. Here is a short video clip showing the difference in signal quality for one of their design MAFs vs a more traditional aftermarket tonsil design available just about everywhere. Click here =>
Pro-M MAF Signal Quality.
A significant challenge for EFI systems is the quality of the voltage signal from the MAF. As the voltage bounces up and down the ECU will continuously chase it, recalculating injector pulse widths to compensate, so the proper air / fuel ratio is maintained. The snafu is the air flow is not really changing, the air flow meter is just not capable of providing a clean signal to the ECU. The continually moving mass air target creates havoc with the fueling logic in the ECU depending on how well the ECU can mute or if you will smooth the voltage perturbations from the MAF.
The second video (it follows the first one) speaks to, among other things, reversion at the MAF attributable to a cam with adequate overlap to give us our rumpity rump idle. As the camshaft overlap causes the air flow to change direction at idle in the MAF, traditional MAFs essentially double or triple count the air mass moving back and forth over the meter tonsil causing the EFI to add more fuel and you guessed it, surge back an forth from rich to lean at idle. The fuel delivery see sawing makes idling the engine more challenging that it needs to be. The second video right after that first one speaks to how the Pro-M meter design defeats the reversion problem.
A MAF will add about $400 to the cost of your EFI system but after you first use one you'll never want to do build anything but a MAF based system.
Ed
p.s. another nicety that's free for nothing is the ability to change intake manifolds, throttle bodies, heads and even camshafts and the tune remains the same because everything you just did only affects the volume (mass) of air the engine injests and of course the MAF catches the change in air flow, reports it to the ECU and the ECU makes the proper fueling corrections all without any intervention on your part — pretty cool, all things considered ...