Beating the Baro... MAP reading trickery?

[GasSavers_RoadWarrior]

Hi folks,

An interesting little nugget popped up the other day while I was looking for something else and I've been tossing it round in the noodle since. ECUs that use a MAP sensor generally take the reference barometric reading as soon as you switch on the ignition. This tells it whether the air is denser or thinner than normal and it adjusts fuel trim leaner or richer accordingly. It also will re-callibrate to atmospheric when you go wide open throttle (WOT) because plenum pressure should be atmospheric at that point.

Anyway, that got me thinking that if it sees a lower pressure at ignition on, then it's gonna run leaner until it re-callibrates (and it may do that while running according to O2 feedback, but we may as well start off with an "edge" right?) So, my usual routine at starting up would be to turn the ignition on, check the idiot lights, then start the motor. This gives it 5 seconds to see what the real pressure is... we can't have that can we? muhuhahahahaaaaa

So, I've been putting the key in and twisting it to start immediately, figuring that if the ECU is slow getting it's reading, the motor will be pulling some vacuum before it gets it, leading to a lower pressure reading, leading to a leaner initial fuel trim when it goes into closed loop.

I don't know if this is going to make THAT much of a difference, and I probably won't be able to tell at the moment how it's doing, but it kinda feels like a good thing to be trying.

Also been wondering if one could put a check valve on the MAP vacuum line, that you can work from the cabin, just before you shut off, you close it... leaving the MAP line in vacuum, then when you start up, you leave it closed until the engine fires, then open it up again... I don't know if that would be too extreme a value though and the ECU would mark MAP bad and use defaults, because it would probably be like you were flying at 30,000 feet.

I'm thinking this might be something to bear in mind for those of you that do engine off coasting, with a MAP based system, if you turn the ignition back on immediately it might retain the MAP value, also if you pop the clutch to restart the motor after you turn the ignition on, it gets a real baro reading. If you pop the clutch and then turn on the ignition, it might read the vacuum as the baro value and run leaner (Or not at all if you're pulling real high vacuum at this point) Those with vacuum gauges might be able to figure an optimum feather point for the throttle on restart such that the ECU believes the baro value and is capable of running on it.

Those who are electronically minded might figure out a resistance bias to the MAP reading that is set when you turn the ignition on for the first time, then is unset by a relay kicking it out when you engage the starter.

Anyway, thought this would be something interesting to play with.

Road Warrior