Silly elevation question for MAP only cars

[kingtal0n]

I just thought of something


When I first turn the ECU on it samples the MAP, right? And then it records that as atmospheric pressure- 0psi.

So what happens when I drive up 3000 feet on a mountain, and it samples the air there and sets that to 0psi. Won't it be wayyyyy off?

[cobaltssoverbooster]

if no baro then it will be way off. thats what the trims are for.
That exact method is how edelbrock, fast xfi, and others run their carb conversion systems. initial map read and o2 correction.

[kingtal0n]

What do you mean if no baro. Are you saying some cars have an additional sensor?

[Rocko350]

edited and moved it down.


Chris

[Ben C]

Yes, some cars have an additional baro sensor. Lsa engines do for example.

[Ben C]

There is a baro update setting in the ecm. Usually around 3600. So if you go WOT or make a 75% throttle upshift, it will relearn the new baro setting during the drive. This needs to be defeated to a much higher rpm while tuning boost for obvious resons. you will be in positive pressure when the ecm tries to relearn and the fueling will go real rich on the next throttle transition.

Chris

You have any more info on that? I can't understand how that would work, and never seen it mentioned before.

[Rocko350]

Ben C

I'm gonna edit that for clarification. I have it in tunercat 2 and can confirm with snap on scanner it works. Looks like we don't have it here in hp tuners.

There is a baro update setting in the ecm. Its not in Hp tuners that i've seen. In Tunercat 2 (jet DST) its usually set to around 3600rpm and have to raise it to a fairly high number like 7000 so we don't go dead rich after a wot run and return to idle in boosted engines. No reason to mess with it or want to . The newer processors from 01 and up are more than capable of adjusting for the baro changes during normal fueling corrections during a drive cycle.

Chris

[kingtal0n]

Whats funny is how I got around to thinking about this in the first place.

Sometimes, my engine needs to crank twice to start. I let it go 1-2-3-and let go of the key expecting it to fire to life. But sometimes it needs to go all the way around (maybe the cps 'stops' right on top of the trigger tooth sometimes).

So when this happens, I guess when I re-key (re-crank the second time) the barometer reads the engine vacuum from the PREVIOUS cranking event (the first, failed crank) and when the engine roars to life the second time around its completely rich across the board because it read that "vacuum" was 0psi I guess, and now any "vacuum" is actual vacuum MINUS the difference from when it re-set. So it 0psi winds up being really like 2" Hg of vacuum and the REAL 0psi winds up at like 2psi, etc... Because I re-cranked too quickly.

its quite a difference around idle/cruise. I will see 13.8's instead of 15.2's I am used to.

So essentially its "learning" that I am at a higher altitude when I re-key the second crank event too soon, while there is vacuum in the manifold from the first cranking event. I guess.
If I shut it off and restart it properly it goes back to normal a/f ratios.