RGSS

iTrader: (2)

Are the speed density tunes skewed by altitude changes? If i tune my car at 5000 ft ASL with a baro reading close to 83 KPA and then go racing to a track at 100 ft ASL with a baro reading close to 100 KPA will my tune be completely off?
I am guessing yes, because Idle MAP readings and WOT MAP readings will be offset, but this is just a guess.
Somebody with experience or an insight on this sbject please chime in.
thanks

jimmyblue

iTrader: (11)

No, that's stuff from the carburetor days where fuel flow was set by
venturi vacuum. MAP sensor tells you the real manifold pressure no
matter where you are.

RGSS

iTrader: (2)

The Idle KPA readings change a lot with 5000 ft altitude differences. The VE map usually usually has a low area just around the idle values and then increases dramatically with MAP readings to handle fast throttle changes. I dont know how to go about that.

white01ss

iTrader: (37)

For idle I would say it would pretty much stay the same. But during WOT the MAP sensor is only going to see the pressure at that altitude. So at 5000' WOT would hit the cells around 83 kpa vs 100kpa for 100'.

For an NA car, atmospheric pressure is the only thing pushing air into the motor so the the tune would need to be tweaked under both conditions. I would speculate that once you've tuned under one elevation that you can leave those settings in the tune when tuning for the other elevation.

For example, at 83kpa the MAP sensor won't even see 100kpa so those cells would not get touched. Conversely, under WOT at 100kpa the MAP sensor will go right past 83kpa so those cells tuned for 5000' wouldn't get used.