Speed density tunes offset by altitude?

[Ricardo]

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
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[mecanicman]

I only have had oppotunity to play with about 1500 ft in altitude differance. Idle does not seem to change much, wot does. You can suddenly hit cells you could not before. My baro reading is 91kpa at home. So when tuning wot here I hit the 90kpa cells in the ve. A trip to a neighboring city at lower altitiude results in hitting 93-94 kpa cells and now hitting the 95 kpa cells in the ve. I take the last line i could hit, copy it to remaining rows I did not hit, then multiply by 1.025, 1.05, ect for each row till i get to 105kpa(2.5% increase for each increase in baro). Its not perfect I am sure, but seems to work well for the differance in altitude I see.

[Ricardo]

Thanks for your input. That is what I do too. I take the highest line I can hit in WOT at home (80KPA) and then multiply it by a factor for the remaining ones.

Now, Idle is a completely different story. 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.

[SSUte01]

I only have had oppotunity to play with about 1500 ft in altitude differance. Idle does not seem to change much, wot does. You can suddenly hit cells you could not before. My baro reading is 91kpa at home. So when tuning wot here I hit the 90kpa cells in the ve. A trip to a neighboring city at lower altitiude results in hitting 93-94 kpa cells and now hitting the 95 kpa cells in the ve. I take the last line i could hit, copy it to remaining rows I did not hit, then multiply by 1.025, 1.05, ect for each row till i get to 105kpa(2.5% increase for each increase in baro). Its not perfect I am sure, but seems to work well for the differance in altitude I see.

That's pretty sound advice. I have always used 3% for "rows" of increased Baro and when I have actually got to MSL to test it has been well within tolerances.

I wouldn't expect idle vacuum to vary that much. However, if you tune the car at the highest vacuum (least MAP) then you should be able to dial in all values of increased MAP, so that should your idle end up in the next "row", you've already nailed the VE there.

[Ricardo]

Well, I have now experimented and can confirm that the VE tune does not remain accurate with altitude (BARO) changes, at least with my 411 PCM running a 2002 F Body OS.
I had my VE dialed in at 5000 ft ASL and the last time I was at the track at sea level the car ran rich. The VE tune was approx 10% rich in average all around.