Elevation change on a SD tune car.
06-01-2007, 07:25 AM
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Elevation change on a SD tune car. Hey guys. I had my car tuned at 4500ft elevation, and now Im back home at less than 500. I am tuned SD. Should I be concerned about the car running lean with the elevation change? Ive been too worried to driver her and Im having withdrawls! 
06-01-2007, 09:48 AM
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The MAP sensor still knows what's in the manifold, and that
is what matters. As long as the tuner made some sensible
extrapolation to MAP=100, from the MAP=90 or so at higher
ground that he could see,in the VE table and the spark maps.
is what matters. As long as the tuner made some sensible
extrapolation to MAP=100, from the MAP=90 or so at higher
ground that he could see,in the VE table and the spark maps.
06-01-2007, 12:33 PM
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jimmyblue has the right of it as usual. Probably the biggest misconception about speed density is that it is blind to altitude changes. This is 100% false, for the reason above.
The SD calc isn't perfect, and your trims will shift with drastic altitude changes. I run SD and my trims vary between my place in NM (about 5000 ft) and here in Rochester (400 feet or so). However, not by very much, and in the opposite direction that you would assume. My trims are tuned slightly negative here in Rochester, but when I drive to NM they shift to slightly positive. Go figure.
The SD calc isn't perfect, and your trims will shift with drastic altitude changes. I run SD and my trims vary between my place in NM (about 5000 ft) and here in Rochester (400 feet or so). However, not by very much, and in the opposite direction that you would assume. My trims are tuned slightly negative here in Rochester, but when I drive to NM they shift to slightly positive. Go figure.
06-04-2007, 08:07 AM
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Originally Posted by Silverhawk_02TA
My trims are tuned slightly negative here in Rochester, but when I drive to NM they shift to slightly positive. Go figure.
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06-04-2007, 08:42 AM
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Originally Posted by P Mack
One effect that is never talked about is the effect of elevation on the exhaust stroke. The pcm calculates the density of the air in the intake manifold so theoretically trims shouldn't change. But at higher elevations, there is less atmospheric pressure pushing back on the exhaust. So for a given MAP value, you could actually have slightly higher VE at a higher elevation, which would make your fuel trims positive at elevation. Just an idea.
What's the most manifold pressure it can see at WOT at low altitude?
What's the most manifold pressure it can see at WOT at high altitude?
At high altitude the max map value that can be achieved is lower. So if it ever DID reach as high of a map value as it can at low altitude, it's REALLY sucking some air and needs more fuel.