Effects of elevation on Boost readings

[Tenchuu]

so i was looking to counter the effects of me not having a boost controller on my car, and living at 3650 feet i had originally plugged in .4 BAR on my boost gauge to come out as 5.8 PSI, but since then i found a elevation correction calculator here:
http://www.dragsource.com/index.php?navselect=calculators&calctoview=8

With the local elevation of 3630 feet where i live that same .4 BAR equates out to 4.2PSI.

to correct this i would need to get a boost controller and turn it up until my gauge (in BAR) reads about .7 BAR to be at 8.6 PSI which is more respectable for a RB25DET.

I stand ready to be corrected. ant this whole theory is assuming that elevation has an effect on a mechanical boost gauge (have on older apexi mechanical gauge that reads in BAR)

to be honest i am surprised that i didn't fins something like this already by searching.

[Tenchuu]

apon further inspection, i have read that i would need to keep it tuned to whatever the safe factory rating for the turbo was as (ie if it is 8PSI @ sea level then i need to tune it to 8PSI at elevation or else then turbo will be working harder than i want it to (ie out of it's efficiency range)

again this is working on the assumption that a mechanical BAR boost gauge is not a system that is sealed and calibrated to read at seal level, and is not effected by elevation, of which i have no clue and would appreciate a definitive response.

[Tenchuu]

A little read for anyone searching for this:
http://www.clubwrx.net/forums/showthread.php?t=61581&page=3

i will need to re-read it and then confirm it to call it good on this subject though.

[Grenade180sx]

double post fail :(

[Grenade180sx]

Let’s start by noting air is the working medium of internal combustion engines, and the more air (mass of air) processed by the engine, the more power it will make. We also all know air density decreases with elevation, so engines lose power the higher they operate.


Let’s also note supercharged and turbocharged engines are naturally aspirated upstream of the supercharger or turbo. In other words, we’re still relying on something (atmospheric pressure) to drive the air into the blower or turbo. So, the higher we go, the less power the engine makes unless something changes to process more air through the engine.


As you noted, the naturally aspirated engine loses power because it can’t process any more air—the engine can’t magically grow displacement or increase rpm.



Likewise, a mechanically driven supercharged engine doesn’t get any larger, doesn’t rev any higher nor does the supercharger turn any faster. Therefore, the mechanically supercharged engine doesn’t process any more air and begins losing power as it rises above sea level just as a naturally aspirated engine does.

This effect is typically masked in automobiles because the supercharged engines have so much power at sea level that it’s easy to use full throttle earlier and longer while driving in the mountains and not notice the difference.
The same is true of turbocharged engines, except for one thing. Turbocharged auto*motive engines are often fitted with relatively large turbos with excess capacity at sea level, plus a wastegate to limit turbo boost. Take such a car into the hills and the wastegate simply stays closed earlier and longer, and voilÃ*, a sea-level air mass is forced into the engine. Well, it is until the engine’s critical elevation is reached; that’s where the turbo system runs out of capacity due to its sizing, or insufficient atmospheric pressure to feed it, and then only sea level or less power is possible.
The important point is a forced-induction system has to increase speed or have reserve capacity to maintain sea-level horsepower at altitude. This can be done with a mechanically driven supercharger if fitted with a transmission (once common on aircraft), or as we saw, more easily with a turbo. Likewise, some turbo systems are carefully sized to limit sea-level boost to a predetermined level and have no wastegate. Such engines cannot spin the turbo faster or earlier, and they lose power with elevation starting immediately. But again, they likely have so much power to start with, you can use more throttle earlier and not notice any drop for the first 2000 to 3000 feet of elevation.



credit road and track