FYI: Altitude Tuning Differences
I did a little research into tuning for altitude and wanted to share some information with you guys. I know some people on here are new to turbo cars and the tuning theory that goes along with them so I thought it's better to start now to provide people with a resource for information.
First, we need to start out by understanding what atmospheric pressure (ATM) is/means. ATM is referred to in the USA in standard units of PSI (pounds per square inch). By definition, this means that ATM is the amount of pressure the molecules of air in the atmosphere above your head at a given altitude exert on one square inch of ____ (insert object here). At different altitudes, there is a different quantity of air molecules exerting pressure which means that ATM goes DOWN as you go UP in altitude. The table below shows the effect of altitude on ATM (expressed below or above mean sea level (MSL):
MSL (ft) ATM (psi)
0 (SL) 14.696
So, as you can see, there is quite a difference depending on where you are in altitude in the amount of ATM you would be working with. The most important aspect of all the information here is that the air at higher altitudes is less dense than air at lower altitudes. I think that explains the ATM portion well enough, so now on to what this means to us!
When a turbo car "makes boost", it is COMPLIMENTING (adding to) atmospheric pressure. When you look at your boost gauge in your car (if you have one) it only expresses the relative pressure that the turbo is generating. This means that the gauge reading does NOT express any correlation to changes in ATM! So, lets look at the absolute pressure, ATM+boost essentially, in relation to our cars:
MSL (ft) Abs Press(psi)
*Note that this table expresses normal ATM plus 18psi boost*
So, what this means is that if you tune your car at say 3500' at 18psi and drive to a place where it is 1000' you will experience an approximately 2psi equivalent jump due to air density! Conversly, tuning at 1000' @ 18psi then ascending to 3500' would result in a 2psi equivalent drop! Granted, if your car is stock, the engine computer will compensate for this and make adjustments accordingly. BUT, if you use a MBC or EBC, you "set" your boost level and the computer does not compensate anymore! A 2psi equivalent drop/gain CAN make a car run rich/lean depending on other variables (not good)!
Now, on to what this means in terms of your turbo. The turbo in any vehicle acts as an air pump. It sucks ii, pressurizes, and expells air not giving any notice to changes in ATM. BUT, changes in ATM do effect the density of air the turbo sucks in and, therefore, the density of the air it puts out. What this means is that going up in altitude requires your turbo to work harder to obtain the same intake charge density, or psi equivalent, that it did at a lower elevation! Also keep in mind that both sets of values expressed in the charts above DO NOT take into consideration humidity or temperature (both of which can effect tuning as well)!
The stock turbo in our cars is only capable of pumping so much air (read as efficiency range). What this means as far as our discussion so far is that our stock turbo will have different limits just in reference to altitude. For example, it has been stated that the stock turbo runs out of steam after 18psi in places relatively close to sea level (500-800'), so what this means is that when you take that same turbo and ask it to pump less dense air, it will be able to generate LESS boost. Different locations have different altitudes and therefore this changes the options/viability of using certain turbo configurations as well as changing the way a car is tuned.
I could go into a lot more detail here but I wanted to mainly provide some conversation points and get some of the various tuners represented here involved so that people can be more directed on their pursuits/desires for their cars based on their locations. I will answer any questions to the best of my ability or find you answers to your questions if I cant but I'd like to see some input here from some tuning vendors as far as what they have experienced/know about this whole deal.