Recently bought a stage 2+ 2000 s4 6mt with the apr 93 chip. Bought the car at sea level and drove it back to Colorado at about 6000 feet elevation. According to the boost gauge at sea level it would spike at 17-18 and then hold 15 lbs. After i got it home and drove it for a couple weeks I cant spike past 13 and it will hold at 10 lbs. Could this be from the car adjusting to the altitude? I realize how elevation changes will affect power but Im new to turbo cars but i thought that altitude should not affect the amount of boost that the turbos will make? I did a compression test and have found no leaks. the dvs and tbb are fairly new and no cel. Also the highest octane available here is 91. Will this hurt anything since i have the 93 map? Kinda confused at this point, im at about 70k miles and everything runs and feels great, just less power and boost now.

 

That's weird man. These cars use a Bosch Motronic ECU, which does a pretty good job at compensating for differences in air density. The turbos should compensate for the altitude, they have to work harder to produce the same amount of boost that they would at sea level, but nonetheless, they should be building more than 13psi.

I live in Denver and at this altitude, NA engines will see about a 12% reduction in CHP. You're seeing ~30% reduction in boost alone. Do you have a 91 program on the ECU? If not, consider having APR (or one of their dealers) set you back on a 91 program. Honestly, I don't think it matters, the ECU should automatically advance the timing a bit for lower octane fuel, but this would be a good place to start.

Where in Colorado do you live?

 

something is definatly wrong and i think its more of octane than altitude issues.

 

im in Colorado Springs. does the 91 map change the amount of boost that the ecu allows or is it all in the timing. Are there any good tuners in the Denver/ Cos area?

 

A Colorado Springs company that does APR http://www.europeanperformancespecialists.com/

I have no idea how good they are. I have GIAC, and the stock program (helpful in the winter with the icy roads to prevent too much power and torque).

Good luck and keep us posted on the results.

 

You are going to want to switch to the 91 oct. APR program. If you don't have it then you need to get it. I'm pretty sure it came standard with a APR flash but I could be wrong. Do you know how to switch to different programs with the cruise control button? I know for my setup when the CEL flash twice then I'm on the 91 oct. program. When it flashes 3 times then I'm on the 93 oct program.

Either way if you are using the 93 oct program with a 91 oct gas then yes that will produce less boost...

 

I can switch between the stock and 93 octane maps using the cruise control button. should the 91 map produce the same boost as the 93 map? ill contact apr or one of their dealers and see what they can tell me.

 

You will produce the same but your timing will be retarded to prevent knock since the ecu is mapped at 93. You might feel a little difference in power but your boost shouldn't be cut.

 

Not all did.


No, it won't. You'll produce the same amount of boost no matter what octane you put in the tank. But you can get detonation if your octane is lower than recommended for your tune.
The ECU and N75 control boost. Not octane content.


Meatbiscuit, hit the nail on the head.
The motronic compensates for air density, humidity, temperature, etc.
I remember reading somewhere that for every 5000 ft. above sea level you can expect a 1% increase in turbine shaft speed.
The turbo is spinning faster to make the same amount of boost.

Your ambient pressure isn't 14.7 psi anymore as it is at sea level.

Let's say for example you normally run 15psi max.

And say for example it's 12.7 psi ambient pressure at 5000 ft., then your turbos have to make up for the 2.0 psi loss on that you lose in ambient pressure on top of the 15 psi the ECU is requesting.

This means that your turbos are working as hard as they would to make 15 psi @ 5000 ft as they would to make 17 psi@ 0 ft. (sea level)
for example .

 

Red 01 hit the nail on the head. I'm in Colo Springs myself and am all too familiar with the effects of altitude. At our altitude (6000 ft), expect about 3PSI less than what you'd see at sea level.

I'd recommend getting your car "Altituned" by Avalon Motorsports up in Denver. They have tuning at altitude down to science.