I just got back from a 1700 mile road trip from Dallas, TX -> Fort Collins, CO and back. On the way there, I drove rather aggressively to try to make time. Fort Collins, CO is about 4800 feet asl, and Dallas is on the order of 500 feet asl. I noticed the car was sluggish up in Colorado, but I attributed it to the altitude. Is this correct??

Secondly, on the way back, I drove **very** aggressively -- sometimes 120+ for 20 miles at a time (there was no one on the roads, and I was not on a major highway). I noticed that when I go back to Dallas -- the car seemed very 'peppy' -- inadvertent third-gear chirps without really trying to.

Did the computer adjust the tuning of the car based on the 850 miles of high-speed driving?? Will this affect the LS1 Edit that I had done?? Is this just my mind playing tricks on me due to the sluggishness experienced in Colorado??

Oh yeah -- why don't they sell 93 octane in Colorado??

Thanks,

--dc.

 

:lurk:

 

Yes, the altitude will affect performance on any car, including the C5.

Yes, your agressive drving on the way back was 'learned' and subsequently affected further driving to be more 'peppy'.

No, this 'learned' driving will not affect any parameters downloaded by your last Edit tune. Never will. The trim registers that maintain your data as you drive is different from the tables you access and 'tune' with edit. BTW, the registers are held statically by battery power; disconnect your battery and you lose all trim averages.
Robert

 

I live at an altitude of over 7,000ft. Within 2 hours I can drive as high as 9,300ft or as low as 1,200ft. So I have a little bit of experience with it.

The pcm uses the MAP sensor to determine the ambient air pressure before engine startup, after that it is used to read absolute pressure inside the intake manifold. Think of it this way, when I start the engine in phoenix (1,000-1,500ft) and I drive home, the engine runs progressively richer until I stop the engine and it can read the ambient pressure again. As you decend from a high elevation the engine is running leaner, which will produce more power until it gets to the point that it starts detonating.

The other contributing factor is that sometimes you cant see that you're driving a 1-2% grade, but you can feel in how much throttle you apply.

93 octane is not sold at higher elevations because it is normally not required. The thinner air reduces cylinder pressure, which reduces the chance of detonation.

Eric

 

Thanks dmcantrell! Interesting thread. :yesnod: :thumbs:

 

Thanks y'all -- you've been very helpful!!

--dc.

 

The manifold absolute pressure sensor acts as a barometric pressure sensor not only during start-up, but also during WOT acceleration. When you go WOT, the PCM sets an internal clock and after a specific time out period (1-3 seconds), it reads the MAP sensor reading and determines that this reading is barometric pressure for the altitude the vehicle is currently being driven.
Now, if you do not go into WOT mode (in a Corvette, is that possible?), the PCM will only have the initial start up barometric pressure reading; however, via the O2 sensors and MAP sensor (pressure inside the engine changes with altitude), it will try and keep the air/fuel mixture as stoichiometric as possible. It does a pretty darn good job of leaning out the engine based upon O2 sensor readings alone.
The biggest problem with driving in high-altitude with a naturally aspirated engine is that horsepower drops off three percent for every 1000 feet gained in altitude. This is due to a three percent drop in air density for every 1000 feet you travel higher. If your PCM was not doing a good job controlling the air/fuel mixture, you would have noticed a smoke plume coming from the rear of your vehicle. This was normal in the days prior to computer controlled air/fuel mixtures, but now with computers monitoring not only the barometric pressure and absolute pressure inside the engine, but also the O2 sensors, the smoke plume is gone, or should be.
It's not just altitude that affects air density, the atmospheric pressure, temperature and humidity all affect the density of the air. On a hot day, or at high altitude, or on a moist day, the air is less dense. A reduction in air density reduces the amount of oxygen available for combustion and therefore reduces the engine horsepower and torque. This is one reason why your car will be a bit faster on a beautiful, 60 degree day with no humidity, versus a hot, humid 95 degree day. The air charge entering the engine on the 60 degree day is denser, thus more oxygen, thus more HP.

Back in 1969, I drove to the top of Pike's Peak, and I had to stop twice to manually adjust the air/fuel ratio with a screwdriver. In 1986, I drove the same road in a Ford Sable, and at that time Ford had the best electronic controls in the automotive world (EEC-IV), and I had absolutely no problem with transversing the road. It did start to feel a bit sluggish at around 10,000 feet. (That was also the same altitude that my wife started to ask why, she went along with my idea to drive, versus taking the Cog Railway.)