James Bailey

Attitude is all that matters

Sailmed

Attitude and latitude!

Bill Ball

It makes a substantial difference. The LH adjustments don't help as you have inherently less air at high altitude and it reduces the fuel deilivery to keep the mixture stoich. Therefore less power. The general rule is 3% per 1000 feet. And a supercharged car is equally affected.

So, I think you will notice the car has a lot more grunt at sea level. I sure do when I return from a race in Nevada which we run at 5000-6000 feet. The car really wakes up on the way back to the Bay Area. One related issue - you may ping/kock at sea level when you don't at high altitude. I've had that experience directly and tuning fuel did not help.

heinrich

Bill, again Kibort will tell you, "the car will have a lot more grunt" is all in your head, and the math and chemistry of it just doesn't add up. There is no difference at sea level or up at 10,000 ft, the car is performing the same, it's the effervescent blood bubbles in your brain that retard your actions to such extent that you perceive less "grunt".

Seriously, when I drove someone else's S4 across the Rockies, I was VERY concerned at one point in the mountains that there was a SERIOUS PROBLEM with the car. I woke my copilot to discuss WTF was wrong, when he pointed at a sign we were passing .... Elev. 10,000 ft.

It makes a huge difference.

Lizard928

No, a SC will build however much boost it can as per the speed it is spinning. This is controlled by the pulley sizes.
A turbos boost is limited by the wastegate which vents excessive exhaust gasses once desired boost is reached. Because of this it always pumps in the same pressure irregardless of elevation.

If you built a wastegate system that dumped intake pressure after X psi then you would not be affected. But there is no need to do that.

jpNcos

I live at just under 7k feet and my turbo car is really sluggish up here.

S4ordie

bigs - Barometric pressure is air density. The fact that the weather changes it from location to location is another one of those variables that have an effect on performance. Not a whole lot when you are only talking about a 3900 foot elevation difference, but when it is cumulative to many other things they begin to add up.

I am not disputing driving your car over the Continental Divide versus at sea level will result in better performance when near the ocean, it defintely will. All of my comments are relative to "bigs'" question relative to where he lives now, Provo Utah and where he will be, Arlington Texas. The altitude difference and the resultant 3hp decrease per 1000 feet of elevation is not enough to override other variables such as temp/air density/humidity in a way that make the car perform notable more powerful.

Bill - I certainly am no expert in the workings of our cars, especially of the LH at different altitudes, but if it funtions like most other mid 90s FI systems it is doing a lot of metering and adjustment. Just consider for a moment how aircraft compensate for air density by going more rich/lean depending upon altitude to get optimum performance. Why planes get best performance (fuel burn and speed) at higher altitudes as opposed to sea level. This is why I mentioned how George's car, even yours running at 150mph, at higher altitude must consider the aerodynamic efficiencies due to thinner air as opposed to just higher engine hp output. Your cars move at speeds commensurate and indeed faster than most modern production NA personal aircraft (Cirrus/Cessna) where the difference between flying just above the ocean as opposed to 5,000 - 6,000 feet asl makes a significant different in final speed and performance. The faster you go at higher altitude the greater the aerodynamic benefit of thinner air. I don't know but there must be some crossover between hp decline per 1000 feet of elevation versus aerodynamic effect as elevation increases. Perhaps, just perhaps, a car like George's, has a higher final top end speed at 5,000' than at sea level due to the aero advantage of thinner air. Tough to know unless you can assert consitency of all environmental variables.

IcemanG17

Bigs
Granted I'm not an expert..but I noticed the difference from 14ft to tahoe at 6200+ft quite a bit....oddly enough it was the fact that I almost always would get a 1st gear kickdown start...vs the 2nd gear start I get down here......& thats just cruising around.....

Lizard928

Well the turbo will produce the same boost at sea level as it will at higher elevations, unlike the SC system. However this can be hindered by the engine management system not being able to compensate for altitude.

And you will still see some loss from a turbocharged engine, just not nearly as much.

Sailmed

Bigs,
It will go down hill faster then uphill....
Did that help?

bigs

That's why I'm figurin' on an extra day for the trip back home!

docmirror

Ok, sports fans, this has been fun.

The two charts you need are the pressure altitude chart(for STP of 60F, and 14.696PSIA @ SL):

http://www.engineeringtoolbox.com/ai...ure-d_462.html

And then the density altitude formula(not exact, but close, cribbed from wiki):

Density altitude in feet = pressure altitude in feet + (120 x (OAT - ISA_temperature))

Where:

OAT = Outside air temperature in °C
ISA_temperature = 15 °C - (1.98 °C/1000 ft × pressure altitude in feet)

From this, an enterprising person with some time on their hands can calculate the delta of pressure given similar humidity. As someone has already mentioned, the lower air density at altitude will present less wind resistance that near sea level, which is one reason planes fly so high.

Since I've been imbibing from the nectar of the gods, the calculations will be left to the student as an exercise. Your papers are due no later than July 31 @ 09:00 GMT. Idiots like me just look this stuff up in the chart, and follow the line prior to take off. But, that's specific to the plane and weight involved.

anonymousagain

Bigs - I just drove past your neck of the woods on Fri, visiting a friend in Alpine en route to Rhode Island. Actually, I think my friend saw your car at Motorsports while picking up a tow eye for me, since he said it was a very nice red GTS.

Anyway, you will absolutely notice the difference! SATA felt sluggish or heavier and clearly down on hp. Even my wife noticed how much more hp returned as we dropped altitude. You will have a BIG grin on your face as you drop...I'd place a bet that the 40+ hp estimate is accurate.

Enjoy!

Bill Ball

Sure the top speed is less affected due to thinner air reducing drag, but acceleration is terribly affected at altitude. We see this driving in the race in the time it takes us to get up to speed inititially and when coming out of slower turns. The car feels like a slug compared to sea level, but yes, even stock, my car would do 165 at 5000 feet if given enough time.

As regards the LH, it is nothing more sophisticated than taking the O2 readings and adjusting the mixture for stoich under idle and cruise conditions. So, less air, less fuel. When you go WOT or heavy acceleration, it goes off the O2 loop and into a fixed enrichment map, so you could go overly rich at high altitude. I had that problem in the last race and we had to do a bunch of on-the-spot Sharktuning during practice runs. Anyway, the description that the engine is nothing more than an air pump applies. There's no magic to overcome the lower air mass.

mark kibort

No, Kibort will tell you that the physics dont lie here or fool here either. You lose boat loads of hp at 4000 to 6000ft. I got the holbert car and drove over the pass. On a level road, it was near 12 seconds 60-100mph, while at sea level when I got to sacramento, it was 7 seconds. dyno confimed at sea level, that hp was in the 290rwhp range.