Trying to understand horsepower at altitude
 

I live in the Denver area, at about 5700 feet.

I just bought a 2001 996 TT, and I really like it. It seems really fast, although I’d have to say that it's quickness is not my main focus.

I took it to the local P dealer, and did a bumper to bumper check out. It passed with flying colors, as well it should with only 9k miles. (The car certainly was not cheap, but you get what you pay for)

But here’s the question. At this altitude, we often see the normally aspirated cars loose about 15% of their rear wheel hp due to the altitude.

However, the service rep here at the P dealership, who seems really knowledgeable, said that is not the case, as Porsche turbocharged cars make up for the difference via the boost to .8 bar, so the rated hp would be the same as cars at lower altitudes.

I can confirm that the boost gauge shows .8 bar at, or slightly below redline, so it seems that his info may be accurate.

So, now I’m thinking that I have a car that produces the same hp as this car would produce at sea level. Is that the case?

Is there any counterpoint to his statement? But, it seems like to me the air is thinner here at 5700 feet, so even if you compress it, it still has less air input, so less air output, or does it just compress it more to get it to the max pressure (psi) that it should have to get it to the rated horsepower?

Tnx for any input. Just trying to learn.

Bill

www.billoxley.com

No an engineer by any means, but I think you have to compress nore "thin" air to get to the same absolute pressure.

It's about a 3% loss on normally aspirated per 1000 feet. Forced induction does not have that issue. Congrats, enjoy the new ride.

thats what turbocharging does: gives you sea level air pressure at altitude. As you go higher the air thins so there isnt enough air for proper combustion. I cant remenber the fuel to air ratio is but in order to generate max power the engine needs the proper fuel to air ratio

You win!
 

Your car running at 5700' ASL is probably as fast or faster than at sea level. The turbo makes about the same power at higher altitude AND the drag is less because the air density is less. A non turbo car (N/A) will lose quite a bit of power at higher altitude.Hope that helps answer your question.
You may have an interest in doing a "google" on the subject.

Cheers

R

I think it comes down to the turbo spinning a bit faster and stuffing a greater volume of air until it gets the pressure up to the desired level. Eventually, as you got high enough, the turbo's efficiency would drop off, and even spinning it's little heart out, you'd lose power.

But I know bugger all about turbos, so wait for someone who knows.

I'm with P-Mac (an have similar brain capacity). I think your turbos are working harder and getting more of the lesser density air in for about the same power as sea level.

Turbocharged engines will also experience a loss of power at altitude...but not nearly as much as a N/A engine will.

Yep. The turbos produce ~0.7 bar above atmosphere...the higher the altitude the lower the ambient pressure in the intake and the more boost the turbos have to make to achieve the engine's desired 0.7 bar at the engine.

Yes, but only above the turbo's critical altitude...which is pretty high.

and don't forget the fact that turbos LOVE that cold air! Ever notice how much faster your turbo is on a very cold day (compared to say 100 degrees out).

Just out of curiosity, which P Dealer was the car at?

picture included
 

It was a private party purchase, but I took it to Stevinson Porsche for the inspection.

Here's a picture before the wheel change. I didn't care too much for the chromed turbo wheels.


http://www.billoxley.com/images/996TT.jpg

Have to agree...those chrome rims would look better at the bottom of that pond, fish would love them.

Re: picture included
 

Congrats on the purchase. :)

Just out of curiousity, is that the Turbo that was for sale by Z Best Limousine? I swear their Turbo looked just like that.