I currently live in a Denver suburb and my home elevation is about 6000 ft.

Moving to Vancouver WA / Portland area where I will be at sea level and will have regular access to 92 gas. Only have 91 right now.

Have exhaust, intake, tune and am consistant at 1.0 Bar on my 03' X50 car.

Any performance gain to be expected with the altitude change and better octane?

Just curious. Thanks,

CS

 

The general formula for HP loss is 3% for every 1,000 feet above sea level. You should notice the difference. I moved to Colorado from Indiana. My home in Indiana was at roughly 800 feet of elevation. My home in Colorado is at roughly 8,000 feet of elevation. I can definitely tell the difference in my normally aspirated car. In your turbo, the difference moving down to sea level will probably be less noticable overall, but you should feel it.

 

In a 996tt, assuming you are using the same fuel, your car will develop the same boost numbers (in your case 1.0) at 6000' as it will at sea level. Performance will be the same. If you want to split hairs, in theory, the car would be a touch faster at 6000' since the air is thinner and there will be less parasitic drag. The negative is that you will have slightly reduced cooling due to the thinner air. Higher octane fuel will give you better performance assuming your tune is set up for it.

 

You're forgetting about the efficiencies of the turbos. In order to get the same 1 bar pressure in thinner the compressor has to spin faster than it would at sea level which often times puts the compressor out of its efficiency zone and adversly affects overall performance. That's why a custom wheel design for high altitudes will help.

 

uhhhhh, peformance can and will be degraded significantly as altitude goes up. At 6000', atmospheric pressure is ~11psi (off from ~14.7 psi at SL). Since the 996 is targeting absolute pressure, it has to overcome the 3.7 psi differential to make 1 bar gauge pressure (2 bar absolute). So a 1 bar flash is really asking 1.3 bar from the compressors. With K-16s already at the edge of their compressor maps on a flashed car and considering loses through inlet pipes, intercoolers and the y-pipe, a flash a high elevation will push them to stratospheric rpms and into the <60% efficiency range (basically off the map), driving charge temps though the roof and plummeting overall efficiency. Bigger compressor will do better but some loses are still present.

Anecdotally, I picked up 6-7mph in the 1/4 mile between Albuquerque (5300') and Bakersfield (625'). (Density was ~8000' and 1000', respectively) and dropped 3 seconds from my 60-130 times.

While turbo cars are generally less affected than NA counterparts, power output still drops as altitude increases.

 

Seems like there are varoius opinions on this. For the record, I'm running K24 turbos, not K16, if that makes a difference.

Thanks for all your replies. I am thinking that due to the denser air and higher octane gas 92 vs 91, I should see some improvement. My tune is set up to recognize the octane and adjust accordingly.

 

We lose about 12% of power here. If there's any doubt, put your car on the dyno with no correction. You'll see the truth in it.

If it were me, I would get the car retuned once you get to sea level. You'll both make more power and get better gas mileage than running a high altitude tune.

 

Thanks,

Will likely make the 3 hr drive up to Kevin @ Ultimate just outside Seattle for a new tune, maybe a nice K24/18g setup as well.

Just thought it was interesting to see so many different schools of thought on this.
How since the car is tuned and runs at 1.0 Bar here, it will also run 1.0 Bar at sea level and thus no power gain would be seen.

 

My car boosts to 0.7 bar at sea level, I've seen 0.9 bar at 8k feet. Stock tune.

 

I'm wondering if your car is just automatically adding the .2 Bar to make up the difference for the thinner air, thus power output remains the same, even with the higher boost.

 

Since you have K24s and not K16s you shouldn't notice much if any difference in corrected power between the two altitudes. If you do dyno your car without a correction factor in Colorado then the dyno isn't taking into account the density altitude. The efficiency of the turbo does dictate when (in this case altitude) you'll start to lose power. I'd say the only time you would noticeably start to lose power is in the summer when it's hot and the density altitude is very high. I think you'll notice between 6000 ft. and 700 ft. altitude is your off boost to on boost spool time and how your engine reacts when off boost.

 

It's because the DME is targeting the absolute pressure, which at high altitudes, will show up as increased gauge pressure. You still have the efficiency and power losses.

 

SAE, an NA correction factor, overcorrects the snot out of turbo cars at high altitude -I made 490 rwhp SAE on a K16 car on 91 octane at 8000' DA. Yay me a K16 record! ...but then the car trapped 115 mph, indicative of about 380 true rwhp. At sea level, I was running 122mph and making 450 rwhp SAE. Shiv wrote a nice article summing it up a few years back.

On another note, please explain how increasing PRs and dropping turbo efficiency does not affect power output...

 

So with the K24s should I see the boost come on sooner at sea level?

Are you talking about corrected power as read by the dyno? I believe that (in theory) the SAE corrected numbers should be the same no matter what altitude I'm at. I know I lose power duie to heat and thinner air, thats why I figured I'd get some back in lower altitudes where the air is naturally denser.

I'm interested in actual output differences and if I'll feel any difference in the car at sea level.

 

On another note, please explain how increasing PRs and dropping turbo efficiency does not affect power output...[/QUOTE]

If you'll read what I wrote again you'll notice that I said it does affect power output.