john weires

I'm wondering what the loss from the rated 475 hsp would be at 7500 feet?

Obviously the dyno reading would have to be compared with
the dyno'd rear wheel horsepower of the GT3 at normal/low elevation.

My guess is about 20% loss.

MileHigh911

I have been told in the past, that a turbo will lose .5%/1000 ft, and a N/A car loses 2-3%/1000 ft. So, assume 2.5%x 7.5=18.75% So your guess is close. But...not a lot of racing happens at 7500 ft. I remember you are moving to CO, where are you moving to?

BBMGT3

i've also wondered the same about air temperature. except that I would guess turbos would lose more at higher temps than NA cars. I have an air intake temp sensor and from session to session sometimes notice as much as 7C difference in temperature. Always wondered what impact this had... I'm sure its something

Nb* motec sensor on race car.

GrantG

Unfortunately, I think your loss will be more than 20%. It's close to 20% down here in Denver (15-18%).

This calculator says you'll lose 107hp (or 22.5%):
http://www.wallaceracing.com/braking-hp.php

doubleurx

Yes, unfortunately! The math is

Altitude in feet X 0.03 / 1000 X hp at sea level = power loss.

That said, my 500 hp 996 TT feels slower than my GT3 at 6500 feet. The only test I've done other than seat of the pants, is an uphill climb from a stop sign, onto a 2 lane highway. There is a marker up the hill (7%) a ways down. I can hit 70 mph in the turbo, by the marker. 80 comes quicker in the GT3. So, I question the calculator.

I can't imagine the GT3 with 85 more HP. Looks like I need to take a trip out to the coast!

GrantG

Yeah, when I had my 2.7RS+ motor built for my lightweight 73 911, I picked it up in CA at sea level and the difference is dramatic. Turbo's definitely lose less peak power, but their lag is way worse at high altitude. I also had a ride in a 997.1RS on track and it felt like a totally different car than the ones I've experienced at altitude.

GrantG

Here's an even more sophisticated calculator and with Temp of 70F, 30 Inches of Hg, and Dew Point of 45 degrees, you lose 25.9%...

http://wahiduddin.net/calc/calc_hp_dp.htm

john weires

I have had a vacation home in Estes Park for a long time.
My wife and I are adding on to it so we can live full time there.
We are probably at least a year and a half away from the actual move.
In the meantime, I spend 10 days there every month.
10.5 hours total drive time, most of it on I-80 through Nebraska and Iowa.
The road is so straight and flat, with cruise control and good alignment you can almost take a nap.

It's nice to have passing power. Every time I drive up the canyon from Loveland to Estes Park on HWY 36, there is always somebody driving below the limit afraid of the curves. Hey I just remembered, there is at least one high altitude racing event at Pikes Peak!

mark kibort

its kind of a loaded question.... what are you really trying to determine.
what it feels like, vs sea level ?

keeping the temp the same, you get about 25% difference in air density, so that would be about 25% difference in HP (all other things kept equal)
But, if you look at the temp that 7500ft might have vs that same day at sea level, the difference would only be 20% because at 7500ft the temp would be close to 55 degrees.
Then again, if you compare when you are usually driving at 7500ft, the temp might be 30 degrees, while at sea level at 100F, the difference might only be 14%
So ,It depends on what you are really looking for.

here is a better more simple density calculator:

http://www.denysschen.com/catalogue/density.aspx

mark kibort

try 60-100 times or 60-80 times, they are easier to control and repeat with a stop watch. your turbo should have a huge advantage over the NA GT3 due to the turbo, negating some of the effects of altitude.

I guess it depends on the turbo system. if your turbo has absolute boost controlled, then you might not see any loss at all.

probably about 25% loss for same temp, but the 7500ft difference, so that's about 120hp loss!

the dynos should never change much, unless you look a the actual data, uncorrected.

I did this test at 7000ft and then at sea level.... all the times were pretty expected. 60-100 10 seconds at 7000 and then 6 seconds at sea level. dynode right after this test, and it was near 300rwhp for a 3200lb car.
9 seconds at sea level was what we got with cars with 100less hp.

Petevb

Power falls off faster than air density.

Power falling off at the same rate as air density would imply you'd only hit zero output in the vacuum of space. In reality an IC motor will stop working far before the ambient pressure gets that low- get to an "effective compression ratio" down below around 4:1 and most IC engines can't get out of there own way- too much friction, much less effective combustion.

Hence the need for more complex formulas that take other factors into account.

GrantG

Nice place to drive! Do you plan to do any track days at High Plains Raceway?

http://highplainsraceway.com/

Great track and probably the most abundant cheap track days in the country (~$80 for 4 hrs most Fridays during the warm months and weekends in the colder months).

MileHigh911

New for 2015, $1,000 for annual open lap day pass. The pass price would be the cheapest consumable!!!!!

MarcusG

Well,


Since you're all at the same altitude it's all things equal. This is the only time I can joke and say my 991 S makes more hp than your GT3 at 7500 ft.

Still don't like the sound of it though.

My parents live up above Colorado Springs. I think their house is like at 7,800 ft. What a bummer.

doubleurx

Maybe, but there are many other benefits to being in the mountains. I'm a car nut, but I'll never leave. I love where I live.