My understanding is that a normally aspirated engine will lose power at about 3% per 1000 feet. That gives me at least a 15% loss here in Denver. :sad: :sad:

What would a supercharger lose up here? What would a Turbo lose?

Thanks!!! :cheers:

 

I hear you on that. I lost about 11% real world perfomance between Houston and Salt Lake City on my non-F/I car. The concept of S/C and TT is a faux atmosphere. Theoretically you should be able to tune out performance differences between a F/I car at sea-level and at your altitude.
:cool:

 

It will loose close to the same percent. My car made 501 rwhp on the dyno at sea level and I judging by my mph in the quarter I am at about 380 or so here. In actual practice I think that the loss is greater than 3%.

 

I think you will still see an issue.. The problem is that the air is thinner... less dense at 5000ft than at sea level. The only power adder that is not effected is N20, since you are adding oxygen with the nitrous.

 

So does that mean NA I will be 15% down but once the nitrous is flowing I am running without any altitude loss?

BTW Thanks Guys!! :cheers:

 

From my flying experience I can tell you that a loss is a loss as far as density
altitude is concerned. Even with power adders like nitrous and forced induction you can compensate for altitude,but to say that you can produce as
much power at 5000 feet as at sea level is just not true.UNLESS you are able
to produce/introduce enough additional oxygen to replicate the air density at
sea level.
I don't know how you'ld do it.Maybe a system capable of producing 18 psi
but limited to 12 psi (wastegate) could completely compensate for it up to a
point where the turbo/supercharger began to fall below the threshold of the
wastegate (12 psi). I want to say that a NOS system could be tuned to do it
at a given altitude,but would have to be tuned again for a different altitude
when the car is moved.Very interesting topic.Can't wait to read some other
opinions.
I remember once at Lima, Peru (13,000 ft) that the air was so thin that the
engines on the L-1011 I was flying on had to be spun up to almost 40% power with small turbines (ground power units) befor the air in the combustion chambers would support combustion. If left to it's own devices the airplane could not be started!

:crazy:

 

:iagree: I really have no experience in this area with cars, but in airplanes this is true. The jet we launch off of Denver Centennial airport every Sunday night takes significantly longer to get off the ground than anywhere else. I also had extensive experience flying Piper Navajos before I started flying jets. Navajos are turbocharged, and there is a definite difference between the max manifold pressure we could get in Denver as opposed to the plains. Also, as the Navajo climbs higher it will reach an altitude where the turbos can no longer compensate for the thinning air and manifold pressure drops off. Obviously that altitude is well above 5000 ft., but the principle still exists that it is losing power as it gets higher. I really don't know how this could be tuned out. I would think if it could be done airplane manufacturers would have done it. I'm curious to see any ideas anyone has as well. :cheers:

:lurk:

 

When I had my Z28 with the ATI procharger I was making 10 pounds of boost max with a 12 pound pulley. You typically will loose 2-3 pounds of boost up here. Turbo's don't loose as much.

 

:lurk:

 

Just increase your nitrous jets 15% :D The other thing to keep in mind is that everyone else is loosing 15% also; you are still one of the top dogs here :)

Keith

 

Okay, lets take this the other way. You install an ATI, make the appropriate changes to produce 8psi here at altitude (6000ft), you take the care to say Okla or Texas, something under 1000ft. Is the car safe to run at full throttle and max boost as set at 6000ft?

Lonnie (Okie in Colorado) :D

 

Tuning a car at altitude and going to sea level would open a can of worms. The other way not as much of an issue.

If for example a car makes 700RWHP up at Denver. The last time I tuned there the correction factor was 22%. That means that real RWHP is 573.
This creates the following issues if you go to sea level:
1. Fuel pump capability
2. Injector capability
3. Mass Air Flow meter peaking
4. Cylinder Pressure and timing curve.

It's a whole lot easier to make 573 RWHP than 700 RWHP as far as the support items are concerned. When I see big dyno #'s from people at altitude I do not correlate that to what a car would make at sea level.

You could get away with doing a 600 RWHP setup in Denver without even changing the fuel pumps. Also the MAF will probably not peak so there are no real tuning difficulties. Also that stock bottom end is hardly stressed since the real HP #s are what matter.

Then you have timing concerns. Regardless of boost the real concern is dynamic cylinder pressure. You can typically run more timing up at altitude than you would at sea level. Also the octane requirements change drastically. For a long time the mountain areas had lower octane fuel at the pumps.

If you have a built car and move to a lower elevation, i would strongly suggest dynoing it to make sure it is ok still. On the flip side if you are moving to a higher elevation you are probably going to give up some hp unless you retune it for the altitude.

THe computer's are pretty capable in these cars, but huge altitude swings with heavy mods need to be addressed.

 

Mike,

What you are saying makes since and is in line with what I thought. Taking my soon to be 500/500 car from Colo Springs to Ok or TX for short visits will just require that I use some common since when it comes to the throttle. What I was thinking was incremintally going through the RPMs up to 7psi and limit myself to that so as to protect my heads and bottom end.

Am I on the right track or should I just cool it down south?

Thanks for the great info,
Lonnie :D

 

Good scoop!! Thanks for the info guys. Any more??

:lurk:

 

Bill---

You shoulda moved to Texas :smash: :lol:

 

This is absolutely true. As I mentioned in an earlier post, for those living in notably higher than sea-level altitude, the only F.I. option that's viable is an appropriately large turbocharger(s) with electronic wastegating. Appropriately large for altitude could mean more lag at all conditions regardless of altitude.

More words of wisdom. At higher altitudes, the lower air density reduces the aerodynamic drag for given road speed. For airplanes, this is why high altitude is an efficient flight regime. Hence one cannot estimate horsepower for trap speed differences exclusively. Also, this suggests that if one has a certain trap speed target in mind, you don't need quite as much engine output to attain it at altitude.

 

Regarding aircraft, an engine that develops 200 bhp at sea-level at a certain rpm develops only 165 bhp at 5000 feet altitude if the only contribution to the density difference is that due to altitude. If however that engine is far below its rated power rpm, then one can simply raise revs to compensate.

However running at rated power rpm for a considerable duration increases the engine overhaul frequency tremendously. For continuous cruise, the maximum recommended rpm is 75% of rated rpm, designated as a performance cruise rpm. 55% of rated rpm is generally regarded as most fuel efficient. And 65% of rated rpm splits the difference for good performance and economy. If it's not enough, then turbo or mechanical supercharging is on order. Although they both add cylinder pressure stress, the generous low rpm boost development nature ***less peaky*** of appropriately sized turbos and appropriately sized/ratio'd mechanical positive displacement superchargers allow one to run lower rpms in exchange of higher gas pressure forces.

 

I was overruled :lol: :lol: :cheers:

 

understand the program :lolg: :lolg:

 

No one has mentioned compression ratios, if I have 11 to 1 in denver wouldn't I be running more CR in Texas and running a dangerous level, doesn't the CR play a factor in losing rwhp at altitude, if I have 11 cr at sealevel doesn't that drop significantly in Denver and account for a lot of the power loss. I guess what I'm saying is if I have 500rwhp with 11 to 1 CR at sealevel and I moved to Denver I may only have 9 to 1 CR and certainly a loss of power, so couldn't I shave the heads a little or use diff pistons to bring the CR back to 11 to 1--


[Modified by NEPTUNEBILL, 7:22 AM 9/24/2003]