troppo

OK, I think I have heard at least 4 different things from 4 different sources and am looking for a definitive answer.

I live at 5500 ft (denver) and am wondering how much less hp/torque my stock 993tt is making than at sea level.

What do you guys think?

911/Q45

My info says power stays the same to 10,000 ft, then the ECU dials back boost to keep the turbos from overspeeding.

troppo

so, that would mean that boost would be increased automatically by the dme to make up for the lower air density...but I only see .8 bar on my boost gauge.

ca993twin

If you are using the digital stock gauge, it won't read any higher then .8 bar no matter what. But that actually may not matter... with the higher altitude the ambient pressure drops. The ECU will see this as less boost, and will not gate the boost or dial back timing, and the boost will climb to normal (.8 bar) levels. This is what Don (911/Q45) is alluding to. But the turbos have to spin faster to maintain that boost level in the lower ambient pressure, and that's where you see the 2nd part of Don's statement. It sounds perfectly reasonable to me, and accounts for the popularity of boosted cars at high elevations... no significant power loss.

kennyboy

This is interesting - so perhaps what may happen is peak power may not change though the power curve could be different?

i.e. the turbos have to spin longer/faster to reach peak turque so the car would take longer to reach peak torque therefore is slower?

Though maybe this all happens so fast it doesn't relate to a measurable difference on the road.

ACEparts_com

The air will be less dense so the turbos will spin up faster (less resistance) - no difference in performance

Kevin

The turbochargers will spin up to 30-35% faster to reach the target boost level.

However, the efficiency of the turbocharger can and may fall out of it's ideal range.

The end result is more heat load placed on your IC, and possible damage due to overspeed.

Running a stock K16 at target boost levels of 1.1bar at that elevation and higher, will wreck a good turbocharger.

md11plt

From what I have experienced, the off-boost performance is less than that closer to sea level. The boost comes on harder and quicker at higher altitude. It is an amazing experience, really. It is not so much a shove as a good swift kick! I believe the turbos are capable of making more boost than the engine can use, so once you reach .8, the waste gate opens and all the excess is dumped. I don’t believe the .8 is differential pressure, I believe it to be total pressure so it would be the same regardless of altitude.

If I am wrong, please enlighten me.

Tom

troppo

Tom-
So the turbos will just spin faster to achieve the .8 bar of boost?


Kevin-
So are your k16/24 hybrids (that I am saving for) have the ability to spin faster without overspeeding?


thanks!

md11plt

I don't know if they spin faster. The way I understand it, the turbos are capable of X pounds of boost. The engine can only take Y amount of pressure so Z amount goes out of the waste gate. I think the relative lack of power off boost combined with the boost comming on a bit quicker due to less air density (Jason Elam at Mile High) make the boost feel more like a 930. Kind of an all or nothing. Great fun!

Kevin Ross

G'Day folks

Can I bring into the discussion the affect air temp has too. The hotter it is the lower the air density - just as altitude has this affect the higher you go... we all know just how good our cars feel on a cold, misty day versus the hot summer temps. Some say this is more seat of the pants than actual fact though. Does the turbo and ECU combo sort this out for temp too?

Cheers

kurtv

One other indirect effect of turbochargers at high altitude is that you can probably only get lower octane fuel. Octane ratings can vary greatly from region to region. For example, the minimum octane rating available in much of the United States is 87 and the highest is 93. In the Rocky Mountain (high altitude) states, 85 octane is the minimum octane and 91 is the maximum octane available in fuel. We have this same effect in Texas at you get to the panhandle. The reason for this is that in higher-altitude areas, a typical combustion engine draws in less air per cycle due to the reduced density of the atmosphere. A turbocharger compensates for this, but you still suffer due to the lower octane fuel.

Kevin

Yes, the turbochargers will and are spinning faster to reach the .8 bar target boost.

However, when one upgrades the compressor wheel the end result in a "slower" shaft speed.. We get two benifits, more air at a greater efficiency (less heat) and then a slower shaft RPM = less wear and tear. Compressor wheels have burst going over mountain passes..

Peter S 993tt

There seems to be some conflicting ideas being talked about.
1 - the .8 bar is a relative pressure - to the outside.
2 - the ECU does dial back boost to compensate for temp - the higher the intermix temp - the greater the tendency for detonation - so spark is retarded. FYI Higher intermix temp is due to higher boost pressure and/or higher outside temperatures.
3 - The reason you in the rockies have lower octane is because you have adopted California fuel specifications by your state legislatures - you can change that - contact your state representatives.