[gofast!]

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[Bimmer This]

Because at high altitude, the air is "thinner" causing your turbos to work twice as hard to force more air through it. I think that's pretty much it!

[raceyBMW]

The turbos at higher altitudes will increase boost pressure to compensate for the lower absolute pressure. So yes, technically, our turbos at our altitudes have to work harder to keep the same power as it would make at lower elevations. As far as longevity is concerned, I would assume BMW did lots of high altitude testing. They would have designed the turbos and engine for the most extreme conditions. So basically at sea level, the turbos would not be anywhere near their capacity...which is why it is so easy to raise boost with the piggybacks.

Everyone loses a little bit of power at our altitude, even turboed engines. No way will you find someone who runs a 1/4 mile up here match what someone can do at sea level. We just loose significantly less power than a naturally aspirated engine would since they have no way to overcome the "thinner" air. there are also other things to consider that the turbos working harder means that the oil/radiator fluid will run hotter and possibly increase air temps going into the engine. Hopefully the IC was designed to prevent this at our elevations, but who knows for sure.

And no, its not a stupid question at all, but you for sure would have been told to go search or whatever.

[335TrackAddict]

Turbocharged vehicles lose power at altitude for various reasons.

A turbo doesn't care about psi. It only cares about pressure ratio. If you are at sea level (14.7psi ambient) and want to produce 14.7psi boost, the turbo workload is 2.0 pressure ratio. Right now, my altimeter watch is indicating 12.02psi ambient. For the same turbo to produce 14.7psi boost, the turbo workload is 2.22 pressure ratio ((ambient+boost) / (ambient). It is working harder to produce the same PSI.

The problem is that when turbos go higher and higher in the pressure ratio metric, they produce less additional PSI and more heat. Hot air = bad.

To compound the problem, the intercooler is less efficient. At altitude, there are less molecules moving across the intercooler fins. It is therefore less efficient at drawing heat away from the intake charge.

Some cars (Sti, I believe) have ECUs that are tuned to reduce boost at altitude. That reduces the stress on the turbo by making it work at the same pressure ratio.

BMW will up boost to compensate for lower ambients. However, it does not fully compensate. That implies to me that in stock form, the turbo has enough headroom to spin faster and compensate, to an extent, for the thinner air.

Throw in a tune and the turbo is working incredibly hard at altitude.

[F82_SID]

So then would a blow off valve really help reduce some of the extra stress that is being put on our turbos?

[forza1976]

Originally Posted by gofast!

I think someone posted on here last week that a 335 would lose about 10% of its power here in CO. However, when shopping for cars last fall I looked at a few Audis and all three of the dealers pushed me towards the 2.0T engine because they said it loses no power at altitude and actually runs better than the 3.2 V6. I've heard this 0% loss statistic from a few others as well.

I think half of what the deal is saying here is reasonable as well. The 2.0T does seem to feel better at this altitude versus the 3.2. Due in part to much of what the other guys have stated as well.

However, the Audi is just as susceptible to power loss as the BMW due to the lack of oxygen we have and of course air pressure. He is of course attempting to sell you a car

[335TrackAddict]

Originally Posted by E92_SID

So then would a blow off valve really help reduce some of the extra stress that is being put on our turbos?

Not really. The stress is there when you are under load and producing boost.

The BOV just vents excess pressure when you get OFF throttle and no longer need boost. The stock diverter valves have worked fine for me for that purpose.

[raceyBMW]

The only way to reduce stress on the turbos under load is for the turbos to not be working as hard.

Mods such as catless DPs and an upgraded IC will help the turbos have to not work so hard to achieve a the same manifold pressure.

Catless DPs reduce the backpressure on the exhaust side of the turbo, allowing it to spin more freely.

An upgraded IC with a lower pressure drop across the cooling bars/fins means there are less losses of pressure from the turbo to the manifold, so the same manifold pressure can be achieved with the turbos boosting less. Obviously the other benefit to an upgrade IC as well is cooler charge air temperatures...cooler denser air will always make more power in our cars...which also may have a benefit to the turbos since higher AFRs mean you can run more advanced timing and therefore less overall boost from the turbos is needed.

BTW, someone correct me if I am wrong on any of this, don't want any misinformation going around.