# absolute pressure vs boost



## wanderlust (Feb 19, 2012)

Are our d's intake pressure regulated absolutly or relativly? ie does atmospheric pressure play a roll or is it mearly an absolute pressure. if they are dyno'd at 1000 ft and at 5000ft with everything else =will there be a difference?


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## wxmanCCM (Feb 17, 2010)

All I can tell you is that the European BMW diesel specs are "charge pressure above atmospheric pressure".

The 286 hp 3.0 liter diesel engine (M57) has a stock "charge pressure above atmospheric pressure" of 1980 mbar (1.98 bar = ~29 psi) according to the spec sheet.


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## wanderlust (Feb 19, 2012)

ok so its probably a mechanical valve and elivation probably decreases power

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## boooomer (Apr 23, 2010)

wanderlust said:


> ok so its probably a mechanical valve and elivation probably decreases power
> 
> powered by lemings


Interesting...I always thought turbocharged motors were at an advantage over normally aspirated motors at higher elevations.

I'll have the car in Santa Fe (el 7400') this summer for 2 months. It will be interesting to see if there's any noticeable difference in performance from here in SE Florida.


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## WreckerAFG (Apr 24, 2012)

I've read somewhere (probably this forum) that the 335i increases boost to make up for the loss of pressure in higher altitudes. I wouldn't see why the d would be any different. 

In order for the d to have no loss in power at 7400ft above sea level, the car would have to provide an additional 11psi of boost.


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## Flyingman (Sep 13, 2009)

Umm, Check the math. Atmospheric pressure (at sea level) is 14.7psi last I checked. Also known as 1 Bar in metric.

7600 feet is 2300 meters. Air pressure would be approx 11.0psi or 0.75 Bar.

So the turbo would need to boost about 3.7psi (some 12% of rated pressure) to reach equivalent sea level air pressure. Our turbos have plenty of excess capacity to satisfy this deficit.

Diesel Engines are well known for being least affected by altitude, especially when turbo charged. I doubt the 335d would miss a beat climbing any mountain pass we may run across.


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## WreckerAFG (Apr 24, 2012)

Flyingman said:


> Umm, Check the math. Atmospheric pressure (at sea level) is 14.7psi last I checked. Also known as 1 Bar in metric.
> 
> 7600 feet is 2300 meters. Air pressure would be approx 11.0psi or 0.75 Bar.
> 
> ...


Math is right, I didn't realize I got it backwards lol. Thanks though. I just did the standard 29.92 pressure, and subtracted 7.4 for the 1000ft/1" mercury loss and multiplied by .491 to convert to psi.

Good to know the diesels shouldn't be too affected by the loss in pressure.


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## wanderlust (Feb 19, 2012)

yeah the theory is sound, up the boost to not loose power, im just not certain its actualy done, hense the absolute pressure question. could tell quick if anyone has dyno numbers from denver or other high altitude. should be a substantial difference if boost isnt increased at altitude. I know most supercharged engines do loose hp, but a turbo could just leave waste gate closed to compensate.

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