If I were to drive to Denver, CO, at a mile above sea level when my in-dash boost display reads 1 bar, am I getting the same actual boost pressure that I would get at sea level, or something less due to the high altitude?

 

AFAIK, the dash is gauge pressure. I would see 1.2 bar in Albuquerque on my 1 bar flash as the ECU was trying to maintain a constant absolute pressure (~2000 mbar). Pressure there was about 800mb so gauge pressure was 2000mb-800mb=1200mb=1.2 bar.

 

Yes, same boost pressure as at sea level but the turbos would be working (spinning) harder due to the thinner air. Same principle applies to a hot day cold day scenario. As altitude (or temperature) increases, air molecules will be spaced farther apart.

 

All true. It works the same for turbine engines at altitude. The higher you fly, the thinner the air, so the compressor has to spin a little faster to make the same power. Usually the limiting factor on most jet engines becomes heat. They start running hotter until temperature limits are reached prior to torque limits or compressor speed limits. I imagine a turbo charger runs hotter too under the circumstances, but to a lesser degree, but it is not measured or monitored. I suppose if it were a factor, engineers would put EGT (Exhaust Gas Temperature) gages in your car among other things.

 

guess what 997TTs have?

 

If you hook up your Durametric, it will show you the absolute pressure in mm Hg. Let's call the absolute pressure at idle P_0, and the absolute pressure under boost P. The boost level B is computed as:
B = (P - P_0)/760
where 760 mm Hg is "standard pressure."

If you were at higher altitude, where P_0 is lower, then the same boost level would be reached at a lower absolute pressure P, and correspondingly lower density of air.

Jon

 

Thanks for all the replies. It seems as though most people are saying that the boost readout displays "gauge pressure" (pressure relative to ambient pressure), whereas the ECU operates with respect to absolute pressure so that at higher altitudes, the readout display will be higher (at altitude than at sea level) as the ECU is compensating for inherently lower ambient temperature to reach the same absolute pressure as at sea level. Is that correct?

 

My info is the engine controller will attempt to satisfy the torque demand represented by the throttle signal from E-Gas.

At a higher elevation the engine controller will allow the engine to develop more boost than it would at say sea level provided there are no other limiting factors present.

I have observed this several times in my Turbo. On one section of I-80 in WY I managed to see 0.9 bar of boost for some distance, climbing a good upgrade at over 8K feet.

At lower elevations but under similar circumstances I have seen 0.8 bar and for a sustained period of time.

Sincerely,

Macster.