Higher Altitude = Higher Boost? :confused:

Stealth TDI

Pre-Forum Veteran Member
Joined
Feb 13, 1998
Location
Newport News, VA
TDI
2017 GTI APR Stage 3 (395 hp/376 lb-ft)
Hi,

I'm confused. I've been posting lately about how I found the "bleed" to be sort of useless. I get the same 20-22 psi with or with out the bleed. I remember having the exact boost levels that Rene said I'd get, 20-psi peak and 17-sustained. Later, I bled for 20-psi continuous. Recently, I removed the bleed because I decided it wasn't worth the hassle of fiddling with the adjustment... plus I kept spouting leaks.

I quoted that my boost levels remained unchanged after removing the bleed... 22-peak and 20-sustained. But now I'm at a significantly lower altitude (near Austin) and I'm measuring the "original" Upsolute pressures of 20-peak and 17-sustained.

So now I'm confused. I thought the turbo had to struggle to make the same pressure at altitude due to thinner air. To me, thinner air equals lower available pressure. But my car's not doing that.

Can someone explain why, without ANY adjustments, my turbo produces 3-psi more boost at 4300-ft than it does at 1000-ft?

Thanks,
 

jaydhall

Veteran Member
Joined
Apr 2, 1999
Location
Aurora Colorado
TDI
2012 Passat SE, 1999 NB, 1999.5 Jetta GLS, 2004 Jetta
Hi stealth!

I thought of one. It depends how your presure gague works. If it compares your boost to outside air pressure, then maybe the difference of thin outside air vs what the ECU is producing is the answer. This is just a guess. (As compared to sea level pressure.) Sort of like my air pressure tire gauge. I pump the tires to 42 at altitude and here at sea level it is quite a bit lower.

73 WA6MOK... dit dit
 

Stealth TDI

Pre-Forum Veteran Member
Joined
Feb 13, 1998
Location
Newport News, VA
TDI
2017 GTI APR Stage 3 (395 hp/376 lb-ft)
Hi,

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>Originally posted by jaydhall:
If it compares your boost to outside air pressure, then maybe the difference of thin outside air vs what the ECU is producing is the answer.<HR></BLOCKQUOTE>

I'm pretty sure that's how they ALL work. However, your tire theory doesn't quite apply here because the amount of air in your tires doesn't change... just the pressure over time/altitude. In the case of my turbo, the air is constantly being refreshed.
I know what I'm thinking; but can't quite express it here. Perhaps some of our physics guys will wake up this morning and do some splainin'.
 
M

mickey

Guest
I would imagine the wastegate on the A3 turbo is not controlled by "boost", but rather by Absolute Pressure. The higher you go, the more boost is required to maintain a given Absolute Pressure.

The A4 turbos work off Absolute Pressure as well, but the computer is programmed to give you LESS boost at higher altitudes in order to protect the turbo.

-mickey
 

weedeater

Top Post Dawg
Joined
Mar 17, 2001
Location
Reston, VA
TDI
Jetta, 2001, Baltic Green
When the key is turned prior to the engine actually starting, the ECU looks at the reading from the Manifold Absolute Pressure (MAP) sensor to get the 'resting' pressure. This corresponds to the altitude of the vehicle at the time of startup. The ECU then uses this figure as a basis for controlling the turbo, modulating the 'wastegate' to maintain the required boost for that altitude.

I've read that its a good idea, if driving from/to different altitudes, to stop and restart the vehicle to let the computer know how the altitude has changed in order to recompute the mixture values (gas engines).
 
Top