The boost that they produce is controlled by either a wastegate or a
VNT in the case of 02 TDIs. The turbo itself must be able to boost
well beyond the range it will actually be limited to in the
application.  In the case of altitude, even though atmospheric
pressure is lower, the turbo simply boosts higher to achieve the same
intake manifold pressure, and therefore the same power levels.. it is
of course boosting to a higher pressure ratio than it would at sea
level to maintain the same manifold pressure, but since the system has
this in mind, the turbo itself won't reach it's surge lines/overrev
rpm range.

To add some vanagon content, on the 2nd prototype turbo kit I'm
working on that's installed on brady's van, the wastegate opening is
too small for the gas flow the engine is producing.. this means that
even wide open(where it should not be allowing the turbo to spool),
boost continues to creep higher and cannot be limited to the range it
needs to be in(this is a result of choosing a small turbo to produce
loads of low end and lag-free operation) But I bet at 5k feet it would
be fine since the absolute pressure would be lower... too bad that's
not going to cut it, and the wastegate opening and flapper are going
to need to be enlarged.

Jim Akiba

On Tue, Jul 1, 2008 at 7:04 PM, Mike Miller <mwmiller@cwnet.com> wrote:
> All,
>
> When I had an 02 TDI I was told the turbo produced more boost than the
> engine was set up for at sea level. This allowed full power to 5000 feet
> where the boost no longer had to bled off.
>
> Also told the 944 turbo Porsche was set up the same way.
>
> Anyone know is this is true and if it's true for the TD engines?
>
> Mike
>