WRX vs. 1.8t

[jdbubb@ix.netcom.com (Dan Bubb)]

Let me see if I can explain a little better.
If you have a mechanical wastegate or the ECU controls the wastegate for
a set value of manifold pressure then the maximum pressure in the
manifold will always be the same. Let's assume 15 psi. Then at sea level
manifold absolute (as oppossed to "gauge") pressure will be 30 psi (just
assume ambient is 15 psi for this example). i.e. manifold pressure +
ambient pressure. Now if you go to Colorado Springs the ambient is 12
psi. An NA car will have 80% (12/15) of sea level power. Our boosted car
will have 90% [(12+15)/30] of sea level power.
My 98 Audi 1.8T doesn't even have a manifold pressure sensor. All it
does is measure MAF and adjust the wastegate to acquire a set MAF value
at WOT. So, on a humid day where air density is down it will run higher
manifold pressure for the same MAF. Same thing at altitude. If ambient
pressure drops then  MAF will drop if the manifold pressure is at a
fixed boost level, but the Audi is only looking at MAF and adjusts
manifold pressure up to, again, reach the desired MAF value. So, if the
Suby ECU controls MAP and the Audi/VW ECU controls MAF then the Suby
will be at 90% in the above high altitude example and the Audi/VW will
be at 100% minus some amount associated with lower turbo efficiency at
higher pressure ratios.
I don't know whether the Suby ECU controls MAF or MAP, so this is all
conjecture. Also, athough my 98 Audi doesn't have a MAP sensor, later
Audi/VW's do. I still think they control MAF and not MAP, but I could be
wrong.
So, is that clearer, or just gobbledygook?
Dan 

Neal Tovsen wrote:
> 
> I'm not quite sure I follow this, but I know this is
> getting over my head technically so I could be plain
> wrong...
> 
> I think you're saying the early 1.8t measured the MAF
> and adjusted the wastegate via the ECU instead of a
> self-controlled mechanical wastegate. But mechanical
> wastegates are driven by a diaphram...isn't that
> basically the same thing? Boost is pressure, which for
> the same fluid should equal density. Thus, for a given
> volume/path and speed boost should be directly related
> to MAF. Or am I missing something important? 15psi in
> the manifold at 5000rpm is 15psi at 5000rpm,
> regardless of what the air pressure was *before* it
> was compressed.
> 
> Neal
> 
> --- Dan Bubb <jdbubb@ix.netcom.com> wrote:
> > The early 1.8T's didn't actually measure and control
> > boost pressure.
> > They measured mass air flow and adjusted boost
> > pressure to control that.
> > I don't know if the later 1.8T's control boost the
> > same way or measure
> > and control boost directly.
> > The net upshot is that at higher altitudes the VW
> > engine will run higher
> > boost to attain the same mass air flow the engine
> > would have at sea
> > level. So, power declines at altitude (mostly due to
> > turbo inefficiency
> > at higher pressure ratios), but not much. If the
> > Subaru directly
> > controls boost then it will loose more power than
> > the VW at altitude.
> > Maybe that explains it. Or maybe the Suby driver was
> > lost in turbo lag?
> > Dan
> >
> > -------- Original Message --------
> > Subject: WRX vs. 1.8t
> >    Date: Thu, 24 Oct 2002 12:59:12 -0600
> >    From: "Brandon Smith" <SCIROCCO_SPEED@msn.com>
> >      To: "SCIROCCO-LIST" <scirocco-l@scirocco.org>
> >
> >  Okay to make myself clear, here in tha Mountains of
> > Colorado (well not
> > really, but still over 6,200 ft above sea
> > level...like the base of Pikes
> > Peak) its just been my experience that the 1.8ts are
> > faster than the
> > WRX....why??? assuming because of the turbo lag of
> > the WRX.   Up here
> > the 1.8t pulls harder.  guess maybe VW had visions
> > of their european
> > customers driving around in the Alps a lot.? but for
> > some reason, a
> > 150hp 1.8t in a VW seems to pull harder in the first
> > few gears than a
> > 180hp Audi ???-Brando
> >
> >
> >
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