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WRX vs. 1.8t
Actually, from what I remember from Bosch school, MAF does sense density in
the air and temp not to mention flow so it seems that a change in altitude
would make little if any difference at all. It use to make a LOT of
difference, especially with carburated cars, but for the most part the MAF
has fixed it (along with what the ECUs do with the info the MAF supplies).
Dave
----- Original Message -----
From: "Dan Bubb" <jdbubb@ix.netcom.com>
To: "Neal Tovsen" <nealtovsen@yahoo.com>
Cc: "Brandon Smith" <SCIROCCO_SPEED@msn.com>; <scirocco-l@scirocco.org>
Sent: Friday, October 25, 2002 12:07 PM
Subject: Re: WRX vs. 1.8t
> OK. I think I see my error.
> The ECU can look at either MAP or MAF to provide for fuel or ignition.
> MAF is better cause that's the specific value you want to know. MAP is
> generally proportional to MAF but can vary depending on humidity or
> temperature.
> But, what the original issue I was trying to address is how the ECU
> controls boost pressure. Most cars control boost pressure by directly
> sensing and controlling manifold pressure. So, the pressure above
> atmospheric is a fixed value. My 1.8T doesn't measure manifold pressure.
> It measures MAF and regulates boost pressure to control MAF.
> As atmospheric conditions (humidity, temperature, ambient pressure)
> change a car that senses and directly controls boost pressure will have
> a change in MAF that will effect power. And in the case of a significant
> altitude change, since the manifold pressure is a fixed amount above
> atmospheric, the change in MAF can be large. The 1.8T doesn't respond to
> changes in atmospheric conditions cause it's only looking for mass air
> flow and will change boost pressure as required to get it.
> That explain it?
> Dan
>
> Neal Tovsen wrote:
> >
> > > So, is that clearer, or just gobbledygook?
> >
> > Yes, clearer. But yet, gobbledygook! Hehe.
> >
> > The difference between measuring MAP vs. MAF has
> > always confused me, so that is definitely where the
> > problem lies.
> >
> > I don't quite understand why a given absolute pressure
> > value in the manifold doesn't directly relate to
> > density of the fluid. In my mind, density should be a
> > simple factor of pressure. If it did, it would make
> > MAP vs. MAF irrelavant, which you say isn't the case.
> > But I'll admit that I need to learn more here...and my
> > physics book has been collecting dust on the top shelf
> > at home for years. :)
> >
> > Neal
> >
> > --- Dan Bubb <jdbubb@ix.netcom.com> wrote:
> > > 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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