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WRX vs. 1.8t
Yes, MAF does what it's name implies. It measures mass air flow, so MAF does
compensate for changes in density due to humidity, temp or altitude. So,
it's the best direct input (as opposed to MAP or throttle position) for
correct fueling and ignition.
MAP and MAF are generally proportional, but humidity, temp and especially
altitude will cause deviations in proportionality.
As far as engine power related to how the wastegate is controlled, you can
regulate to a given amount of air flow or a set manifold pressure. At sea
level it makes little difference but, these two strategies will give
significantly different power during high altitude running.
That's the point I keep trying to make.
Dan
----- Original Message -----
From: Rabbit16v <Rabbit16v@attbi.com>
To: Scirocco List <scirocco-l@scirocco.org>
Sent: Friday, October 25, 2002 9:38 PM
Subject: Re: 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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