Re: fifth-gear and miles per gallon [CIS-E]

["Allyn" <amalventano1@home.com>]

OK, here's why there is different flow at the 2 throttle positions:
Think if your running engine. Pistons act as a huge pump. This does whats
called 'drawing a vacuum' in your intake manifold. The only thing stopping
the 'atmospere' from rushing in to fill the vacuum is your throttle valve.
At idle, there is very little air coming in. The intake manifold remains at
a near perfect vacuum. This means for each intake stroke, the pistons only
draw in a little bit of 'air mass'. This small mass is coupled with the
little bit of fuel sprayed in by the injector. You get minimal combustion
and your engine stays at idle.
As you open the throttle, air rushes into the intake manifold faster,
attempting to overcome the sucking power of the pistons. This gives the
pistons more mass of air to suck in on each stroke. Since there is now less
restriction (throttle opening), air flows past the sensor plate faster. This
causes higher fuel pressure at the injectors (cis), spraying more fuel in.
This matches (hopefully) the increased mass of air in the cylinders, and you
go faster.
Now using this info you can see that opening throttle from 50-75% will burn
more fuel. Less restriction, more flow, plate rises, fuel pressure goes up,
more sprayed, more vroom.
Hope this helps.
Al

Allyn Malventano, ETC(SS), USN
87 Rieger GTO Scirocco 16v (daily driver, 170k, rocco #6) running - for now
87 Jetta 8v Wolfsburg 2dr (daily driver, 260k, 0 rattles, original clutch,
driveshafts, wheels :)

----- Original Message -----
From: "16V Jason" <jason@scirocco.org>
To: "Marc Peterson" <mpeters6@rochester.rr.com>; <scirocco-l@scirocco.org>
Sent: Wednesday, October 31, 2001 5:17 PM
Subject: Re: fifth-gear and miles per gallon [CIS-E]


> At 04:48 PM 10/31/2001, Marc Peterson wrote:
> >Assuming no enrichment is used...
>
> Right...
>
>
> >All CIS/CIS-E engines mixed by the air flow meter flap.
>
> Right!...
>
>
> >   If the engine is
> >running at a given RPM, it is pumping a fixed amount of air though it, so
to
> >my understanding 3000 rpm is the same amout of fuel whether it is 1/2 or
3/4
> >throttle, the mixture doesn't change based on the throttle position.
>
> Oh! So close! :)
>
> You're totally right --mixture is determined by air flow sensor.  So the
> mixture (proportion of fuel per air) should stay relatively constant
> through the rev range.  However, at higher throttle openings, the engine
> moves more air, and thus more fuel.
>
> >This is different on Motronic system, obviously.  They have a throttle
> >position sensor.
>
> I'm not all _that_ familiar with Motronic, but I'd be surprised if all it
> had is a throttle position sensor -- doesn't it have a mass airflow sensor
> as well?  I'm pretty sure every single modern fuel injection system uses a
> mass airflow sensor to determine how much air is going in, and then
> correlates that with throttle position for a number of reasons (to richen
> mixture during abrupt throttle movements, full throttle enrichment, etc
etc).
>
> Jason
>
>
>
>
>
> >I think the above is right.
> >
> >Marc
> >----- Original Message -----
> >From: "Fry, Larry" <LEF@chem-tronics.com>
> >To: <scirocco-l@scirocco.org>
> >Sent: Wednesday, October 31, 2001 4:33 PM
> >Subject: fifth-gear and miles per gallon
> >
> >
> > > Hypothetical situation: you have fifth-gear ratio that gives 3800rpm
at
> > > 80mph and it takes 1/4 throttle to maintain that steady speed.
> > > second hypothetical situation: you have, on the same car, on the same
> > > stretch of road, a fifth-gear ratio that gives 3000rpm at 80mph but it
> >takes
> > > 1/2 throttle to maintain that steady speed.
> > >
> > > question: which situation yields the best mileage?
> > >
> > > In other words, which influences miles-per-gallon more, rpm or
throttle
> > > position?  Or is it pretty much a wash?
> > >
> > > Inquiring minds......
> > >
> > > Larry  sandiego16V
> > >
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