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

[16V Jason <jason@scirocco.org>]

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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