[DMCForum] Re: DeLorean MPG (Matt)

["content22207" <brobertson@xxxxxxxxxxxx>]

The whole problem is Martin thinks I am saying things I am not saying
(either I am not clear or there are trans-Atlantic translation
difficulties).

For example, in Message #26140 he seems to think I am saying the CO
screw will affect air sensor plate deflection. I never said such. I
DID say the CO screw will affect what the air sensor plate does to the
metering piston -- an assertion I still stand by (I also stand by the
assertion that it will affect the metering piston throughout its arc
of travel, not just at idle as Martin claims).

In Message #26137 he seems to think I am saying that restricting air
intake will cause a leaner overall fuel mixture. I never said that
either. I DID say that restricting air intake will reduce overall fuel
consumption (fuel/air ratio remaining unchanged). That's how an engine
with a 2 barrel carb yields better fuel economy than the exact same
engine with a 4 barrel, at the cost of performance of course.

There are only two devices that variably affect PRV K Jetronic
fuel/air ratios on the fly: CPR and Lambda. Primary metering valve can
be changed, but it doesn't change thereafter. CO screw can change the
height of the teeter totter, but once set it doesn't change either. If
an owner wants to vary fuel metering while driving, CPR and Lambda are
the only two devices (from the factory) with which to accomplish such.

Also, there is only one fuel/air metering channel -- over the air
sensor plate. From idle through WOT, every foot of air passes over it
(except for a pittance via PCV). Thus any device associated with the
air sensor plate will remain associated with it throughout its arc of
travel, such as the CO screw, which is set at idle but the effects of
which are felt even at WOT. 

Bill Robertson
#5939

>--- In DMCForum@xxxxxxxxxxxxxxx, "Matt Spittle" <mds328@xxxx> wrote:
> Hey there Bill,
> 
> If I may interject here, maybe I can try to clarify what Martin tried
> to say with the math example.  I don't claim to be a PRV expert, so if
> this analogy is flawed to begin with; I have no idea.  
> 
> y = Mx + C
> 
> y --> the total fuel used
> M --> proportion set by the primary pressure regulator
> x --> engine RPMs (we'll just say x goes from 75 to 600)
> C --> baseline fuel consumption set by the mixture screw
> 
> Now if you play with the numbers a bit, you can see the trend...
> 
> If we assume at idle, that C is 750, M is 10 and x is 75, then both C
> and the primary pressure regulator are contributing about 50% to the
> total fuel used.  (this is a *very* crude explanation).  This would
> support the idea that backing off the mixture screw would save you
> fuel.  It's also interesting to note here, that if your mixture
> adjustment is way off, you would really notice it here.  
> 
> Assume then we're cruising at 2500 RPM (x is now 250).  Now C is only
> contributing to 23% of the total fuel used.  Again, if your mixture
> adjustment is way off here, you wouldn't notice it as much.   
> 
> This would seem to support the experimental result that when your
> mixture screw is a little "off" you usually can still nurse the car to
> the nearest garage as long as you can keep it from dying (by keeping
> the RPMs a little higher).  When you do this, you're simply moving
> yourself further along the y = Mx + C graph to where the effect of C
> (the mixture screw) becomes less and less.  
> 
> Now there is a bit of a problem with this example, and it's mostly due
> to the fact that I don't know the baseline relationships between the
> primary press. regulator and the mixture screw.  Maybe the mixture
> screw accounts for 99.9% of the total fuel used at idle?  The only way
> to really get a "feel" for how much it matters is by setting the screw
> to a point where the engine BARELY runs.  In mostly all cases, opening
> the throttle will make it run better, and that seems to be consistent
> with what I've been saying (that the effect of the mixture screw
> decreases as the RPMs increase).  
> 
> Maybe that will help.
> 
> Matt
> #1604
> 
> 
> --- In DMCForum@xxxxxxxxxxxxxxx, "content22207" <brobertson@xxxx> wrote:
> 
> > If high school math remains valid, increasing C will always yield a
> > higher value of Y, all other variables unchanged. You can reduce the
> > other variables, yielding a lower Y, but Y would be even lower if C
> > hadn't been increased in the first place.
> > 
> > I understand the purpose of the CPR. And indeed, it does vary the
> > effect of the air sensor plate on the metering piston. But the bottom
> > line remains -- adjusting the CO screw changes the height of the
> > teeter totter with no ability for further variation short of removing
> > the back window and turning it while moving. If you tighten the CO
> > screw, fuel metering will *ALWAYS* be richer than if you back it off,
> > all other variables unchanged.

---

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