[DMCForum] Re: DeLorean MPG (Martin G)

["Matt Spittle" <mds328@xxxxxxx>]

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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