More truck stuff

[Reiswig, George]

[George Reiswig]  Dan wrote:
	 
> Yeah that's kinda what I was trying to say.  It does effect the computer 
> though.  These all signal with a 0 - 5 v signal so if the larger MAF
> signals
> 4 volts say, the computer is calibrated to the stock MAF and is expecting
> a certain mass of air but is actually getting something more.  So, it runs
> leaner in open loop and maybe in closed loop as well (depending on how big
> the difference is).
> 
[George Reiswig]  This bears out under the experiments I did last night.
BTW, Randy, I was right in my logic but wrong in my premise when I talked to
you last evening...restricting the flow over the bypass hole (where the
hotwires are) makes the engine run leaner.  Makes sense, because the ECM
thinks that less air is coming in.  Duh.

For clarity's sake, hereafter when I refer to the "bypass oriface," I'm
talking about the little ~8mm hole where the hot wires are.  

Interestingly, my engine idled very rough with my air cleaner assembly taken
off the MAF unit.  Put it back on, and the idle smoothed out.  The rough
idle was also due to a lean condition, judging from the O2 sensor output.
Why, I asked, would it run lean without the cleaner assembly on?  If you
recall, my intake is now positioned almost directly above my headers.  If I
put anything below the MAF intake (hand, paper, aluminum), anything that
would block the convection coming off the headers, the idle and mixture
improved.  Apparently, the really hot air coming off the headers is fooling
the hot wires.  An interesting point...one might theorize that, since the
air actually going into the engine is the same temp as that being measured,
the MAF should compensate for the change in temp/density.  But it couldn't,
at least at that flow rate.  I also tried it at part throttle, with similar
results.  One wonders what the actual effective temperature range for a hot
wire MAF really is.

	[George Reiswig]  (SNIP) > computer uses a voltage passed by the hot
wires as its data to determine air
> > flow (is that right, it's looking at a voltage, not a resistance?), then
> 
> On mine, it measure voltage.  Yours is more than likely the same.
> 
[George Reiswig]  So an increase in voltage signals an increase in air flow,
or what?

> > multiplying that voltage by a factor of two should get you in the
> ballpark.
> > Nes pas?
> 
> Maybe.  That's assuming that the relationship between the mass flowing 
> through the MAF is linear with the voltage output.  It probably is or is
> very close to it but I'd want documentation or flow test results to make
> sure.
[George Reiswig]  Actually, all I think I'm assuming here is that the
relationship between voltage out and flow rate is one which the computer
expects, a safe bet.  In other words, suppose that throttle position X gives
you flow rate F, and the MAF registers that as voltage +V.  If you add a
second oriface that also can flow F, then your POTENTIAL (this is the
sticking point, I think) flow rate is 2F.  The computer, in order to put in
the appropriate fuel mixture, should see a voltage of +2V.  In actuality
under these conditions, the MAF would probably see something less than +V,
say +.8V (I'm guessing that opening up the second large oriface will
decrease the flow over the wires by less than half...that could be the
showstopper.  Also, it may be the case that halving the flow over the hot
wires will not cut the voltage out in half.  But, in theory, you should be
able to find the right multiple for the voltage, and the relationship
between air flow and voltage would be maintained)(Getting tired of
parenthetical statements?).  So, you should be able to multiply the measured
voltage by (2/.8), or 2.5, and you'll end up with 2V.  

It could happen!

	[George Reiswig]  (MORE SNIPPAGE) 
> Cost is minimal (although I do like the idea of a Vortec better).  I 
> already have most of the hardware I need.  Total cost including the
> injector (maybe 2) and development tools should come in under about
> $300.  Most of that is the tools.  Once I get one working, follow-ons
> could be as little as $100 - $150 depending on the injector source (new
> or used) and how many I end up using.  I'm leaning towards two right 
> now.  Switched alternately to give a steady supply of fuel rather than
> pulses and still not exceed the duty cycle of a single injector.
> 
[George Reiswig]  This is interesting.  What's the breakdown that you have
on parts?  How much is the turbo, how much is the second MAF, and so forth.
Shoot, *I* could almost afford $300!

As I mentioned to Randy, I looked under some hoods yesterday.  Most vehicles
with similar displacement engines have larger MAF openings.  Toyotas, for
example, look positively massive by comparison.  My guess is that there is a
ratio of bypass hole size to main oriface size.  If the bypass for the hot
wires is 5mm in diameter, and the main oriface is 40mm, that's a 1:8 ratio.
Again theoretically, if you increased one diameter by 50%, you should be
able to do the same to the other diameter and maintain the ratio.  However,
I don't know exactly what the relationship is of the slot inside the main
oriface to the flow over the hot wires.  It looks like it's almost a venturi
setup, such that flow over the main oriface walls will create a partial
vacuum in that slot, sucking air in through the bypass oriface.  I don't
know whether or not I'd have to also change that slot size correspondingly,
but that wouldn't be too hard.  In fact, you could make that adjustable,
just as you could the bypass oriface.

GR