[Supras] Crazy Lex tuning thought + Dawson

[berniek at technicaldevelop.com]

Dear Jon, Mike, Jim and list:

    I've read quite a few stories about tip-in hesitation from idle with any
electronic device which alters AFM frequency in the case of the Karman
Vortex type air meter.  One reason is that there MUST be at least one cycle
of latency (one Hz of latency) while the intermediate device measures the
vortex period, and generates a new period (reciprocal of frequency, faster
to do than frequency measurement), which must and will be asynchronous with
respect to the one from the AFM.  At higher speeds the effect of latency
disappears because everything happens faster, but near idle with intake
manifold vacuum, a typical gasoline engine has an airflow turndown of about
40:1 (idle to WOT), more than that with forced induction.  So if the maximum
vortex freqency is, say, 1200 Hz, at idle it will be 30 Hz with an N/A
engine, probably 23-25 Hz with a turbo at stock boost  Add in at least 100%
for completion of period measurement and generation of a new "false"
frequency and there can be perhaps 1/4 to 1/10 of a second delay in airflow
reading reaching the ECU.  Idle frequency will be lower that stock with the
LEX AFM body, adding to the latency problem.

    My last car was an 84 300 ZX which used a hot wire AFM.  Tip-in was
always a problem with it as well, traced to thermal inertia effects,
although I learned how to drive to avoid its effects.  When I had more time
in the past, I did some careful analysis of the hot wire electronics, and
found that it was of the feedback type, where wire temperature is held
constant measured by resistance, and the imposed voltage to keep it so
served as the AFM output.  But the control loop itself took time to act,
despite the fact that it appeared to have derivative (rate) response.

So what I will probably do other than a mechanical arrangement is to install
a lower resistance air temperature thermistor (with a curve similar to the
factory one in the AFM electronics), connected in parallel with the existing
one by WOT switch or WOT switch operated relay.  That will fool the (Lex)
AFM into believing there is less air flowing than is actually the case.

    Has anyone had experience with the Dawson A/F four LED indicator?  It
reads the last bit of stock O2 sensor voltage near 1V and extracts the
compressed rich running information from the output voltage range of aboug
.9 to 1V.  I'm not looking for the ultimate in accuracy possible with a WB,
but plus or minus maybe 1/2 A/F unit to keep it rich enough for safety.  I'm
not looking for most HP.  Whatever I can get with 14 PSI, Walbro, AFM and
550cc injectors I will take (maybe an upgraded CT26 later).  Here in NJ I
don't have the time to see if I can pass emissions with one cat and a
downpipe, although I do have a cat-back 85 mm HKS exhaust (which the state
inspectors really listened to at last inspection).  So I cannot locate a WB
where temperatures are lower than in the turbine elbow on a permanent basis.

    Thanks.  Any and all information would be appreciated.

        BernieK


----- Original Message ----- 
From: "Hacker J" <jonbhacker at yahoo.com>
To: <supras at supras.com>; <berniek at technicaldevelop.com>
Sent: Monday, October 09, 2006 4:14 PM
Subject: Re: [Supras] Crazy Lex tuning thought


>
>
>
> Bernie,
>
> What you are proposing is effectively what the electronic fuel piggybacks
do already by inserting
> themselves between the ECU and the AFM.  I would suggest that instead of
your mechanical approach,
> you just buy a Apeci SAFC, MAFTPro, or any other supra compatible fuel
piggyback and then you will
> be able tweak the AFM gain as a function of rpm, load and so on with more
flexibility and
> convenience than any mechanical bypass valve would allow.
>
> Jon
> 90T
>
>
> >
> > OK.  First, I'll expect the nice men with white coats and the extra
large leather jacket at the
> > door when this is read.
> >
> > Now for the nutzoid question:  has anyone ever used a boost pressure
operated bypass valve
> > around the AFM (Lex which I'll be using, although the same should apply
to the stock sensor
> > body) to provide a slightly leaner mixture at WOT?  If the bypass is
sized to provide perhaps
> > 15% of the AFM flow, it would lean the mixture from 9.5:1 to about 11:1
when in open loop at
> > WOT.
> >
> >       BernieK
> >
> > > _______________________________________________
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> > Supras at supras.com
> > http://supras.com/mailman/listinfo/supras_supras.com
> >
>
>
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