In reply to a message from Roleyrev sent Tue 20 Mar 2007:
Hi John
When at first I took control of the fuel injection on my car I also
implemented current limit resistors and using the high impedance
approach (HiZ) but I found that doing so INCREASES the opening time
of the injectors, which in itself is not a big deal, but what I
also found was that doing so increased the DIFFERENCES between
opening times of the injectors which had a MUCH more negative
effect.
What I found was that it was impossible to get the car to idle
relatively lean and to run smooth on lean cruise because these
differences in opening times resulted in great differences in the
AFR ratios between the different cylinders under these two
conditions which cause rough running.
This is what motivated (forced) me to implement the true low
impedance approach (despite the great fear I had for frying the
injectors) where the LOW impedance injectors are switched directly
to ground for an initial period (which opens them in the quickest
possible time) and thereafter applying pulse width modulation is
implemented to keep them open. This is a feature that I am
personally very glad the Megasquirt supports (with hindsight of
course).
Keep in mind that my car is one of the very last Pre-HE cars which
I have concluded is a ‘‘halfbreed’’ between ‘‘true Pre-HE’’ and HE
versions. I’m not even sure if I have Pre-HE or HE injectors on my
car. The fuel rail certainly looks like HE but the heads are
certainly pre-HE.
Now that I’ve implemented low impedance switching (LoZ), the car
idles smooth at an AFR of 14 and I can lean it out to 17.5 on
lean overrun with the engine remaining smooth as we know it to be,
although I have chosen to not go leaner than 16.5 at very low
manifold pressures (40 - 15 kpa).
Megasquirt-2 supports both an Inlet Air Controller (IAC) which is a
stepper motor as well as a Pulse Width Modulated air controller
although I do believe one can not have true low impedance injection
switching AND PWM air control simultaneously (since the same PWM
circuit is used for both. The IAC and PWM control is temperature
based though, where I would have preferred it to be rpm based
(which is more complex to implement sw wise) as you confirm the
Linkplus does. My car still has its AAV which is working perfectly.
I have wired the AC pypass valve in parallel to the AC clutch which
results in a very good ‘‘constant’’ idle speed regardless whether the
AC compressor clutch is engaged or not.
Kind regards
Philip–
The original message included these comments:
The LinkPlus G3 does require current limiting resistors in line
with the injectors. WBO2s are not included in the cost, I feel
the least amount of fuel in 1977. Another thing that is very cool
is the ablity of the ECU to control idle speed (via the Bosch AAV
Ex BMW) in a closed loop fashion. This makes the idle respond to
–
'80 XJS PreHE, 5spman, Dual EDIS, Megasquirt-2 (spark&fuel)
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