[alfa] RE: L-Jetronic and Pat Bradens claims!

["John Fielding" <johnf@xxxxxxxxxxxxxxx>]

Hi Ben,

I saw your comment today about the Pat Braden book.  In it you said that the Bosch
L-Jetronic injects the fuel once every 4 revolutions of the engine. Pat obviously was
smoking something if he wrote that!

I have extensive knowledge of the L-Jetronic and Motronic system, and I can tell you that
Bosch do no such thing.  No engine I know of will run properly with only one injector
pulse per 4  revolutions (8 stokes).  Most fuel stategy maps use either 1 squirt per
2 revolutions or 1 squirts per revolution.  Using only 1 squirt per 2 revolutions
sometimes gives a rough idle and lack of torque at the low rpm range.  What
I think Pat probably meant was to say 1 squirt per 4 strokes, which is 2 revolutions.

When developing the fuel table, Bosch and the engine designers work together to get a
good compromise,  and this heavily leans towards emissions aspects in your market
place.  The fuel map is developed from several engines running on a dyno and also
from testing done with the engines fitted in vehicles to satisfy the driveability issues.
I am sure Eric Storhok can give some inside information on both aspects, as this is his
job.

What the WOT switch does is that it signals to the ECU that the throttle is beyond
the 60% open mark.  This causes the software to change its operating mode.  It
ignores the O2 sensor input during the WOT period and uses just the AFM vane voltage
and engine speed information to compute the fuel required.  At WOT the ECU expects
to see the AFM vane voltage reach a maximum, when it does it increases the injector
pulse width to provide more fuel, initially as an accelerator pump.  After a predetermined
"pump-shot" the fuel is backed off to a leaner condition. If the WOT and AFM voltage
stay high it works out the required extra fuel from an internal fuel map derived when the
engine was tested. There is no need to "double pulse" the injectors, they can flow more
an enough fuel by being driven up to 80% duty cycle.

I have a test jig for both the L-Jet and the Motronic ECU's I built some years back to try
and understand how the systems work.  I was trying to modify the Bosch systems for a turbo
race car.  In the end I gave up as so much of the operating system is locked up in
software, which no one has so far managed to break into and modify.  I ended up using a
Megasquirt and tossed the Bosch stuff.

So beware what you read in books, the biggest culprit is Charles Probst book on EFI.  Many
people read this and assume it is gospel.  Worse still they repeat what Probst wrote and
so proliferate the errors.  I know the local Bosch technical training lecturer and he was
very helpful when I was trying to modify the Motronic ECU.  He told me things you will not
find printed anywhere, except in the secret Bosch internal training manuals.  Even these
contain errors as I discovered when I had the ECU on the bench running off the stimulator.

I hope this helps.

John
Durban
South Africa
Alfetta 1.8L turbo - with Megasquirt EFI & Megaspark ignition ECU.
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