Re: FW: Back to the gas question... -Reply

From: JT McBride (James.McBride@GDEsystems.COM)
Date: Mon Feb 24 1997 - 13:10:55 EST


I'm sure the SBEC has both ROM and volatile memory. The basic program
the computer executes is in ROM, the fault codes, and data from what
it 'learns' is in volatile memory.

An 8-bit microcontroller would use look-up tables to find the appropriate
injector duration and timing advance for a given engine speed, O2-sensor
input, temp, pressure, and throttle position. There's a default table in
ROM - what the SBEC uses when installed, and when the battery's been
disconnected for awhile. A modiefied version of this table is written
to volatile memory as the SBEC learns your driving habits - if you are
happy with a certain acceleration curve, it maximizes the efficiency of
the vehicle for that curve. Since most of us can't drive like a bat out
of hell every day (no ticket immunity, among other things), the curve
the SBEC 'learns' is pretty mild, so 'zapping the PROM' or disconnecting
the battery so it loses this memory, will give you back the default
"showroom" curve.

I believe the SBEC has a special program for Wide-Open-Throttle (WOT)
conditions, and this is outside 'learned' programming. You really have
to stomp the pedal to the metal to engage it, but it will always give
you all the engine's good for. Enough WOT after initial acceleration
will get the SBEC to modify the 'learned' curve as inadequate, but as
I say, we don't get to drive that way too often (especially those in
snow country this time of year).

The GM and Ford controllers use a much more sophisticated control law,
where they actually calculate what you need on the fly. This takes a
great deal more compute power, but results in better gas mileage, even
lower emissions, and improved driveability under all conditions. It
costs more monsy too though, which is why mainly Cadillacs, performance
cars like the Camaro Z-28 and Corvette, and the Lincolns from F*rd get
them. I think Chevy trucks still make do with controllers much like
the Chrysler SBEC. One easy way to tell is whether they can take advantage
of higher octane fuel - but not require it.

Jim

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