Folks,
I hope that this help a little bit...
....We have installed, made harnesses and programmed sequential (motec)
after market engine mgmt systems on 928 engines, made lots of power and have
gotten them to pass calif smog! I have also bought, understood, installed,
mapped, etc. other systems onto other engines.
Are they better??...in general yes...but programming them to work for a
daily driver takes about 200+ hours minimum. Programming them to race take
about 20hours. And to race, you can get to the 90 percent level in about 8
hours.
It is much harder to make a street car work correctly, due to all the
factors that must be considered...cold start, hot start, part throttle
decel, part throttle enrich, battery volt creep/duty cycle, canister dump,
flappy, altitude start, warm up, smog...yes, we here in calif worry about
smog...But a race car is easier.
The stock bank to bank fire is a very good system....and it works well for
almost all applications.
Remember, The White Car generates between 425 + rear wheel hp and 470 + ft
lbs (chassis dyno numbers) and uses a stock system with larger injectors and
rrr. No chips, no fus..... Is this the limit, no, I expect the
system to
work into the 600 plus ground hp range due to the size of the MAF meter.
There are other issues at that power level, and one is not the fuel/ign mgmt
system. And the white car uses stock cams and passes smog.
How does it work?...The 928 system works as follows....
When you start the engine, the maf takes readings to determine altitude,
temp, etc. (mass air properties for the moment) and any changes from last
stored data...it then provides a map offset factor to the fixed map. The
fixed map is what is changed when a chip is burned.
This is why the engine runs funky when you disconnect the battery..it must
reset itself....take about 10 minutes. Every time you make a change to the
engine, it takes about 10 minutes to reset.....
The system runs on the maf meter up to some reading (approx. .xxx volts or
so) or up to about 3000 rpm with full throttle, at which point, it switches
over to map running and remains on the map till the rpms or maf reading
drops below that above set point.
O2 sensing/closed loop is done below some rpm...I have not found the point
yet, but guess that is is around 3000 at full throttle or higher rpm at part
throttle. The maf and O2 feed the ecu info to create the map offsets.
The system uses all throttle positions (below the set point) to get the
correct offsets for the map based upon many throttle positions/engine loads
below the maf meter set point/rpm set point. The .xxx volts may be reached
in many positions...full throttle at 2500 or 3000 rpm, or part throttle at
6000 rpm!
With a bigger engine, the maf point is reached at a lower rpm (under full
throttle), then with a stock engine. But that does not seem to affect the
map, at least not on thw white car. For about the last year or so, our
dyno
guy has a pi a/f ratio meter that I just plug into the exhaust to get
readings...and the curve looks very similar to a stock engine.
The hardest part about working with a non tunable stock system is matching
injector size with fuel pressure to get at a good a/f ratio through the
range of rpms above the set point....
One question I am always asked is what really messes up the system?....cam
timing....too much overlap raises hell...as a maf is unidirectional!
I hope that this helps....
--
Marc M. Thomas
DEVEK
650-592-5287 phone
650-610-0557 fax
http://www.devek.net
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>> During part-throttle acceleration the FI system relies on the
preprogrammed maps and the O2 sensor. During throttle opening it also
increases fuel delivery (longer pulse time) to avoid stumbling using a
preprogrammed increase in pulse time. <<
My understanding is that the system is still closed loop during part
throttle acceleration, so the computer controls the fuel flow based upon
the standard inputs, including the oxygen sensor input.
>> However, it's not entirely clear to me that LH-Jetronic-equipped 928s
are 'pure' LH-Jetronic since they are also equipped with the EZK
ignition computer. The two in combination do the same thing as an
LH-Motronic system. LH-Motronic systems ignore the air-mass signal
during WOT and rely solely on the preprogrammed pulse time (fuel)
vs. rpm map.
The two brains in the 928 certainly perform the same function as an
LH-Motronic system and the Bosch book reports that 85-86 928s used
LH-Jetronic while 87 and newer 928s used LH-Motronic. Both Susan
Kirby and Marc Reviel have at one time called the 87+ systems
"Motronic." <<
From "Bosch Fuel Injection & Engine Management", by Charles O.
Probst,
pp 27 & 28:
"In simple terms, Motronic is an engine management system with a single
control unit for control of ignition timing as well as fuel-injection."
"Several cars, such as Porsche 928 and Peugeot 505STX have separate
ignition controls, no vacuum hoses or weights, and are not considered
Motronic systems."
>> >A reprogrammed chip will probably be needed to handle WOT, but that will be relatively trivial, compared to remapping the surfaces.
Now, interestingly enough, Wally seems to subscribe to the "Motronic
camp" while... <<
No, I don't think so.
>>From: Zuffen928@aol.com
(Steve)
>... you don't have to re-map the fuel curves because the engine control system measures engine air flow ... Therefore, the computer always knows how much air is going in and can compute the correct amount of fuel
... Steve subscribes to the "LH-Jetronic camp."
So, my question to you guys is: which one is it? <<
I suspect that Steve is talking about closed-loop operation, I was
talking about WOT.
>> Yup. Yup. Let's also clear up one potential source of confusion:
The
FI computer can't actually control _fuel_ delivery. It controls the
'pulse time' of the injector thereby indirectly controlling fuel
delivery. Thus with higher pressure in the rail the same pulse time
will deliver more fuel (up to a point.) If the computer is sending out
the maximum pulse time and we still can't get enough fuel (or if we'd
like to not pressurize the fuel system to an extreme) we need
injectors that meter out more fuel for a given pulse.
So, when big injectors and an adjustable regulator are fitted we are
pushing more fuel out of the rails but the computer doesn't _really_
know it. And if we've a Motronic system we're relying on preprogrammed
pulse times. <<
The computer DOES control the amount of fuel injected: a) in closed loop
operation, when b) the amount of fuel required is within the system's
delivery capabilities. The computer DOES control the amount of fuel
injected: a) when using the programmed maps, and b) when the amount of
fuel required is within the system's delivery capabilites.
The computer does not control the amount of fuel injected when
conditions are outside the system's capabilities, or when the system is
misadjusted (either accidentally or deliberately).
The amount of fuel injected in open-loop operation can be changed from
that programmed into the system thru misadjustement of the fuel
pressures, changing injectors, etc.
>> If you're going to spend $30k to build a force-fed Beastly motor with
stroker this and forged that why _wouldn't_ you go ahead a spring the
extra 10% for a programmable Motronic (or Motronic-like) system to
assure (and insure) the performance of your investment? <<
From "Bosch Fuel Injection & Engine Management", by Charles O.
Probst, p
28:
"Engine control can be based on actual needs of each engine model based
on large amounts of engine-test data during different operating
conditions stored in the Motronic Read Only Memory (ROM)."
Going to a user-programmable ECU is doable - IF you know what you are
doing, and are willing to expend large amounts of money, dyno time, and
frustration for relatively small gains in one part of the engine's
operating regime, while sacrificing in other parts. Porsche and Bosch
spent a couple of years of skilled engineer time, and a huge amount of
money getting the 928's control system matched to the engine under a
wide range of operating conditions. I think that anyone who believes
that they can take a one-size-fits-all control unit, spend a Saturday
morning on a chassis dyno, and improve that, isn't living in the real
world.
Wally