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?? 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 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 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? timing....too much overlap raises a maf is unidirectional!

I hope that this helps....

Marc M. Thomas
650-592-5287 phone
650-610-0557 fax


>> 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: (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.