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Re: personal D-Jet opinions (long)

On Wed, 28 May 1997, Keith Park wrote:

> I (and I beleive Jim) have all the factory Bosch documentation for the D-jet,
> so yes its all avail.

But, as I understand it, the schematic for the ecu box is not provided. 
Further, to test they system according to the factory method, you need a 
special electronic ecu box tester (I know of one person that has one). 
So, if a part fails in the ecu box, and it's either completely destroyed 
or you can't tell whether it's fried or not, you're stuck with replacing 
the entire box. I'm not pleased with that style of solution. I can solder 
and read resistor and capacitor values, so I'd rather replace just the 
busted part. I can only do that if I have a schematic.

> You can change mixture simply by adjusting the press sens, in an infinate #
> of poositions (unlike the finite # of Jets avail) as well as Idle mixture
> with an adjustment on the ECU.

True, if you remove the epoxy plug from the end of the MPS, there's a 
screw adjustment that sets the spring tension on the soft iron slug that 
moves in response to vacuum which in turn varies the inductance of the 
transformer. Some ecu boxes have a knob for idle mixture, too. 

But, the real problem comes forward when you attempt to get more
performance from the engine. Anything that changes the vacuum will effect
the system, and adjusting the MPS may not help then, since the overall
inductance range may be engine specific. Things that change the vacuum
include just about anything the you would do to an engine, including ratio
rocker arms, cams, exhaust systems, porting and displacement. 

Since these are analog systems, each one is tailored to it's specific
engine, with sensors and ecu box internal values specific to each engine
type. What I've learned is that the ecu box is basically a bistable
mulitvibrator with a variable time constant driving an output stage. The
injectors are fired in pairs (which is less than optimum), and the various
resistive and inductive sensors are incorporated into feedback loops
within the ecu circuit to vary the time constant (and resulting pulse
width). There are other functions incorporated, also, such as high engine
speed, closed throttle fuel shutoff and fuel pump control. The system is
open loop (it does not measure either exhaust temperature directly or
oxygen content), so it has no way to adjust itself to changes in the
engine condition. The designers assumed a specific engine displacement,
cam timing, etc. which results in certain expected vacuum levels and
temperatures within the engine. 

So, if you want to keep the injection, I would have to say that you have 
to expect to keep the engine bone stock. If that's OK with you, then you 
don't have a problem. But for those of us who want more performance, 
either carbs or a programmable fuel injection is the best answer. 

Dirk Wright 		wright@pioneer.uspto.gov 	       1971 BMW 2002
"I speak for myself and not my employer."               1974 Porsche 914 2.0
"A real hifi glows in the dark and has horns."            1965 Goodman House

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