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Re: [T3] Injector interchangability?

On 2 Dec 2003 at 1:41, David V.N. wrote:

> From: "Jim Adney" <jadney@vwtype3.org>

>  > Sorry, I don't think that was a particularly good explanation. I do much
> better with sketches and graphs.
> I know what you are saying.I can visualize it well enough,I think.

> How about this:
>       " Special Cases: I only have one specific case, the introduction of
> the 2.0L motor for the 914 in 1973. It's clear that this motor had a
> different speed correction curve than the 1.7L, but the motor was introduced
> using the 1.7L's ECU. My conjecture is that Bosch and/or VW-Porsche had not
> completed the development and design of the 2.0L ECU, so they had to go with
> the 1.7L's. Higher flow rate injectors were used to account for the basic
> mixture difference, but that still didn't handle the changes in the Ve
> curve. 

I have a bunch of problems with this. The first is that 73 was the second year 
for the 2L 914/4, not the first, so I question the writer's credibility right 
from the start. The second is the "conjecture" that VW/Bosch didn't know enough 
about the D-Jet operation by 1973 to make the adjustments necessary to make the 
transition from the 1.7 to the 2.0L. This would have been a simple change since 
all the intake characteristics would have already been known from the 1.7. It 
was just a matter of extending the operating range by 20%.

Plus, by this time, Bosch had the D-Jet in MANY cars (VW type 3s and 4s, 
Porsche 914s, Volvos, Saabs, Mercedes) and had a LOT of experience with it. By 
73, the D-Jet product lifetime was running down and Bosch was developing the L-
Jet (and probably the K-Jet.)

I'm not sure what the writer means by "speed correction curve" but it's 
probably just his way of thinking about airflow vs the vacuum that the PS 
measures. The jump to "higher flow rate injectors" is what everyone assumes, 
but I'm not sure if anyone really knows. IIRC, there is a major external 
difference in the 914 injectors that might be the only difference, but I don't 

> To accommodate, Bosch made a slight change in the MPS - they tweaked it to
> have a richer full-load response, and they changed the resistance
> characteristics of the head temperature sensor. First, they changed the
> set-point of the sensor at 20 C from 2.5 K ohms to 1.3 K ohms, and added a
> ballast (static) resistor of 270 ohms in series. This brought the cold engine
> (sitting overnight, ambient temp 20 C) resistance to 1.57 K ohms,
> considerably leaner than before - this was usually dealt with by running the
> idle mixture richer, but '73's were always a bit harder to get going when
> cold.

I thought that ALL the D-Jet head temp sensors had the same resistance. I'll 
have to see if I have the info on this anywhere to prove this. OTOH, if he's 
right, that may explain why I seem to have temp sensors with various 
resistances. I had just assumed that it was normal statistical variation, but 
some of that variation seems extreme.

BTW, VW also introduced an in-line resistor to shift the sensor resistance up 
in D-Jet cars that gave certain kinds of trouble. I've never seen one, but Russ 
claims to have one stashed away somewhere. I don't remember it's resistance 

The claim that adding the series resistance "brought the cold engine (sitting 
overnight, ambient temp 20 C) resistance to 1.57 K ohms, considerably leaner 
than before" is just wrong. Raising the resistance richens the mixture, so the 
writer has this backwards.  

> When hot, however, the ballast resistance kept the hot resistance value
> considerably higher (richer) than would the standard sensor.

Now he's got it right, so maybe part of the mistake was a typo, but he then I 
wonder what he was thinking with the argument he makes following that 

> The richer mixture produced good part-load and full-load response, even with
> the more choked-off Ve response of the 1.7L ECU. 

He seems to be saying that the 2.0 and the 1.7 used the same brain. I don't 
think there's any truth in that, but I can check. OTOH, all the brains are 
pretty similar and you can make the argument that they are all just "tweaks" of 
the earliest type 3 B brain. [Our A brain was significantly different, at least 
in construction, although I suspect the concept was identical.] 

BTW, there was mention here recently about the Mercedes brain. That one has a 
nice heat sink, but it's important to understand that it probably HAD to: it 
drove 8 injectors. I think the M-B D-Jet cars were the only ones with more than 
4 cylinders.  

> Note that Automobile Atlanta has sold a "hot European setup" for many years -
> a '73 MPS,  temp sensor, and ballast resistor. Gives you a richer part-load
> mixture for more power - oh, and more emissions and higher fuel consumption,
> too." 

This is odd, because he seems to be saying that the "cure" for the poor running 
of the 73s can be cured by installing these three items which appear to be 
stock 73 parts. I wonder if a lot of these inconsistencies can be explained by 
assuming that the "73" in the first paragraph was a typo and should have been 

> From http://members.rennlist.com/pbanders/djetfund.htm

Wow, there's a LOT of interesting stuff here. It's clear that this guy has done 
a lot of research, but it's annoying that there are still some glaring and 
obvious mistakes, such as the statement that D-Jet was first introduced in the 
67 VW type 3 (it was the fall of 67, in the 68 type 3.)

Jim Adney
Madison, WI 53711-3054

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