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Re: [T3] Injector interchangability?

On 30 Nov 2003 at 23:04, David V.N. wrote:

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

>  > fuel. At some point the PS will max out, however, and then there will be
> no
> > additional fuel as the airflow continues to increase. So you are lean.
> >
> > If you try to cure this by just increasing the fuel pressure you can make
> > things work right at some point, but the mixture at 36psi will be 20% rich
> and
> > the PS will still be maxed out.
> A "ramping" regulator has been suggested to take this into account.
> That is,using a microswitch,(or something similar),a higher pressure
> could be "activated" at some pre-set throttle position.I think I've heard of
> such a thing being used in turbo application.

The problem with this is that the PS is already ramping the fuel delivery at 
the low demand end. What you need is a pressure ramp that starts just as the PS 
ramp stops (when it maxes out) which will be pretty hard to accomplish. The 
better solution is to find a PS that has a wider range, or a PS/throttle valve 
combination which has a wider range, and a brain that understands this new 

> > > >In my opinion, It MIGHT be possible to upgrade our FI to that of a
> > > >different car IF we also upgraded the throttle body either by machining
> out the
> > > >throat diameter to that of the other car, or by using the throttle body
> > > from that car.
> > >
> > > Is the porsche d-jet THAT dissimiliar?The internal volume doesn't appear
> to be that different. (The porsche bieng slightly larger)
> >
> > Internal volume has nothing to do with it. It's simply the throat diameter
> and
> > possibly a small effect from the air cleaner flow properties. I don't know
> what
> > the relative throat diameters are, or what effects that might have.
> I am assuming that you're referring to air velocity past the port for the
> MPC.
> Hmmmmm.If the relative diameters of the throat are close then perhaps it's
> only a matter of placeing the sensing port of the MPC at a distance that
> approximates the distance inside a T3.

Virtually all the pressure drop occurs across the throttle body. Once past that 
point I think we can safely assume that the whole intake air distributor is at 
the same pressure. Air velocity inside the IAD doesn't really matter much, but 
there will be a velocity-dependent pressure drop between the IAD and the 
combustion chambers for which the FI system needs to have built in 

You still need to get a PS that has a wider range. This means a wider range of 
input pressures (vacuums, actually) with the usual range of outputs, since I 
suspect that ALL the D-Jet PSs have the same output range.

Then you have to adjust the brain, or find a different brain, which responds to 
the usual PS outputs with proportionally wider injector pulses (or find 
injectors which put out proportionally more fuel for the same inputs.

So it just might be possible to do with a different PS and different injectors. 
Finding the right combination of parts would be the tough part, and it's not 
even certain that an appropriate combination already exists. I'll agree, 
however, that there are enough options out there that it should be possible to 
come close.

Jim Adney
Madison, WI 53711-3054

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