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Re: [T3] Injector interchangability?

On 1 Dec 2003 at 11:08, Jim Adney wrote:

> 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 
> combination.

I'd like to try to explain this a little better. See if this makes any sense.

The PS (pressure sensor) sends a signal to the brain which the brain has to 
interpret. Just for the sake of understanding lets think of this signal as 
being a 0-10V signal. That's not really what it does, but what it really does 
is much more complicated and this is equivalvent.

Let's suppose that 0V means very little airflow and 10V means full 
1600cc/WOT/5500RPM airflow.

Now you increase your displacement to 2100cc and take it for a ride. You get it 
all the way up to 4000 RPM and everything is fine, but the PS signal is now at 
10V. Everything's fine because the airflow is the same as the 1600cc engine 
drew at 5500 and the FI is giving the right amount of fuel for that amount of 

What happens as you continue to speed up? The air flow increases, but there's 
no more signal available from the PS so the fuel delivery stays the same as it 
was at 4000RPM. You're now running lean.

Could you get a pressure sensor that will go to higher voltage? Well, no, all 
the pressure sensors go from 0-10V, and even if you could make one, the brain 
is only programmed to understand 0-10V. Different PSs just have different  
vacuum response slopes.

What you would need would be a different PS that had a wider vacuum range (with 
the right starting point) and a brain that interpreted this correctly. And even 
with all of this we still might need different injectors with different fuel 
delivery rates.

Sorry, I don't think that was a particularly good explanation. I do much better 
with sketches and graphs.

Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA

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