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Re: [T3] Injector interchangability?

On 2 Dec 2003 at 12:35, Martinez, Leon B Civ 203 wrote:

> Well let's see if Understand this , the PS only cares about vacume or
> what we call absolute manifold pressure , in Idle the vacume may be at
> 15 for example then at acceleration it is at 4 lets say , well this
> vacume energy sucks into the Pressure sensor which moves the bellows
> and electrical accordingly inside . at constant velocity the engine
> may have a vacume of 14 on flat ground and 9 on hills or load, the
> lower the vacume the closer to resting position the P.S. guts are when
> engine turned off , the more load, the less vacume going to the P.S.
> the more voltage (?) sent to the brain to tell it more fuel  by
> lenghtening  injection time (tiney fractions of seconds more .  When
> engine is hard accellerating the vacume drops and sensor goes more
> towards resting position and this P.S. position tells the brain keep
> the fuel comming some more ,by even longer injection times. , to me
> that is basically what the pressure sensor does and does not care
> about anything else except for vacume. 

Well, if by "resting postion" you mean the position it returns to when the 
engine is off, then I think you are exactly right.

The rest of this gets really complicated, and I have to rethink it all the way 
thru each time I try to do it. I'm afraid I haven't been doing that correctly 
recently and it's been bothering me that I KNOW I came to some different 
conclusions a couple of years ago when you guys pushed me on this before.

So I'm going to go back to the conclusion I came to then: I think it should be 
fairly easy to compensate for larger displacement by simply upping the fuel 
pressure in proportion to the displacement increase.

So if you want to go from 1600  to 2100 cc then you should increase the fuel 
pressure from 30 to 39.4 psi.

I'll try to think thru this all over again and see if it could really be this 
simple, but I think it just might be. Here's the reasoning:

Suppose we have 5" of vacuum driving along in our 1600. That means that each 
cylinder is pulling in 400 cc of air an 5" of vacuum (less some pressure drop.) 
If we then look at our 2100 cc engine also running with 5" of vacuum we now 
have each cylinder pulling in 525 cc of air at the same pressure. The only 
change is that we now have more of it, in proportion to 525/400. If we change 
nothing, the FI will still meter out the amount of fuel that would be required 
by 400 cc of air at this pressure. 

If you could find injectors that metered more fuel by exactly the ratio of 
525/400 (2100/1600) then this would also work, but it's a lot simpler to just 
dial up the pressure to 39.4 psi. This is exactly equivalent, and it has the 
advantage that you can dial it in to whatever you want.

There's one uncertainly, and that's the fact that this assumes that the 
pressure drop between the IAD and the combustion chamber is linear with 
airflow. I don't know if that's true or not, but even if it's wrong, the linear 
assumption for small % increases of displacement should be pretty good 

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

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