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Re: [T3] Injector interchangability?

----- Original Message ----- 
From: "Jim Adney" <jadney@vwtype3.org>
To: <type3@vwtype3.org>

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

Is this possible with the stock pump?

> Suppose we have 5" of vacuum driving along in our 1600. That means that
> cylinder is pulling in 400 cc of air an 5" of vacuum (less some pressure
> If we then look at our 2100 cc engine also running with 5" of vacuum we
> 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
> nothing, the FI will still meter out the amount of fuel that would be
> by 400 cc of air at this pressure.

Seems correct to me.

> 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
> dial up the pressure to 39.4 psi. This is exactly equivalent, and it has
> advantage that you can dial it in to whatever you want.

Am I correct in assuming you're referring to Lbs/Hr ?

> 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 seem to remember that it is not but probably close enough for our

>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
> approximations.

I hope so!    8^}

"....the whole world , Chico. And everything in it."

David V. Nelson
1969 Fastback FI (no matter what!)

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