Joined: Nov 2016
Charles, when dealing with fuel injection, using a progressive linkage is counterproductive. With a carburetor, if you snap both blades open (like a double-pumper) you have too much air and not enough fuel. The squirters pour in gas to help compensate, until the airflow going by the boosters and venturii can compensate, and the air starts moving by fast enough to suck fuel into the stream. With vac secondaries, it uses the engine's vacuum to sense need for more air, and does the same thing.
But EFI actually doesn't require vacuum at all. It knows the RPM, knows exactly how much air is roaring through at any given time, and just adds fuel. It doesn't matter whether there is vacuum in the manifold at all to draw fuel out of the venturii. It doesn't even care whether the air going through the throttle body is going fast or slow.
A progressive linkage with EFI can potentially mean a lot more complicated math in how much gas you're supposed to be adding because the equation is not linear anymore, but more of a curve. That's why virtually no throttle bodies I know of do anything but open all four holes at once, and even with a dual-quad setup, a progressive linkage works worse with them instead of better.
And good on you for using manifold vacuum, since that's really what proper timing needs anyway. *high five*
I smile a lot. It makes people wonder what I'm up to...