gas mileage Mechanical vs Vacuum Secondaries

kielbasa said

I've always heard that vacuum secondaries get better mileage, wondering if anyone knows how true that statement is?
Just curious, I've been trying to figure out why that would be the case (assuming steady state driving with a constant throttle opening)...

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The problem is…… "assuming steady state driving with a constant throttle opening"…… Doesn't happen in the real world.

Vacuum secondary is an on demand system (it delivers when the engine asks for it) and will generally give better MPGs in real world driving (all things being equal).


Mechanical secondaries are just that, mechanical. When you reach a given throttle position (regardless of engine demand) it starts dumping fuel (all things being equal).

This thread became far more complicated then it ever needed to be.