On the SU carbs there are two issues with air horns. First is the
transition from free air to what is essentially a tube. A very abrupt
transition,
in electron terms, an impedance mismatch, causing lots of drag. The curved
inlet of the air horn matches the flow reducing the drag. The second issue is
the vent holes for the piston dome. Without the air horn they are very close to
the air inlet. This causes the signal to the dome to be reduced, confusing the
carb. The air horn seperates them giving the dome signal ports a better
sampling of true ambient pressure.
keith
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