Next time you have your generator running, take something (not your hand) and see how hard it is to deflect the exhaust 45 degrees at the outlet. That is how much force it would take to make the exhaust not go into the 4" wide elbow. If the wind is blowing that hard, do you need a Gen-Turi??? :S
By having the 1.5" gap the temp is 15 degrees cooler than without a gap. Increasing the gap further did not decrease the temp and that's the reason I go 1.5". The gap on a factory Gen-Turi is about the same 1.5"... maybe their "engineering" is good enough for most??? :W