Why does everyone seem to require a time slip for things like this?
But you don't understand bottleneck? That outlet is 1-7/8" on most /6s. So how can something larger than that, bolted on behind the said bottleneck, flow any better than that which flows THRU, that size hole????
I definitely wouldn't want anything smaller than the opening for sure.
For 2 tenths difference, that could be coincidence. Or temp. Or reaction. Or any number of things between the 2 runs.
The larger pipe size certainly won't hurt flow... I get that. But I don't get how it can help it either, unless everything is opened to that dimension.
