Exhaust extension for flow testing

[Fast One]

I'm doing a six cylinder straight six head at the moment, flow numbers are turning out quite well, however the runners in this head are of a short nature & listening to the exhaust air movement on the bench I feel that there is some shearing away of the air flow at the exit & not giving an accurate readout, so decided to get a pipe extension, try it out this weekend.

Was basically wondering how many of you fellows that do flow bench testing use the pipe extension & how much difference it makes.