[ledgebmx]

Here in NC one of the cars I used to work on had a 3" cutout on a 20b FD. The owner did it for performance, cost and noise reasons. It's expensive to purchase/fab a 4-5" exhaust that is quiet enough to pass noise laws but the 3" exhaust restricted the potential of the engine. He gained around 80rwhp with the cutout open at WOT. I think he dyno'd ~650rwhp @ 17psi all said and done.

I think the biggest factors are exhaust restriction and tuning issues. I would imagine you would tune with the cutout open and just deal with not racing with it closed very often. It's possible to tune for both if you're not running an exhaust that's too small (IE 3" on a 20B). You'd have to keep an eye on the tune but that's a given with these cars!