(First posted on MX5 nutz forum in February 2016)
I read that a hot side supercharger setup like I am building can suffer surge on overrun and that is remedied with the use of a bypass valve just as you will find on the original setup of these superchargers on the BMW Mini. It basically works like a dump valve on a turbocharged car to release the boost pressure when closing the throttle. But unlike some dump valves it needs to be open when the engine is at idle.
Instead of using a BMW Mini bypass butterfly valve though, I decided to use a standard single piston recirculating dump valve. This should allow enough air to flow to leak air back to the inlet side of the supercharger and will be held open by the inlet manifold vacuum on idle. The moment the throttle is opened it will close and hold boost. I was toying with the idea of controlling the pressure of air to the vacuum/pressure line that controls the dump valve as a way of controlling boost. Not sure I want/need to now but this still should work fine as a bypass valve.
A support for the inlet added underneath to reduce the stress on the join to the SC inlet.
Fabrication always takes longer than you hope it will!