The starting point, Vauxhall Omega V6 2.5 throttle body, nice and cheap on ebay for £5 ! After chopping the throttle body in half with a hack saw and removing excess bits with an angle grinder it was time to turn it down on the lathe. This is how it will work in the exhaust. Here's one half down and the other half still with the rest of the metal on it. Now both are cut out on the lathe. Test fit with valves closed. Valve open! Hold drilled through exhaust tip, throttle body metal fitted in test fitted.