[Doug Clark]

I concur with Jac Mac. Also don't underestimate how much this'll screw up the carb settings. When I built a closed box setup for the TZ250 I used to have it was a hell of a job to get it right with a fully closed box and I was never 100% happy with. That was with a bike though and car drivers are less sensitive to throttle control-reaction than bike riders so it may not be an issue.

Also make the box as big as possible, the bigger the box the slower the air in it and therefore the higher the pressure inside it & the inlet should have the smallest cross sectional area of the whole thing, the idea is to gradually slow the air and bring it up to pressure in the box.

I made mine by making a polystyrene model of the required shape, wrapping it in FRP and then using gasoline to disolve out the polystyrene, ugly but effective. Seal the polystyrene with glad wrap or paint to stop the resin getting into it.

There is math somewhere to work out proper size of the box and the inlet area you should be able to find that on the web, if the inlet is too big it can also cause issues.