I've just read an article about air inlet bellmouth geometry for IC engines (carbs or FI). It found that a simple round bell mouth increses airflow by around 16% over a cut pipe and a properly shaped eliptical trumpet improves airflow by nearly 20%. That's the airlflow capacity of the carb or throttle body, not the whole inlet tract. More air, if properly jetted / mapped = more power. The effect increases with the sequare of RPM so at half max RPM you only get 1/4 of the increased flow.

Everyone with R1 engines and the rubber trumpet removed from the bell mouth boss take note, it might be worth having some 4mm radiused bosses made up.

and not just R1's - Blades could be improved as well

See DIY Si post on this thread:

http://www.locostbuilders.co.uk/viewthread.php?tid=46508

David

Beat me to it, I was just looking for that. Scanned from one of the Vizard books by the way. Probably the mini one I think.

The comments on that link say that you must roll the edge of the flare over. The article I read compared a flare at 90° to the axis and a full bead. They found no real difference at the optimal flare radius.