Butterflies, Barrels or Slide Throttles
Before the mid-1980's, carburettors and mechanical fuel injection were the only choice to fuel a racing car engine. Selection of air valve type was simple and apparently carved in stone. Carburettors used butterflies and injection used slide throttles. Motor bikes and their history are a special case but what follows is true for any performance 4-stroke petrol engine.
With the advent of electronic fuel injection and a more adventurous (better funded) approach from the leading engine designers, it was discovered that butterflies, whilst sometimes (but not always) giving slightly less power than slides, inevitably gave better lap times. The explanation was simple; butterflies give more progressive throttle control, improved transient conditions and aid mixture quality throughout the RPM range.
As a result of these discoveries most (possibly all) of the leading car race engine manufacturers switched to using butterflies. Lap times continued to tumble but there was a problem brewing for the future.
As peak RPM increased year by year, the required induction system length reduced. At the same time the ideal butterfly to valve distance increased. Over about 15,000 rpm, the ideal butterfly position falls outside the induction system - clearly useless. Enter the barrel.
The barrel has some but not all of the attributes of a butterfly. Opening is reasonable progressive and like the butterfly, it is easily packaged. The great advantage is that it can be made as a continuation of the port shape, regardless of profile (slides would overlap), and thus be placed near or even in the cylinder head. This allows for a very short system to suit the 18,000+ RPM which is now common. Any compromises (poor idle control, tendency to stick, poor flow vector control, etc.) were once believed to be offset by the sheer power available at these RPM however most (possibly all) of the leading car race engine manufacturers (e.g. F1) had switched back to using butterflies by 2006. It follows that barrels on a sub 15,000 RPM engine will suffer from the compromises without gaining the possible benefits. **
The main advantage of the barrel, maintenance of port profile, can be obtained by using fully profiled butterflies. These are made to precisely fit the port profile and are shaped in cross-section to achieve the required characteristics, minimum drag, controlled turbulence or whatever else best suits the application. This is now the preferred solution for top end engines used in Formula 1, World Super Bikes and some sports-racing engines. Jenvey Dynamics supply these for specific applications since they must be designed to suit the engine and cylinder head used.
Butterflies are best wherever they can be used. Jenvey Dynamics have a design and make service for engine - specific profiled butterfly bodies.
Barrels are suitable for engines running at over 15,000 RPM and must be designed to suit a particular engine type. Jenvey Dynamics have a design and make service for engine specific barrel bodies.
Slide throttles are best reserved for classics, if the rules prohibit a change.