Exhaust expansion chamber for 4 stroke?

[Frank/G.speed]

A while ago Todd posted the Cylinder Head Tech article from 
Motorcyclist June 1996, one paragraph was:

"	Plain pipe ends do a poor job of returning the energy of an
emerging sound wave, which is why horns have flared open end-to
get better energy recovery and thus amplitude.  Megaphones, the
exhaust pipe horns known in engineering as diffusers, are vastly
more efficient in this regard.  Racing two-stroke engines
expansion chamber exhaust systems have elaborate blow-down
diffusers, because of their heavy reliance on this
vacuum-cleaner effect to pull air through the transfer ports."

Are there any problems with expansion chamber exhaust?!  Such
as narrow power band?  Why isn't it used on 4 stroke engines?

-- 
Frank M. Lin	  | G.speed - Z.Speed Trust HKS DC-Sports Razo/Carmate Soei
fmlin at ccnet.com   | Web page - http://www.ccnet.com/~fmlin/gspeed.html
EG2, 2:0x @ Sears | Info sheet - "finger fmlin at ccnet.com > gspeed.info"

Fastest 1/4 Mile Honda's - http://www.ccnet.com/~fmlin/eg2/fastest