They are usefull when you want to shift a power curve lower, but an improvement in gross HP figures will not come from this alone.
It (in effect) lengthens the intake track. Taking into account sonic reversion theorys, a longer intake track (as also long(er) header primary tubes) will shift the power curve lower in the rpm range.
#1 '97 CD7 SE Accord Coupe San Marino Red / #2 '96 CD7 LX Accord Coupe Arctic White / #3 '96 EJ8 EX Civic Coupe Black / #4 '88 LXi Coupe (spare)
One more "What's the Top Speed..." thread and I think I'll flame someone