Cafe Racer & Scotto are on the money, but I'd not increase the ID of the tailpipe/stinger any. While lengthening it will increase backpressure - it'll only be very
slightly & the benefit will be reduced noise. With that in mind, you may wish to simply use hi-temp automotive heater hose slipped over the lil tailpipe it's got to extend it, easy & cheap and quiet too. The reason I'd not increase the ID is being a tuned pipe that ID is part of it's design (or should be if it's a good one) - increasing the inner diameter of the tailpipe may reduce the effectiveness of the chamber.
taddthewadd - if yer talkin' about the setup in the above pic... that'd be my lil concoction
TBH, I did that mostly for aesthetics and not so much for performance (tho it doesn't hurt anything) as it's an ol' vintage bike & I was tryin' to "blend" stuff in. The headpipe (which is cut frm a stock kit muffler BTW) is about 10" instead of the optimum 12" and that too was for looks, to place the belly of the chamber under the pedal crank and to get the angle of the glasspack to match the chainstay's.
As for length & performance I'm afraid I didn't do a "before & after" on that bike, yet Pablo & Ghost0 have said that a mere 2" off that setup is well within optimum & I think Junster had a pretty good explanation fer peeps lookin' to get the most outa their pipe;
Originally Posted by Junster
...The exhaust sound wave expands down the front taper then hits the rear taper (which is usually at double the angle of the front) reflecting the wave back at the header tube. The idea is to get the header length "tuned" to have the wave hit the exhaust port just before the piston closes it. Stuffing some of the energy back into the port along with some of the exhaust gases as the piston goes up. The faster the engine is spinning (or the shorter the piston stroke is) the shorter the header needs to be to deliver the charge at the right time. To long a header and it's late, to short and it's early. A little long and you get a broader band. As they get shorter the rpm range is more sensitive but the pipe adds more to the HP. It's a balancing act.
So... having a 10" headpipe gives a far broader band than the Spooky pipe w/no headpipe at all, but is a touch "peaky" compared to the full 12" - it'd "get on the pipe" a lil earlier with 10" over 12" which is good fer me 'cause my ride weighs about a zillion pounds & I need the acceleration lol but if yer after max top speed then ya prolly wanna be closer to 12" - but remember, these are marginal
changes, something you'd prolly not notice w/o a dyno.
Tacomancini - I've a buddy w/the spooky pipe and while it does help performance, it's freakin' LOUD lol, but hey - some dig that sorta thing