not all cans are the same internally -far from it
basically a short head pipe then a large interval from end of pipe to baffle (say 4-6") will give best NON tuned performance - the larger diameter (higher volume expansion room) is also a big help.
the "tuned cans" are trickier to get right --than proper tuned pipes .
getting more power is easy --getting good even throttle response - not as easy . For my money the big JMB beat any of the cans on the market - they are NON tuned . just make shortest possible header and go fly