All things being equal I'll throw my hat in the ring for this one too.

In my opinon bigger is not always better extractors and big pipes
help clear away all those nasty gasses but engines also like some back pressure it helps gernerate more horses I learnt this from my old speedway days (go the hemi !!). Stick with sensible 2.5" pipes on autos sound good and work well with a sports muffler, free flowin' cats and straight 3" pipes won't.

Bigger is not always better. I managed an exhaust shop in the ACT and we did a lot of experimenting with various exhaust configurations, dyno tuning every change. With pipe diameters you must remember, double the diameter equals 4 times the flow. We found that on standard to mild engines, with basic tri Y extractors fitted, that a well designed 3" single produced more horsepower and torque than a twin. This was put down to the scavaging effect of the collector pipe into the single 3", allowing the extractors to scavange each side of the engine, as the extractors are designed to do between cylinders. On the tuned length 4 into 1 extractors, a twin system was the go. One thing you must have however is a good fast flow through the system, which allows the exhaust to heat up more, and last longer. If the exhaust is too big, you'll pick it straight away by the black stain on the back bumper, just check out some of those 4 cylinder boom boxes with the 6" drain pipes out the back and look at the mess of the rear end.

well i am even more confused... it seem's different stroke's for different folk's... i think i will go the twin 2.5 straight though with hi-flow cats... it is a mannal.. i want it sounding like a true windsor but not that loud that it atracks to much atension from the boy's in blue... i must likely getting it done this week.. so for those that are interested i will let you know the out come.... thanx everyone for there opinion...

iv got pacemaker comp headers on my 220 and duel 2.5 manta with 100cpsi cats sounds tough as I find it seems to have lost power in low rpm Im doing a few more mods then il get a sct flash tune then see how things go

Yes the 1 3/4" primaries are simply too big on those headers. You need at least another 50 cubes to make use of them or have much more high winding 302.

my experience is ... save the coin and do it right the first time.

sounds like you could benefit from new headers ' Pacemakers' ... than cats, resonator and muffler.

on my ute ... im running Pacemaker Comp, tuned hi flow cats, merge into 3" single... than I ve got a mid hotdog resonator ending with a megnaflow hi flow muffler and it sounds tuff with no drone.

Though on a technical side, apparently a twin 2.5 exhaust will flow 1000 ccpm where the single 3" will flow 700 ccpm.