 SLP cat back and offroad x too much loss in back pressure?
It's not reduced backpressure that hurts performance, it's reduced velocity of the exhaust gases that hurts. Backpressure is bad and should always be eliminated or reduced to the lowest possible level. But.....
Free flowing exhaust, (LT headers, 3" no-cat mid-pipes and no mufflers) = high HP at high rpm, but you'll see a drop in low-end TQ. Why, he asks? With that kind of system, at lower rpm the exhaust gases will lose velocity. Velocity of the exhaust = momentum, which helps draw out any remaining exhaust gas in the cylinder as the piston reaches the top of its stroke in the exhaust cycle. At the same time, because of value overlap (intake valve is opening while the exhaust valve is closing) both valves are open at the same time and the exhaust gas momentum will actually help draw in more fresh air charge while it's drawing out the remaining exhaust gases. Thus more oxigen molecules to combine with the fuel as it is injected and ignited = more fuel burned = more power. For a street motor, the challenge is to balance free-flow (for high-end HP) with the need to maintain good exhaust gas velocity (for low-end TQ). For an NA 4.6L, 2-1/4" to 2-1/2" pipes and a free flowing cat-back will fill the bill nicely. If you're going SC'd slap on those LT's and 3" pipes because that blower will give you all the intake and exhaust gas velocity you'll need.
In a similar discussion on another board, one guy reported he dynoed his GT with the typical exhaust mods, against the same car with his cut-outs opened up. With the cut-outs opened, he picked up 10-15 HP up top but lost 10-15 ftlbs of TQ on the bottom.
What it comes down to is, what are you trying to build? Race motor that will spend most of it's time in the upper rpm band or a street fighter, light to light, daily driver?
__________________ ....Jakes
'02 Vert, KB blower, FIPK tube w/KB CAI, Accufab 70mm TB, MRT H-pipe w/cats, Magnaflow cat-back, FRPP "G" springs, KYB struts/shocks, MM caster/camber plates | 
