I was just thinking "why does the exhaust pipe end back behind the rear tires instead of just behind the cab or forward of the rear tires?"

It makes since to be that you don't want the hot exhaust discharging to the engine compartment or the truck cab. But if you you want to minimize friction between the pipe and the exhaust gas why not shorten the pipe run?

 

Some guys do but it also moves the noise closer to the cab.

Friction isn't really a factor to be concerned with.

 

Then why increase the pipe size?

 

Mostly because of the desire to eliminate crush in the downpipe from the turbo.

 

I just went through the same thought, I know its an older post, but anyway. I brought my new 4 inch system out in front of the rear wheel, had to put a couple small v's to make slight angles in the pipe to get past the cross members, torsion bar, tranny, rear bearing. but once past that I installed a straight through 9x5x16 muffler with offset, then used a 45degree mandrel bend to send it out from underneath... still heaps quieter than the original 3inch system without the muffler! cant say it made any great power difference tho!