Hello,
I've had a 3" exhaust installed on my HJ61 to help the 12H-T breathe a bit better. The results: impressive.
I can pull up long hills a whole lot better and hang onto the higher gears longer. In short, it gives more torque and makes for a more enjoyable drive.
The standard dump pipe on the 12H-T is a good flow design and does not need replacing if the new exhaust is following the same route as standard.
The new exhaust exits through a hole cut in the guard, and down to the rear on the outside of the chassis rail but inside the line of the body. It then ducks back inside the chassis rail at the rear wheel and follows the regular route with a resonator at the outlet.
I did this to give me more room for extra tanks inside the chassis rails.
Although the existing dump pipe is okay, leaving it in place would have led to too tight fit against the chassis, so the new exhaust was built onto a flange on the turbo.
I'd recommend this mod to anyone wanting more torque. It is also an important step in unleashing the power of these engines with extra boost and fuel - neither have much point with standard exhaust.
Tim