Exhaust pipe diameters range mostly from 3" to 5" on diesel pickups and other forms of light trucks. The 3 rules to remember are:
1. Smaller size diameter creates more resistance and consequently more heat.
2. You get less resistance when the exhaust diameter is increased, when you have less or smoother bends in the pipe or your engine is moving less air through the exhaust system.
3. Less resistance = low end torque less heat and more horse power up in the power band, which is usually around 1800 to 2600 rpm.
Remember that a 5" exhaust on a truck with a few mods offers little or no improvement over a 4" exhaust and that you may end up with even less torque. This means 0 gains, therefore, the 4" becomes the optimal choice.
A 5" exhaust only becomes practical when moving massive quantities of air. This refers to the use of only the largest turbos made or an engine with a bigger capacity for air such as large commercial semis and large heavy equipment. The Bigger the block, the larger exhaust size.
The 4" exhaust offers the best of both worlds, for your truck, because it costs less than a five inch exhaust, you get approximately a 200 degree decrease in exhaust gas temperatures and a 20 hp increase in the power band. The loss in low end torque is unnoticeable, particularly when you add a chip or programmer to the system. Going with a 4" exhaust will almost always be the better choice for diesel pickup trucks..
When choosing a new exhaust for your truck, remember that if you want stacks on your truck, as well as a turbo back exhaust, check out our stacks page or give us a call!