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A clip from the latest Diesel Power magazine

Pipe Dreams

Diesel enthusiasts already know the great power potential and more efficient fuel consumption offered by a diesel truck.All that power and economy doesnt come without picking the right parts though. An after market high flow exhaust system has to do more than simply make your truck sound better. Point in case with a 4" vs. 5 or 6 inch system, how big do you need to go? To answer that first we need to go over a few basics.
On most contemporary turbodiesel engines, the exhaust comes right off the exit of the turbo and after market exhaust manufacturers offer more than one tubing size to vent the exhaust more quickly.
One advantage of moving exhaust gasses quickly and efficiently is to reduce buildup of heat. Turbo diesel engines generate alot of heat, especially when the engine is under a heavy load. A high flow exhaust actually allows your diesel engine to run cooler and adds the potential for more boost when towing.
Large diameter tubing also reduces backpressure, making it easier for the pistons to shove that exhaust out of the cylenders and resulting inmore horsepower. Less backpressure allows the turbin to spin more efficiently, therefore it will build boost quicker and run cooler.
Interestingly enough however there is a point where bigger isnt necessarly better. While engine testing indicates that going from 3-1/2" to 4" can reduce backpressure by as much as 98%, ther can be less than 2% reduction above 4" exhaust. SO while a 5 or 6" exhaust looks cool it wont give you much more than just a coolness factor
 
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