The first mod made to our 2004 Duramax was a manifold. Essentially GMC/Chevrolet designed the exhaust manifold (one side only) with a serious restriction. You could see that they intended to route some mechanical part close to the manifold but in the actual shipping version of the vehicle there is no reason for this restriction.
All engines need to breathe both in and out, any restriction will have an impact on engine power output. Within the diesel platform, this balanced flow of exhaust should also reduce engine gas temperature (EGT), this is a good thing.
The following images show the simple change.
[pe2-image src=”http://lh6.ggpht.com/-SsQRelVraG4/U7zOok2H2uI/AAAAAAAAB2A/75OOdzrN3Ms/s144-c-o/Manifold_Duramax_Compare.jpg” href=”https://picasaweb.google.com/105339013868907141437/TruckStuff#6033924797250394850″ caption=”” type=”image” alt=”Manifold_Duramax_Compare.jpg” ]