ISB, Duramax and the Powerstroke are all pushrod engines with complicated valve train arrangements.
The ISB has a pretty industrial intake design which did not focus on the airflow characteristics.
I don't have enough experience with the 6.7L PSD, however from what pictures I've seen online the intake path is a bit more of a compromise to allow for the inboard exhaust ports. Having the turbo closer to the exhaust ports is an advantage though.
The newest Duramax I have experience with was a 2008 LMM. Fantastic engine but also the second oldest engine in the pickup market. I consider the ISB the oldest because while displacement and much of the engine was updated, it's still the same engine family and design language. It wasn't a substantial departure from the 5.9L.
I have great respect for engines that have a long production life, proving the engineers knew what they were doing. However every once and a while someone comes along and upsets the cart. The 3.0 ecodiesel has similiar design features to the ISV but it's too small to be a real player as a towing engine.
The ISV has chain driven dual over head cams with a fully rollerized valve train for less parasitic loss. Look at the valve arrangement, the valves are all parallel to the cam shafts. This reduces the number of unique parts. It is over-square, meaning it can take advantage of advanced fuel injection systems, insuring all the diesel is burned in a shorter stroke. It's a 5.0L engine so due to the smaller bore, the area the head bolts need to clamp is reduced. What appears to be large bolts with oversized flanges further distributes the clamping force. 5.0Ls is also just enough displacement that if you increase the pressure ratio you can make some real power.
The intake design is where I believe we see some influence or perhaps inspiration from Nissan.
On current pushrod 32 valve diesels the valves are rotated about the cylinders to improve the swirl effect. This is where the ISV stands out. By having what appears to be a tuned intake system with siamese ports the valves can be arranged parallel to the camshafts. With correct tuning the velocity of the intake charge can be increased as it approaches the valve. The higher velocity would increase the momentum of the gas and it would try to continue the same way that it was coming out of the ports. This would improve swirl and the volumetric efficiency of the engine. Just because the charge air is at a higher pressure than ambient, doesn't mean it can't benefit from thoughtful intake design.
I have a Q45 with the Nissan VH45, it has a very similar intake design with tuned length ports. Mine is a later model where they discontinued the siamese ports. It has incredible torque right up to 4000rpm, where it has a little dead spot and then horsepower takes over. A diesel engine has a much narrower RPM band to have to tune for. ISV: 4200-1200=3000RPM, VH45DE 6900-1200=5700RPM
I think it's going to be the tuners who unlock the potential of the ISV, they are going to be what sells the Titan XD. They've got it all hyped up as a heavy duty half ton but what it really is, is a strong, blank canvas for the tuners.
