As for the catchcan setups, its physical location is not important as it is inline with the Positive Crankcase Ventilation (PCV) system. it is meant to eliminate oil vapors from returning to the intake manifold during PVC operation. Those oil vapors stick to the intake manifold, the sensors, and create deposits on the backside of the intake valves. Ever heard of SeaFoaming your intake? That's 90% of what you're trying to clean out, oil deposit residue from your PVC system. Also, the engine does not burn oil in with the same efficiency as gasoline so those vapors entering the combustion chamber manipulate the pre-catalyst o2 sensor readings, degrading performance and economy.
As for the MDS system, some don't notice it for many reasons. With an aftermarket exhaust system it is very noticeable with the note change. Lower mileage motors with more efficiently operating engines are better at handling the transition between 8-4-8 cyls obviously due to strength. Trucks with a higher numerical gearset may not be as noticed due to mechanical advantage, the list continues...but two things are for certain:
One: using the tow/haul function puts unnecessary stress on the transmission that can lead to an undesirable latent side effect. The tow/haul function manipulates the shift parameters in the transmission to meet an temporary intended purpose. Sharper shifts and delayed converter lockup create heat, and heat is a transmissions #1 enemy. Shifting to manual mode eliminates these things.
Two: When using manual mode or even with deactivating the MDS system using a tuning device, you're still utilizing the same MDS lifter sets internally. Research what MDS and DOD (GM's version) lifters do longterm to a camshaft. Locking out the lifter so it doesn't actuate the pushrod and subsequent valve unloads the roller on the lifter. The lifters are designed to be used with a preload, anyone who had adjusted valves on a roller lifter motor knows this. Without the intended preload the roller partially slips along the cam lobe. Over time, throughout hundreds of thousands or millions of revolutions the roller can develop a flat spot. Once that flat spot develops, its over. It just became a flat tappet lifter, and the cam is NOT profiled for a flat tappet lifter. To further make the matter worse, activation and deactivation of the MDS lifter weakens the internal spring that makes it work, leading to the same unloaded roller tip issue as previously mentioned. Once the roller tip develops a flat spot it will wear the cam, developing a misfire, or in some instances the roller tip has suffered catastrophic failure.
Am I saying everyone's MDS system is going to lead to catastrophic engine failure? I am not. But it is not an unheard of occurrence either. Eliminating the use of the system aids in the longevity of the system. But my personal suggestion is that if someone is in need of internal maintenance such as an oil control valve or MDS solenoid replacement, take the opportunity to eliminate the possibility 100% by swapping or having the MDS lifters swapped out.
