This is a point very worth mentioning. Supercharged V Naturally Aspirated. Everyone is unique, but whenever I make a recommendation for someone over the phone, I take into consideration their entire scenario, I even sometimes get personal (married, kids, age of owner, etc.). And, I always consider customers long term goals and budget. But, one of the most important things to consider is whether or not the truck is, or will be, supercharged.
Naturally Aspirated engines (especially engines with dual VVTI) will generally respond more dramatically to exhaust changes than SuperCharged engines. When naturally aspirated and dual VVTI, a lack of scavenging causes reversion to occur, and it happens very aggressively.
Ha, to sum it up, reversion is really worse with a naturally aspirated and dual VVTI motor compared to an engine that is naturally aspirated without dual VVTI. Long tubes give the engine way more scavenging (scavenging is literally the exact opposite of reversion) than shorties or stock manifolds. The scavenging not only draws out the exhaust gases, but when the engine ECU ‘commands’ the dual VVTI into valve overlap, that scavenging actually sucks in the intake charge through the valve overlap.
This suction gives the column of air in the intake manifold kind of a head start, and when the piston moves down the column of air rushes in faster than it would on it’s own. Kind of a mini supercharger phenomenon.
Matter of fact, the ecu is expecting a certain amount of this ‘supercharging’ action when it commands the dual VVTI into valve over lap mode, if the scavenging isn’t there, again, reversion occurs. Reversion actually pushes the intake charge out of the cylinder and back up into the intake manifold, literally doing the opposite of ‘supercharging’. And that is bad, haha.
When you’re supercharged, all of this is out the window. There is a huge pump sitting on top of the motor putting positive pressure against the intake valves, and thru the exhaust valves during valve overlap. You will NEVER see reversion when supercharged. The exhaust doesn’t matter as much when supercharged. You can get away with shorties 🙂 When supercharged the first rule is to not underbuild the exhaust. But, it’s almost impossible to overbuild it (within reason obviously). No matter what you do to the exhaust the supercharger is increasing volumetric efficiency far more than the exhaust is 😉
You can make gobs more power when supercharged, with Dual VVTI by building a BA exhaust system, with great scavenging characteristics (for example, JBA Long Tubes, BAmufflers 3″ catted mid pipes, BAmufflers 3″ race dual catback exhaust, with BAmufflers dual 3″ race tailpipes exiting behind each rear tire slash cut at a 45* angle). You need the great scavenging to get the exhaust out as fast as possible since the BullyDog 93 tune minimizes valve overlap so that you don’t lose boost thru it;) This builds more boost, quicker. But, it gives the exhaust system more work to do. It needs to move the exhaust away from the engine quicker than it would stock because it has less time to do so.