Running a 2003 Ram RCSB with a 5.7, AEM intake, flowmaster exhaust, superchips 91 tune. Would an aftermarket 85 or 90mm throttle body be a good addition for power and would it work adequately or better with my existing canned superchips tune?

 

It would if you free'd up the exhaust. Increasing air in doesn't do much good if you don't increase the rate at which it can leave the cylinders. Headers, especially long tubes that pull (scavenge) exhaust gases, would do wonders. You would also need to ditch the factory scrunched down Y pipe and the ultra restrictive chambered Flowmaster muffler.

 

I don't believe in larger Throttle Bodies unless you are pushing serious power. The point of a throttle body is to restrict air flow 95% of the time unless you are driving around at WOT all day. I think a larger TB may give the appearance of more power, as it will flow more at the same throttle position as the stock TB, but you can get the same affect by just opening the stock TB more with your right foot. Until you have ran out of airflow with your TB at WOT, you probably don't need a larger TB.

If I had to guess you probably wouldn't be at the point of needing a larger TB until after you do th exhaust work mentioned, and some work to the heads and intake manifold.