Depends, LTs offer a tuning and flow advantage over the stock manifolds, and with a set of LTs in a NA application you get the most benefit by changing cams and engine tune to better suit the scavaging characteristics of the header. On an otherwise stock engine the greatest benefit comes from the reduced back pressure that the engine has to fight against (back pressure = negative torque), then on to the rest fo the exhuast. If the stock cats offered no impedence to flow then a set of aftermarket cats will do little if anything for power (although if they change distance from the stock location this will have an effect on resonant tuning in the exhuast - I'm not saying Ford placed the cats at a specfic point to take advantage of this, but anytime you have a change in diameter or you introduce something that would create a hiccup - ie; H/X pipe, prochamber, mufflers, exhust system length, ect. it alters the tuning characteristics of the exhuast). Off road pipes definitely dont do much if the stock cats flow well enough, except spew crap into the atmosphere. One very negative aspect to this is the safe mode operation of the engine. Ford sets the computer up to run rich and cool the exhuast valves and converters down if certain parameters are met. This isn't a failure type event but one that is designed to increase the longevity of those components, without the cats there to capture the extra fuel and evaporate/burn it, the extra fuel gets sent down the exhuast system and shows up as the soot you see sometimes on the tips and rarely on the bumper.

The stock exhaust system on the S-197 after the manifolds really isnt a cork compared to older exhaust systems with a comparitively stock engine. If say you were gonna super-charge or otherwise significantly increase the power output via stroking and/or increasing RPM potential (and conversely increase the volume of gases passed through the system) it would become an impediment.

When it comes down to it, aftermarket exhuast systems basically do one thing for a stock engine. They offer loud, louder, and loudest sound options. Beyond a small reduction in backpressure, they really dont do that much (well without tuning for the change).

 

very informative. Thanks for taking the time.