I have a 2003 3.0 auto ..here in Cali its all cat convertors...would i improve anything by fitting a magnaflow muffler....or leave it stock
Thanks

 

You will change the sound when changing the muffler, but you can't gain any power by modifying any factory exhaust system part.

Contrary to many who believe it does gain power, the math just isn't on their side, like Cold Air Intakes(CAI) boost power, they don't on any fuel injected vehicle.

In the "old days" with carbs, CAI did work because they weren't installed at the factory.
And in the "old days" scavenging exhausts(tuned exhaust) were not installed at the factory, so tuned exhausts DID add power.

But car makers learn from the hot rodders, CAI and tuned exhausts have been standard features on all engines since mid-80's.

Your 3.0l exhaust(headers) are tuned for best mid-range RPM performance, you can change to low-RPM range headers, loosing the mid-range but gaining better off the line performance, you don't gain power, just change when the power is available in the RPM range, you can also get racing header that change it to a high RPM range

Muffler, as long as pipe diameter is equal or larger, won't effect performance in any way, but it can sound better.

And cat converters do NOT effect performance either, just FYI, another popular myth, lol.
If cat converter was clogged then yes, but working cat doesn't effect anything but cleaner air.

 

Thanks RonD
So there is no gain in power or gas mileage... stay stock and save $175/200

 

Nope, no gain, just sound.

Switching to an electric radiator fan will give you more available power to rear wheels, isn't alot but isn't 0 either, it can also improve MPG.
The fan that is powered by the water pump/engine uses about 2-5 horsepower after engine warms up radiator.

E-fan does use more alternator power which does require a horse power draw on fan belt, BUT, the mechanical fan draws the power all the time, the E-fan only draws the power when radiator needs the air flow, so while driving e-fan is usually off because there is plenty of air flowing thru the rad, so it is not using horse power, when you slow down or stop e-fan comes on to maintain air flow thru the rad for cooling.
Extra power to rear wheels when driving, no extra power to rear wheels when stopped, win-win.