Stock Exhaust vs Cat-back Magnaflow
 

So I'm wondering, what is it that makes a cat-back set up, such as a magnaflow cat and resonator and muffler upgrade, more powerful than stock?

I realize the exhausts sound differently, but is an upgrade of that nature truly more powerful than stock, and if so, what is it that makes it more powerful? Are those components built differently?

The magnaflow cat might net some more power if it flows better but as far as the rest of the route - size being equal I don't think anyone can honestly say it is better than stock.
That huge stock muffler actually flows quite well - as evidenced by the cutaway one member did.
Now once you throw in pipe size and configuration changes the door is wide open.

the aftermarket system, if done correctly, will result in a reduction of backpressure which will allow the engine to be more efficient in its use of a given amount of fuel - thus increasing the hp output of the engine. Not done correctly, the aftermarket system can actually decrease hp.... (i.e. reduce the backpressure too much and you can actually suck unburned fuel out of the exhaust system...)
-Bill

not sure about that, but I know that a poorly designed system (ex: too big piping) will give less power because the system does not pull the exhaust gases out of the cylinder (scavenge) as efficiently, not because there is too little backpressure. You don't want any backpressure, in fact, you want a pressure lower than atmospheric at TDC to help pull exhaust gas out of the cylinder and the fresh charge in.

check the very, very short header pipes on Top Fuel and Funny Cars -- the ignition event actually extends out of the pipes (ie flames!). I realize that this is an extreme case....
-Bill (realizing he should probably pull his exhaust system design handbook out...)

nitromethane burns slower than gasoline so when the exhaust valve opens the fuel is still burning ;) . (I've been doing my reading recently too) :D

I can't exactly tell you that an aftermarket system will be better, because so many custom systems are different and not many people have done a before/after test with a dyno to get some actual numbers. I will be doing a system this summer, so I'll be able to tell you how it sounds/feels, and be able to give you some exact numbers on the ever popular and increasingly accurate butt dyno :rolleyes: :D .

Without removing all three of the OEM cats, there isn't much power to be gained by installing a cat-back exhaust. Maybe 5 hp.

Sure... The Stebro exhaust system has been said (by the president of Stebro) to produce about 7-9lb-ft torque and roughly 9-11hp...

Care to know the price??? :D:D:D I don't think ya really want to know... :rolleyes:.... We'll just say that for three pieces, the price has four numbers and a coma after the first one :eek: :eek:

Good point!! :D
-Bill

And I say to him - show me the dyno chart :rolleyes:

If my time slips are any evidence...the exhaust would be a big help.

For mt 15.3, basically the only mod I had was exhaust. (Intake resonator removed, tranny resistor unplugged)

I have the 3 OEM cats in place, with a single pipe coming back, into a Y-pipe, in to 2 flowmaster mufflers, and then out.

Here's pictures.