i've been thinking about installing my 4" magnaflow and running a full 4" exhaust from my 3" down pipe. would this be benificial or would the 3" down pipe take away from purpose of doing the 4" exhaust. right now i have the stock exhaust with a magnaflow muffler. stock, my exhaust goes into a single in dual out muffler then it y's back into one pipe and goes out the side.

i just got the trick kit and don't want to redo the down pipe already. plus mine has rear heat and a/c so not sure if i could fit much bigger of a down pipe in there.

 

It's probably more beneficial to go bigger near the turbo more than anything. If you're running a 3" dp until it gets underneath the truck and then goes to 4" you'll probably end up with a much louder exhaust than performance gain.

 

In an exhaust you want high velocity with little back pressure. As exhaust exits the exhaust port (or turbo) it is cooling down and shrinking. Since the air molecules are physically occupying less space but filling the same diameter pipe: velocity slows down... It's possible that it could get to a point where so much velocity is lost that the air will stagnate; this is counterproductive since the engine has to work harder to get that air moving again and out of the tail-pipe. Because of this, it's not logical to increase the exhaust pipe diameter the further you get away from the exhaust manifold/header/turbo.

Dynomax claims a single 3" exhaust pipe should support 700HP in a naturally aspirated engine. Given those are "aftermarket" horsepower numbers (i.e. highly optimistic), I would say anything under 600hp would be fine with a single 3" exhaust pipe. Anything 4" is overkill and IMO just for show (and it also weighs more). Going from a 3" pipe to 4" pipe equates to the same (i.e. illogical and potentially counterproductive overkill).

 

i understand what you are saying. but at the same time why is it that my stock exhaust goes from a single pipe into the muffler and duals out at the same diameter then goes back to a single pipe right before it exits? i mean going from a single to dual at the same diameter is the same thing as going to a bigger pipe, no?

 

A 3" single will out flow 2.5 duals

 

Im actually putting on a 4" diesel exhaust on today

 

If i'm not mistaken, one of the exiting pipes on the stock muffler flows very little if any at all. I've heard that one pipe is the exhaust exit and the other is off of a reonating chamber in the muffler that is designed to quiet exhaust and muffle the drone.

 

I ran 10.65 with a single 3'' pipe and muffler. Im thinking about trying a 3.5'' system after the 3'' Dp. Im going to try running it uncapped at the dp this weekend. If It speed's up then I will go to the 3.5''.

 

maybe so. i never looked at it. but it has a 3" single in 3" dual out on it now. the stock pipe is almost 3" its bigger than 2.5" but smaller than 3".

 

what turbo did you go with? i think parish8 had a 3" downpipe and 3" exhaust but when he opened the cut out he gained a stupid amount of power.