Ok i have been searching and i have read both good and bad things about neck downs. but can anyone really explain to me the good or the bad more in depth..
power loss no differance
This is what i am talking about .....

this is a pic of the catback i got and the DP does the same....so what do you guys think? (Its All JIC Bullet Products)

 

I don't like the look of that, and if you think about it, restriction is restriction is restriction.

I think that it shoudl affect performance to some degree, but the question would be how much.

Thing of the exhaust gasses as water, would water flow through that pipe as fast as a straight through pipe?

 

Are you running the stock cat? If so, that's going to be way more restrictive than a couple of minor kneck-downs.

If you're running a test-pipe or HFC, I doubt the kneck-downs will make that much of a difference unless you're trying to tune for "mega" power (upwards of 400 whp).....I wouldn't sweat it.

 

im running a 3" cat delete

 

Neck-downs defeat the purpose of nice mandrel bends and straight pipe-routing. I have the Injen O2-housing-back exhaust with no neckdowns right down to the 3" test pipe. All for $850!

 

Yes it will affect power, but not much. My advise...don't sweat it.

 

If you can get it cut off and weld matching pipe, do it.. Basic physics....read about an orifice or venturi's and how they work. They are flow limiting devices. They cause an increase in pressure at the begining as it goes through the venturi and then pressure goes back down and it will have the properties to limit the amount of volume of a gas.
It's kinda hard to explain. Sorry for the crappy job....just read up on them...that little neck down will hurt if the size of the pipe before the neck down is bigger and then after the short run goes back up to the original size (i.e. 3"--->2.5"--->3"). If it is the same size (i.e. 3"-->2.5"-->2.5") then don't worry about it.

PS-I edited this because it was late at night when I originally posted this and it needed some examples ;-)

 

why bother with neck downs. it just looks restrictive