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I will be taking my car to a muffler shop to get a catback system, but I cannot decide whether or not I should use 2.25 inch or 2.5 inch piping. I want most horsepower and torque possible, so I am worried that the I will be losing some needed backpressure with the 2.5 inch piping.

I am also in a dilemma with the choice of mufflers. As for now this is my first choice:

http://www.performancecenter.com/branduser/?productID=146551

And:

http://www.performancecenter.com/branduser/?productID=146579

I know it says that the second one is recommended for trucks, but would it work? and would it be better to use?

Any other recommendations are welcome.
 

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Woh, that's WAY too much for an exhaust unless you need the stainless to prevent rust in 100 years.
 

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The 2.5" exhaust will always make more power on a 2.3L Focus provided the muffler is not drastically restrictive.I went from a Powertone 2.25" to a Borla 2.5" and picked up 1.5whp and 2.5 lb. torque.The Borla also had an extra muffler where the Powertone did not.I repeated this experiment on a friends car and the gains were identical.
 

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This loss of backpressure thing is getting out of hand. Its a myth on these cars, no such thing. Every focus I have dyno'd has made more power and torque with a 2.5" exhaust over a 2.25" exhaust.
 

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focaltech said:
This loss of backpressure thing is getting out of hand. Its a myth on these cars, no such thing. Every focus I have dyno'd has made more power and torque with a 2.5" exhaust over a 2.25" exhaust.
FC CAI and custom Sebring 2.5" equaled 88.89mph nuff said.
 

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focaltech said:
This loss of backpressure thing is getting out of hand. Its a myth on these cars, no such thing. Every focus I have dyno'd has made more power and torque with a 2.5" exhaust over a 2.25" exhaust.
Its mis-labeled. Back-pressure is bad, you don't want it. Its exhaust velocity you want to preserve. Slap a 3" on a NA engine, and your exhaust velocity will just plummit.

http://www.apartment107.net/db/forums/showthread.php?t=213
 

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Yeah but someone once said that backpressure is a by-product of exhaust velocity. Or was it the other way around? Either way if you didn't have backpressure how would you end up with faster exhaust velocity? This is how I see it.
 

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Think of it this way. Back pressure starts at a certain diameter, and increases as the size decreases. Exhaust velocity decreases while pipe size increases. You want a size that is going to minimize backpressure (or have none at all), but maximize exhaust velocity. Usually its the intercept point of those two plots.

But thats in the simplest sense. Cats, mufflers, resonators, and different type of bends add complexity to this, so its not so straight forward.
 

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When you see a civic with black smoke marks all up the backside of the car coming from the exhaust, that tells you 2 things:

1. They need to wash the damn thing
2. There is just about 0 exhaust velocity

Older cars used to have this problem, like police cruisers with sporty exhaust, you used to have to blow the excess crap out the back by reving it real high after cruising around town for a couple hours.... because if you didnt, the buildup could cause problems.
 

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Update: 2.5" equaled 91mph I'm convinced 2.5" will make more power if you want to be a car length back still go with 2.25 I gained torque I didnt lose any and besides this car has torque to spare for a little help up top on the HP side.
 
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