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Old 01-08-2011, 04:07 PM   #1
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Thinking about going 2.5" the entire System

As the title says I'm thinking about goingt 2.5" the entire system.
I can buy the complete exhaust piping from a friends 2010 SS.

Here is what I already have:

- Injen short ram intake
- BBK ceramic coated longtube headers
- BBK highflow cats
- Borla Pro XS mufflers (2.25 in/out)

I am very happy with my current setup and want to keep the sound exactly the way it is now.
But I was wondering how it affects the HP/lb ft when you go from 2.25" to 2.5". Will there be any
gains together with my components?

If it is worth the conversion performance wise but will change the sound I would probably add a
resonated X-pipe.
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Old 01-08-2011, 08:34 PM   #2
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I don't think you'll see much power difference. I can see increasing it for more exhaust flow in a forced induction/nitrous fed set-up.
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Old 01-08-2011, 09:19 PM   #3
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Originally Posted by usa1camaro1969 View Post
I don't think you'll see much power difference. I can see increasing it for more exhaust flow in a forced induction/nitrous fed set-up.

Thanks for your post. So you think it is not worth it?

I thought more diameter = less backpressure. But I also read that the 2.25" diameter is the perfect for the V6. But then again - with stock intake, stock headers, stock mufflers, ...

I'm realy not shure if I should give it a try.
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Old 01-08-2011, 09:45 PM   #4
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You might see some difference with the LT headers. If you want the 2.5" system, go for it. More flow can never hurt!

I don't know what the SS system sounds like, but I've got the 2.5" cherry bomb exhaust on mine and it's worth it even if only for the sound.
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Old 01-08-2011, 09:58 PM   #5
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Could it be that with the bigger diameter I'll loose some low end torque?
Not shure but I think I once read a post in here.
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Old 01-08-2011, 10:21 PM   #6

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On headers, the larger the dia. the larger the gain. NO LOSS OF LOW END TQ, JUST THE OPPOSITE.
Most shops toss the SS exhaust after a cat back. Hope you can score it for a decent price.
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Old 01-08-2011, 10:38 PM   #7

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I tried the SS mufflers after I had the entire system at 2.5" sounded awful. I have the video through the ARH link in my sig.

I than had the MRT 2.0 mufflers welded on.....had to cut down the 2.25" inlet cut down to the 2.5" part on the mufflers so they would fit the 2.5" piping.
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Old 01-09-2011, 08:00 AM   #8
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dual 2.25" pipes will handle up to 1200hp so ... Prolly no gain.
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Old 01-09-2011, 11:43 AM   #9
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not really any hard evidence either way on this as to gains that I have seen. I have talked to D/T, Magnaflow and My long time muffler man, and general thought is on stock applications at least you would lose bottom end torque and gain 1-2 hp on top? But with headers and freeing up the system in front first, then might pickup power and not lose any bottom.Only way to tell is someone has to do it and see if there is a difference on the dyno you can for sure say came from the pipes of course.
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Old 01-09-2011, 05:44 PM   #10
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Interesting post (source:

Originally Posted by PT145SS View Post maybe this answers my question:

Backpressure: The myth and why it's wrong.

I. Introduction
One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "[COLOR=#22229c !important][COLOR=#22229c !important]Engines[/COLOR][/COLOR] need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.

II. Some basic exhaust theory
Your [COLOR=#22229c !important][COLOR=#22229c !important]exhaust [COLOR=#22229c !important]system[/COLOR][/COLOR][/COLOR] is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A [COLOR=#22229c !important][COLOR=#22229c !important]4 [COLOR=#22229c !important]cylinder[/COLOR][/COLOR][/COLOR] motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.

III. Backpressure and velocity
Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.

The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust [COLOR=#22229c !important][COLOR=#22229c !important]gas[/COLOR][/COLOR] velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.

Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and [COLOR=#22229c !important][COLOR=#22229c !important]Nissan[/COLOR][/COLOR] use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.

IV. So how did this myth come to be?
I often wonder how the myth "Engines need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power, therefore the [COLOR=#22229c !important][COLOR=#22229c !important]motor[/COLOR][/COLOR] must need some backpressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a much narrower pipe - in that way he would not have lost all his flow velocity.

V. So why is exhaust velocity so important?
The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the [COLOR=#22229c !important][COLOR=#22229c !important]moving [COLOR=#22229c !important]vehicle[/COLOR][/COLOR][/COLOR] - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on [COLOR=#22229c !important][COLOR=#22229c !important]vans[/COLOR] [/COLOR]
and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s).
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Old 01-09-2011, 05:46 PM   #11
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So after the first replies and some more reading I would say that without intake+headers it doesn't seem to be worth it to go from 2.25" to 2.5".

But with the LT headers there probably are some gains + some said that it will also smoothen out the sound
(without the 2.5" to 2.25" and back convertions).

Still not shure if it is worth a try..
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Old 01-11-2011, 01:53 PM   #12
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Yep. I think you've pretty much answered your own question there. I'd keep it as it is.
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Old 01-11-2011, 03:57 PM   #13
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What if exhaust would shrink to 2" cat-back system?
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2.25, 2.5, borla, piping, ss exhaust

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