Does a 3" catback really make any difference over a 2.5"?

MalcolmV8

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You have to factor in you are running a dual 2.5 inch exhaust. I bet the numbers you looked up would be right if you had a single 2.5 exhaust (like a honda civic or something).

No they were for dual. They list for single and dual. Google the charts and see, none of them are even close to each other which makes me think each exhaust manufacture / magazine etc. just makes up charts for marketing. There doesn't seem to be any real science or testing behind them.
 

SlowSVT

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Sizing and selecting exhaust system components is usually more of an "educated guess" without actual testing. I think where you are at a 3" system starts to makes sense. The factory system is 2 1/4" with some nasty kinks in the cat back, most mid-range Terminators run 2 1/2" systems, a 700+ hp car with a 3" system would seem to be a natural progression. If a high revving NA engine was built to take advantage of a 3" system it would be a dog in the low and mid-range but a PD blower would largely cancel out any stumble related to having a big exhaust at part throttle. I would consider straight thru mufflers as well like a pair of Magnaflows but with headers and no cats but it's going to be loud. Then again a baffled muffler adding a little backpressure could actually work better. Only the dyno knows!
 

MalcolmV8

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Sizing and selecting exhaust system components is usually more of an "educated guess" without actual testing. I think where you are at a 3" system starts to makes sense. The factory system is 2 1/4" with some nasty kinks in the cat back, most mid-range Terminators run 2 1/2" systems, a 700+ hp car with a 3" system would seem to be a natural progression. If a high revving NA engine was built to take advantage of a 3" system it would be a dog in the low and mid-range but a PD blower would largely cancel out any stumble related to having a big exhaust at part throttle. I would consider straight thru mufflers as well like a pair of Magnaflows but with headers and no cats but it's going to be loud. Then again a baffled muffler adding a little backpressure could actually work better. Only the dyno knows!

I agree. My current system is aftermarket nice and smooth flowing 2.5" bassani X and borla stingers which are straight through of course. So it flows very well and of course is loud as you said. That's why I installed some straight through free flowing resonators in the X pipe and it makes it acceptable.
I've heard long tubes make the exhaust note quieter. If that's true I may get away with the Kooks off road X as it ships. If it's to obnoxious I'll put some resonators on it just like I did my bassani.
Cat back will stay straight through as well. Chambers definitely cut down noise but our engines are giant air pumps. Everything we do to them is to improve air flow, bigger TB, blower, exhaust headers etc. etc. so probably not going to then stifle that flow with chambered mufflers :)
 

JuStAkId

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I agree. My current system is aftermarket nice and smooth flowing 2.5" bassani X and borla stingers which are straight through of course. So it flows very well and of course is loud as you said. That's why I installed some straight through free flowing resonators in the X pipe and it makes it acceptable.
I've heard long tubes make the exhaust note quieter. If that's true I may get away with the Kooks off road X as it ships. If it's to obnoxious I'll put some resonators on it just like I did my bassani.
Cat back will stay straight through as well. Chambers definitely cut down noise but our engines are giant air pumps. Everything we do to them is to improve air flow, bigger TB, blower, exhaust headers etc. etc. so probably not going to then stifle that flow with chambered mufflers :)

With lt's my car it is very quiet at idle. Anything over 2750rpm it screams though
 

c6zhombre

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I say with the boost level you are running and going to add long tubes.....I'd go all out and get the 3" setup. My thinking is you will gain some top end hp over the 2.5". Not an obnoxious amount...but maybe 20.

It probably will cost you some low end...but for as much as you drive on the street, I doubt you will ever miss it. It's not like you are going to lose 50rwtq below 4000. I believe it will actually allow you to hook up better while adding that extra oomph and flow for high speed runs.
 

Givens

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If its 3 inch at the header and 3 inch off road x pipe but then you tapper down to 2.5 inch than I don't see how you gain in HP over a 2.5 inch system. The restriction would be at the 2.5, can someone explain this to me?

Or am I missing the boat if so it wont be the first time...
 

MalcolmV8

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If its 3 inch at the header and 3 inch off road x pipe but then you tapper down to 2.5 inch than I don't see how you gain in HP over a 2.5 inch system. The restriction would be at the 2.5, can someone explain this to me?

I'm not entirely sure either. The general consensus is that the headers and x pipe are of most importance... there after not as much... I don't really know how that is either. I have a few theories but I'll let someone who actually knows answer that.
 

GodStang

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According to I think it was Flowmaster our true duals at 2.5" are good to around 650rwhp. Take that for what it is worth. I went through all this research back in the day when I already had Kooks 1-3/4" to 2.5" to 2" Bassani. After talking to Kooks and some others and seeing a good list of guys going to 3" that where at my power level / mod level and seeing the gains they got on the track I decided to go with Kooks 1-7/8" and 3" all the way back.

If the mods in your sig are really your mods then yes I would think 3" would do you some good. If you were sitting on 600rwhp and 17psi I would say not so much. I ran into needing a new K-member to run the 1-7/8". So that is something to keep in mind that you may also run into.
 

MalcolmV8

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According to I think it was Flowmaster our true duals at 2.5" are good to around 650rwhp. Take that for what it is worth. I went through all this research back in the day when I already had Kooks 1-3/4" to 2.5" to 2" Bassani. After talking to Kooks and some others and seeing a good list of guys going to 3" that where at my power level / mod level and seeing the gains they got on the track I decided to go with Kooks 1-7/8" and 3" all the way back.

If the mods in your sig are really your mods then yes I would think 3" would do you some good. If you were sitting on 600rwhp and 17psi I would say not so much. I ran into needing a new K-member to run the 1-7/8". So that is something to keep in mind that you may also run into.

Appreciate the feed back and yes sig is current. Sounds like 3" all the way is the way to go. It's only money right? lol.
 

Bdubbs

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According to I think it was Flowmaster our true duals at 2.5" are good to around 650rwhp. Take that for what it is worth. I went through all this research back in the day when I already had Kooks 1-3/4" to 2.5" to 2" Bassani. After talking to Kooks and some others and seeing a good list of guys going to 3" that where at my power level / mod level and seeing the gains they got on the track I decided to go with Kooks 1-7/8" and 3" all the way back.

If the mods in your sig are really your mods then yes I would think 3" would do you some good. If you were sitting on 600rwhp and 17psi I would say not so much. I ran into needing a new K-member to run the 1-7/8". So that is something to keep in mind that you may also run into.

Interesting that you needed a different k member for those headers. I'll be getting ARH LT headers and debating on the 1 7/8 basically for long term goals. I'd like to stick with the stock k member really, hope I can!
 

GodStang

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Interesting that you needed a different k member for those headers. I'll be getting ARH LT headers and debating on the 1 7/8 basically for long term goals. I'd like to stick with the stock k member really, hope I can!

I emphasize "may need one". The 1-3/4" Kooks I installed in 2005 I had to notch my stock K member because it would bag against it and drive me crazy. They technically fit but the vibration once started drove me crazy so we notched it. Going to the bigger 1-7/8" Kooks I figured no way the bigger ones would fit on my car with stock K-Member knowing the smaller would not. I never mocked up the 1-7/8" with the stock K-Member. When then 1-3/4" came out so did the stock K-Member and the MM K-Member went in and so did the 1-7/8".

Old thread:

http://www.svtperformance.com/forums/showthread.php?806823-Old-Kooks-vs-New-Kooks&highlight=
 

MalcolmV8

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Not only that but on one of the vendors sites (might have been ebay even, I forget) it said to accommodate the 3" kooks x-pipe I needed to replace the rear crossmember. I assume that means the transmission crossmember? but not really sure.

When I spoke to Kooks tech support the other day they told me the 1 7/8 long tubes fit with the stock k-member. Maybe they've revised their header since 05.
I'm thinking of swapping the factory motor mounts for some urethane ones to reduce engine movement in the engine bay anyways. You may remember my issue with the 2.9 Whipple crusher intake sandwiches the IAC motor into the A/C line and shock tower so bad when the engine torques to the sides it destroys the line. Hard to tell in this pic but it's crushed in there hard.

IMAG1164_resized.jpg


So I've been working with James at http://www.cobraengineering.net/ to build a new IAC adapter block that off sets the IAC motor and turns it at a 45 degree angle as well to get off the A/C line. This is something he'd make available for all Whipple Crusher owners once we have a good design working. To further reduce chances of movement and crushing it I was already thinking of urethane motor mounts and now thinking about possible long tube issues it makes even more sense.
 

MalcolmV8

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Hey Guys, So I've been searching around and can't find a full 3" exhaust. All the ones I find taper down to a 2.5" connection to connect to a stock X pipe. Maybe I'm just missing the options but who make a true full 3" cat back?

I have a 3" collector on the long tubes and a 3" x pipe so I need a full 3" cat back that has no tapering down anywhere.

Thanks
 

03 Indy Cobra

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The only thing that l noticed about this cat-back is that it looks like it starts at 2.5 and then goes to a full 3 inch all the way back.
 
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GodStang

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What material and IRS or Straight axle? I have a complete SS IRS one that is 3" every where from collector back.
 

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