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Twin 2.5" Exhausts or 3.5" Exhaust 

 

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 Post subject: Twin 2.5" Exhausts or 3.5" Exhaust
Posted: Tue Jun 14, 2005 4:43 pm 
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As a result of the rebuild i have reached a point that the exhaust is starting to become a restriction. Currently I have a single 3"and am looking to go 2 x 2.5" or single 3.5" (or even 4")


Single Larger Exhaust
PRO's
- Eaiser to install and connect to mufflers etc...
CON'S
- Will drop lower and make the car too low, may be hard to find 3.5" or 4" mufflers (other than trucks) to suit cars that aren't really big.
- Noise i think it will be really loud

2 x 2.5" Exhausts
PRO's
- Will sit lower and be eaiser to run (i think although not sure)
CON'S
- Custom mufflers will need to be made as i'm unaware of any to suit 2 x 2.5" pipess.


Not sure what to do. Any mufflers will need to be through ones as baffle style clog the system too much and just end up with the baffles flat against the back of the muffler (i have tried this before)


Has anyone done a 3.5" or 2 x 2.5" ? (ideally one that want to sell)

 

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Posted: Tue Jun 14, 2005 4:49 pm 
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I would go twin 21/2 inch pipes. The single 4" is just asking for trouble. As soon as you get pulled over it will be obvious that your exhaust is extreme and it would have to create clearance problems. Would also have a massive droning sound from a single 4".
Mufflers for 2.5" should be easy to source - my turbo 2lt uses this size exhaust and i had a huge range to choose from.
4" would grabe everyones attention and be a bit different but if you want to use the car on the road the twin pipe set up would be a lot more practical. Thats my $2 worth but i would go to a good exhaust place and get a few quotes and suggestions. YOu will need to go somewhere that knows a bit more about cars than just fitting catalogue items. Let us know what you choose.
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Posted: Tue Jun 14, 2005 5:02 pm 
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Thanks i have an exhaust place in Morwell that has done some good work for me in the past so i am considering them, but open to suggestions.

My perference was for 2 x 2.5" for clearance and noise reasons.

There must be someone here that has used a set-up like that before.

How are these people getting 460rwkw with single 3" exhausts and standard clutchs and gearboxes?

 

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Posted: Tue Jun 14, 2005 5:40 pm 
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THe bloke that put my engine together is setting me up with a place in Hallam thats doing up a twin 2.5" system for the XE, using straight through mufflers. That in mind I dont think u need to go custom they are out there, PM me if ur after any specifics and I'll dig em up.

The guy that built mine runs twin 3" on his.... so dont be affraid of going larger if u need to, but he pushes a fair few more kw's around than most, 600+.

 

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Posted: Tue Jun 14, 2005 5:55 pm 
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pyroay wrote:
Thanks i have an exhaust place in Morwell


dude i didn't know u lived in the valley!!! i haven't seen your car buzzing around at all, i feel like i haven't truley lived yet.... so what exhaust are u talking about?
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Posted: Tue Jun 14, 2005 5:59 pm 
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Boy thats supprising, i work in Morwell and drive it to work everyday, I live in Traralgon and am always driving around between Warragul and Sale...

Exhaust place in Chickerall street.

 

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Posted: Tue Jun 14, 2005 6:08 pm 
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i'd deffinately go twin 2.5"
you'd have the advantage of lots of V8 owners running twin 2.5" or even twin 3" and thus the stuff is easily available.

 

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Posted: Tue Jun 14, 2005 7:27 pm 
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Well hope this helps:

Some quick calculations:

1 x 3" pipe = 4560mm2 cross sectional area, 239000mm2 wall contact area per meter of pipe

2x 2.5" pipe = 6334 mm2 cross sectional area 399000mm2 wall contact area per meter of pipe.

1 x 4" pipe = 8107mm2 cross sectional area, 319000mm2 wall contact area per meter of pipe

So.......

you will have 28% less exhaust gas velocity in the two 2.5" pipes however wall contact area will be 40% higher

you will have 44% less exhaust gas velocity in the single 4" pipe however wall contact area will be 25% higher

Since pressure drop is proportional to the square of velocity (and wall contact area size is roughly linear in this application) then you would be much better off going with the single 4" from a performance point of view.

So its performance Vs Practicality :)

 

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Posted: Tue Jun 14, 2005 7:36 pm 
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personally i would go 2 3" pipes straight through to behind front wheels (1 each side) and oval tips... would sound f**k insane!

but alas johnny law wouldnt agree

 

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Posted: Tue Jun 14, 2005 7:41 pm 
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lol thats basically what I got now, yes it sounds nuts but I think the fumes are starting to get to me!

 

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Posted: Tue Jun 14, 2005 8:10 pm 
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aaron, you're insane...... has anyone told you that recently?

i've seen a twin 3" on an EL GT, so i dont think twin 2.5" would be too much of a drama

 

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Posted: Tue Jun 14, 2005 9:01 pm 
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I think you need to get under the car with a tape measure and see how much ground clearance you have, on my car I'm thinking of 4in off the exhaust housing and under running as close to the floor as I can get it, then into a 5in Cat, it should fit in the normal cat location as the floor is slightly raised there, 4in out of the cat then into a large Dynomax muffler. Not sure about what to use to get over the diff as 4in might not fit between diff and floor... maybe a pair of 2.5s tacked together.
Ive passangered in a R32 gtr with 4 in jap system and it was pretty quiet cruising and very nasty sounding on boost.

I reckon the main restriction is the first few bends off the exhaust housing so even if you just used 4 in then into a 5in cat and through to the existing 3in system you would see the benifits up high.

Cant remember the company the makes these new 5in cats but they are getting awesome benifits from them
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Posted: Tue Jun 14, 2005 9:18 pm 
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Quote:
Exhaust place in Chickerall street


one or 2 of my mates have gone there and been happy with there buys...
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Posted: Wed Jun 15, 2005 9:05 am 
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If anyone can beat $500 for twin 2.5" supplied and fitted (mufflers etc all included) please let me know, definatly something needed in the very near future

 

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Posted: Wed Jun 15, 2005 11:53 am 
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Thought of twin 3" Or just double what you have now? OR a flange made where you can vent exhuast before mufflers for performance events??...
Baffled mufflers are not good for performace on turbo motors..
I still have 3" with over 300 rwkw but intend to fit bypass before cats...
4' is ok what can be done when exhaust is made to have it done in pieces where each seaction is flanged..Then where clearance to ground is an issue, get that section rolled making it oval....

 

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