Optimum diameter for exhaust

[306rhys]

Standard is 2.25" I believe.

Is it the case of bigger is better?

Which would provide more performance for stage 2 - 2.5" or 3"?

I will be running a de cat and no silencers.

Thanks

[mr_fish]

You'll be alright with a true 2.5" I would of said

[306rhys]

Did just find a thread from a while back with all the info so it's pretty much covered already.

http://306oc.co.uk/forum/archive/index.p...15415.html

http://www.koracing.net/viewarticle.php?article=7


Thinking about making welding my own exhaust together, any advice on this? From another thread people said the hoffman centre section includes 'crush bends' what's the best way of limiting this?

[Dum-Dum]

Crush bends are bad, effectively 2.25 would be fine for a HDi if that was the absolute smallest but if your making something you might as well go to 2.5"

That said 3" sounds incredible.

[silverzx]

3" sounds like a wet fart.

[mr_fish]

3" sounds like a wet fart.

didn't stop you buying one

[silverzx]

On a petrol turbo they sound fvcking insane!!!

On a dirty diesel. Just no.

[Poodle]

2.5" is plenty big enough for stage 2, using 3" can actually harm performance, not to mention sounds crap.

[Connor]

2.5" sounds good

[Frosty]

Exhaust on my estate sounds alright (epic) and thats 3"

[306rhys]

Thanks for the replies!

I'll go for a 2.5" system then, I'm looking for a de car, only one I can find is the Hoffman which is mild steel, anyone know where I could source a stainless steel de cat?

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Thanks for the replies!

I'll go for a 2.5" system  then, I'm looking for a de cat, only one I can find is the Hoffman which is mild steel, anyone know where I could source a stainless steel de cat?

[welshpug]

Totally depends on engine spec rpm fuel type etc.

the original standard PSA exhausts taper down, hence they have very good flow and sound qualities for the standard any most tuned engines, you wont find many generic replacement systems with that kind of detail...

[toseland]

the most important bit is the collector and downpipe.. it needs to be larger, 2.5 at least, 3 inch preferable, this should then run into at least 2.5 and then you can taper down when moving away from the exhaust to a free flowing 2 inch (minimum) as the gas pressure will have dropped considerably as it moves away from the exhaust.. i would want to be at least 2.5 to where the cat was orignally however..


ideally, 2.5/3 inch all the way through with no restrictions like a cat!

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Thanks for the replies!

I'll go for a 2.5" system  then, I'm looking for a de car, only one I can find is the Hoffman which is mild steel, anyone know where I could source a stainless steel de cat?

---

Thanks for the replies!

I'll go for a 2.5" system  then, I'm looking for a de cat, only one I can find is the Hoffman which is mild steel, anyone know where I could source a stainless steel de cat?

  Mild steel decats from hoffman are fine....  the only real issue is the mild steel backboxes as they dont usually get hot enough to deal with the condensation and water..    


hoffman decat,  with a stainless cat-back (or from where the cat is/was) will be fine. 

have a look on yell,  or the like.. quite often there will be a little performance place that will do the pipework for you for a lot less than longlife, and it will be exactly to your specifications. 

[306rhys]

the most important bit is the collector and downpipe.. it needs to be larger, 2.5 at least, 3 inch preferable, this should then run into at least 2.5 and then you can taper down when moving away from the exhaust to a free flowing 2 inch (minimum) as the gas pressure will have dropped considerably as it moves away from the exhaust.. i would want to be at least 2.5 to where the cat was orignally however..

I contacted the suppliers of the Hoffman down pipe and they say that it's identical diameter to the oem one which is 2.25", right? Will have a browse round and see where I can get one made up

[pro_steve]

I'm in the process of making a collector/down pipe to go with the standard GT1546S as I personally think this part is quite restrictive. I am getting the flange laser cut out of 10mm stainless and I asked for 2 to be made so it's yours if you want it...

Personally I'm going to use a 60mm 90 bend, ovalised slightly with another bit welded on to account for the extra long oval shape. I'm then going to weld this to a 2.5" v band flange and then go 2.5" the rest of the way out. (I'm using 60mm because I had a bit laying around and it seems to fit well, 2.5" is 63.5mm).

The standard outlet from the turbo has an ID of approx 48mm (and also quarter of the pipe is indented inside) so with something around 60mm you're getting an increase in flow area (ignoring boundary layers) of 56%. Quite a big difference really. I'll update once I've tested.

[toseland]

SDpeak to jonny steve mate, he is a magician with an angle grinder, T316 and some carbide router bits

[306rhys]

That's something I'd be interested in Pro Steve, did message you on facebook about getting a stage 2 map. Just sorting out all the bits for stage 2 then I'll head over to Bristol just need to sort out an exhaust and clutch then I'm good to go.

[Sambarker]

I have a 3 inch turbo back with no boxes and it sounds great