Who knows about performance car exhausts?

NoNameNoNumber · 11 Jan 2020 at 18:24

I have a modified 2.0l turbo producing 354bhp 379ft/lb. I won't say what as I don't want people just to show me what is available off the shelf
The tuner said I could do with a bigger exhaust as whilst the stock is ok, it's costing me 20-30bhp...
I made the downpipe and sports cat myself, 3.5" off the turbo down to 3" and through a 200 cell sport cat.
I was hesitant to get rid of the standard valved system though as it's a daily that I want to keep quiet.

So I'm looking to make a new cat back. 3" all the way, quad exit. Now on to the tricky part.
I don't want any booming/droning etc and I have no idea what the actual cause of it are. Any 'rules of thumb' or general wisdom you guys might be able to offer?

Failing that, I'll just make one with the biggest centre res I can find along with the biggest back box I can find with 1 inlet and two outlets.

Cheers

Edit;
Yes there are several off the shelf options available... But I can tig weld stainless and I enjoy the DIY approach!

MissChief · 11 Jan 2020 at 18:34

What car?

[TW]Fox · 11 Jan 2020 at 18:34

MissChief said:
What car?

It's top secret. He wants us all to guess what may work.

paradigm · 11 Jan 2020 at 18:44

Judging by his posts about remapping, its an older (8j) TTS. There will be literally hundreds of off the shelf exhausts that already exist for his car and state of tune, but buying one of those is clearly too easy, as seemingly is getting a custom one built by a local specialist.

Kenai · 11 Jan 2020 at 18:46

[TW]Fox said:
It's top secret. He wants us all to guess what may work.

He doesn't, he wants someone who understands what causes exhaust drone to explain it so he can try his hand at building his own with minimal drone.

Unfortunately Motors is here to second guess everything and so his thread will probably descend into bizarre conspiracy about secret cars and why he doesn't want to buy the same milltek exhaust everyone else in the world does.

Some people just enjoy doing things themselves and learning new things, even if it's not the best or the quickest or the easiest way to get it done.

sanaxe1 · 11 Jan 2020 at 19:02

I'm guessing it's a Audi TTS

naefeart · 11 Jan 2020 at 19:18

This is quite a specialist subject, car manufacturers and specialist exhaust firms spend countless hours and money in R&D to produce an exhaust. Of course there are also guys who simply weld some old pipe together regardless of noise and drone issues.

Fairplay for trying it yourself, but not sure how best to advise you. I do know that I have introduced unwanted drone at motorway speeds when buying exhausts that replace the center silencer(s) with straight through or 'x-pipe' configurations. For the small hp gains, I was better offer with the silencers in place, more comfortable when doing distance. Valved seems to be the way to go these days if you want maximum sound but want to stay on good terms with your neighbours.

NoNameNoNumber · 11 Jan 2020 at 21:20

2 people with some sense, thank you!

Kenai said:
snip

naefeart said:
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So yeah, 8j TTS
Miltek, scorpion etc all exist... I keep hearing a lot of mixed reviews on all of them though, snapping welds, corrosion, rattling, droning, too quiet/too loud etc etc... I have all of the tools/skills to make my own. I'll just buy some parts and go in blind, worked well enough on everything else I've built

MissChief · 11 Jan 2020 at 21:23

Might be worth asking on a properly geeky forum? You might get someone on pistonheads for example, they have a home mechanics section

NoNameNoNumber · 11 Jan 2020 at 21:54

MissChief said:
Might be worth asking on a properly geeky forum? You might get someone on pistonheads for example, they have a home mechanics section

Agreed, I did know someone with Engine Analyzer Pro, that would've been awesome to run some numbers but ultimately I'll be constrained by what fits... Little car with prop shaft/rear diff etc to get round.
Cheers.

Nasher · 11 Jan 2020 at 22:49

AFAIK you need to change the whole exhaust for something which flows better, including the manifold to see any real gains. Otherwise it's just noise. The shape and quality of the materials makes a difference too.

A loud exhaust will get annoying after a while. You'll hate having to drive it with a headache

I need to change the back box on my car (because the OEM one is rusting away after 7 years), but I'm only going for something 1db louder than stock.

NoNameNoNumber · 12 Jan 2020 at 06:33

Nasher said:
AFAIK you need to change the whole exhaust for something which flows better, including the manifold to see any real gains. Otherwise it's just noise. The shape and quality of the materials makes a difference too.

A loud exhaust will get annoying after a while. You'll hate having to drive it with a headache I need to change the back box on my car (because the OEM one is rusting away after 7 years), but I'm only going for something 1db louder than stock.

The turbo is part of the manifold, so it'll only be a turbo back.
My theory is that if I can replicate the volume of the stock silencers, but go 3" bore all the way, then I hope that it will flow more and only be marginally louder. Turbo cars aren't nearly as fussy about the exhaust length/volume and NA cars.

Philz0r · 12 Jan 2020 at 06:54

NoNameNoNumber said:
The turbo is part of the manifold, so it'll only be a turbo back.
My theory is that if I can replicate the volume of the stock silencers, but go 3" bore all the way, then I hope that it will flow more and only be marginally louder. Turbo cars aren't nearly as fussy about the exhaust length/volume and NA cars.

I don't think going from stock to 3" will keep the same volume.

eviled · 12 Jan 2020 at 10:24

I’ve got a 70mm non-resonated scorpion on the same car, it’s not very social.

If I were to swap I’d go Miltek out of ease.

it would be interesting to see the internal of a the standard backbox

danlightbulb · 12 Jan 2020 at 12:53

I know nothing about car exhausts but interesting thread OP.

I would say the first thing to do is to draw out the current exhaust layout. For each section of the current exhaust, what is the length and bore.

Then I would make new sections in a modular way so you can easily swap length sections in and out, so by using joints at regular intervals rather than a continous length. This way you can experiment with different bores and silencers in different parts of the exhaust by simply removing and refitting sections.

Why not start with a 3" bore across the whole length - no silencers - and see what happens?

LizardKing · 13 Jan 2020 at 00:07

How small is it currently to be costing you 20-30bhp?!

though being a turbo you'll never get a great noise regardless, just go straighter/bigger (oh err!)

encephalopathy · 14 Jan 2020 at 10:50

have a google for "helmholtz resonator exhaust"

Dup · 14 Jan 2020 at 11:07

Could you not re-baffle an OEM back box? I assume there's pathways etc inside you should shorten etc. Would take out a lot of guess work and could potentially be a cheap experiment others could follow.

adam cool dude · 15 Jan 2020 at 14:15

It's comes with the territory of the size I am afraid. I have 3" all the way through from turbo to tip and I bought an off the shelf fujitsubo system which was meant to be quiet and it is still loud. God knows what a blitz system would be like!

Nasher · 15 Jan 2020 at 14:43

Assume all aftermarket ones are loud unless you specifically buy a quiet one with extra silencing