Designing a new lightweight exhaust but need advice

turbotoaster · 5 Apr 2013 at 21:20

Quick question, just hoping someone might have a bit of input on which way to go with the backbox.

Im going to run a lightweight exhaust which will replace all the bends, box and cat.

I have taken off the current janspeed backbox and sports cat and it weighs 13.75kg so we should be able to make something much lighter

ive measured the space ive got and the exhaust has to be 550mm and the backbox can be a maximum length of 460mm.

Now the issue ive got is that I need to keep it at a maximum 105db so Im allowed on track.

ive found 2 boxes that maybe meet my needs.

One has a length of 335mm and a diameter of 177mm which gives an overall volume of 8246240.35mm3
the other has a length of 457mm and a diameter of 152mm which gives an overall volume of 8248462.76mm3

So as you can see they both have similar volume, so in theory would they both have the same noise reducing effects or doesnt it work with exhausts.

Im hoping someone might have a bit of experience on which way round mufflers work, whether to go for girth or length

AndrewNR · 5 Apr 2013 at 21:32

Pretty technical question. Have you tried looking on Pistonheads? They may have a "mechanics" section.

Simon · 5 Apr 2013 at 21:34

Volume is irrelevant to volume.

bitslice · 5 Apr 2013 at 21:53

Exhausts are very complicated (I was researching ways of making a genny quieter a while back). A lot relates to how the engine exhausts its gas and how the pulse travels up and down the tube, then there are a couple of ways of designing a box to either absorb sound or to dampen it by resonating at a particular frequency. Then there are compromises vs db reduction and flow.
It's not quite as simple as making a box xyz big

Gaijin · 5 Apr 2013 at 22:09

So is this for your RX7 then? Did you get it back from the tuner?

Did everyone get their manifolds?

turbotoaster · 5 Apr 2013 at 22:23

Simon said:
Volume is irrelevant to volume.

Your saying the larger the box has no effect on db? thats against everything I have read up to yet, can you expand?

AndrewNR said:
Pretty technical question. Have you tried looking on Pistonheads? They may have a "mechanics" section.

I was also going to post on there but earlier there forum was down for maintenance, ill stick it on there now to see what they say.

bitslice said:
Exhausts are very complicated (I was researching ways of making a genny quieter a while back). A lot relates to how the engine exhausts its gas and how the pulse travels up and down the tube, then there are a couple of ways of designing a box to either absorb sound or to dampen it by resonating at a particular frequency. Then there are compromises vs db reduction and flow.
It's not quite as simple as making a box xyz big

Well as a guide its a supercharged 4 cylinder engine making around 230bhp, due to being mid-engined vehicle, the backbox literally bolts to the manifold.

I know I cant work out the science of my particular setup and as I have space and weight restrictions I just needed to try and work out what would be the better choice to go with.

Gayjin said:
So is this for your RX7 then? Did you get it back from the tuner?

Did everyone get their manifolds?

No, I never got my rx7 back, I lost out and moved on, now I do everything to any car myself, that way things are kept under control.

Im building a lotus exige race car now purely myself

rodenal · 5 Apr 2013 at 22:26

[TW]Fox · 5 Apr 2013 at 22:35

What happened to your M5 Turbo?

Matt82 · 5 Apr 2013 at 22:39

Take it you no longer work in a call centre

Biohazard · 5 Apr 2013 at 22:51

It depends entirely on the characteristics of the engine. Go find a tuning manual, and some advice from a relevant community.

Fuzz · 5 Apr 2013 at 22:55

Far too complicated to even worth worrying about.

Fit the largest back box volume you can get in there made from Titanium and be done with it.
Add a bung for track days if you are still over the limit.

turbotoaster · 5 Apr 2013 at 22:59

[TW]Fox;24069015 said:
What happened to your M5 Turbo?

I actually started ordering parts for it, thinking that 700bhp and cruise control would make me happy.

I realised when I was thinking of stripping it out to go faster that I would end up ruining the car trying to turn it into something it wasnt meant to be or be any good at, and hoping into a hot hatch one day made realise what I really wanted was something lightweight.

Hence I sold that, made some money on it and bought something with a better power to weight ratio and also less than half the weight.

Matt82 said:
Take it you no longer work in a call centre

My god that was years ago!! Didnt realise so many people remember so much of my rx7 build, I guess it was running for such a long time on here.

I would put up a build on here because people enjoy reading build threads(I know I do) but I can imagine more people will be interested in my personal circumstances than the actual car.

Fuzz said:
Far too complicated to even worth worrying about.

Fit the largest back box volume you can get in there made from Titanium and be done with it.
Add a bung for track days if you are still over the limit.

I contacted a company who do titanium exhausts but turns out they buy them in from abroad so cant make me a custom one without me making it myself in stainless/mild and shipping it abroad and even then they want £1200 for a backbox!!

I hoping the box in stainless itself will weigh about 5kg but need to research other branded exhaust manufacturers to see if I can get a rough weight guide.

Cant really bung it for racing as it will cost to much power, would be fine for trackdays when im setting up the car etc, but ill need all the power to be competitive, even though it will only weigh 680kg

bitslice · 5 Apr 2013 at 23:04

If money isn't an issue then you could test out a few dumb designs to see what is possible using components from eBay.
Make box the maximum size possible, insert a perforated pipe right down the centre (same diameter at manifold pipe), fill with exhaust wadding, weld it together.
Measure dB.
If that isn't even close then you will need a proper design.

Biohazard · 6 Apr 2013 at 13:06

Or just incrementally add expansions chambers in the midsection until you've muted it enough.

Which is just totally gay.

/strokes decatted straight-through midsection with glee

turbotoaster · 6 Apr 2013 at 21:23

the backbox literally bolts onto the exhaust manifold, there is a about 10cm from when the manifold joins up to where it enters the box so it basically has to do everything.

I gave a call to the exhaust company today and they are going to weigh it for me on monday but hey think its around 5kg so a nice 8kg saving