Any exhaust guru's....? A Mass-debate session

[Nova_Tek]

Always thought about how exhaust system differ in sound from one another what makes some aftermarket exhausts sound really sweet and other sound totally sh*t.

So...

What gives an exhaust system its note / sound and how to achieve this... discuss.

[Adam]

Pipe Material
Silencer size
Number of silencers

[Nova_Tek]

for example if we had 2 identical systems with 2 silencers at the same size, same diameter pipe, same tailpipe, one was made of mild steel and the other stainless.

Which system would sound better?

When I say better I mean a system that is not raspy has a good continuous sound and is not loud as funk?

[Adam]

Define "better" ?

Ive had numerous exhaust systems, and if you like loud/bit of rasp get a Sportex/Ashley 2 box mild system. Ive had a couple.

Currently i have a 3 box, stainless Scorpion system which is much quieter than the Ashley, but sounds much better imo. Deeper tone, and no rasp at all.

[Jonlem]

Its a good topic this as there is a lot to know on the subject.

Where an exhaust is even mounted can play a huge part in how noisy it is IN the car which in my eyes is just as important as what it sounds like from behind. My mates old stainless system on his nova had such a rasp that it would fail noise test, a similar exhaust in mild was much quieter.

My Audi for example has a 3" straight through system, one BIG Miltek silencer at the back, its loudish from the outside but stupid loud in the car, very boomy from the rear if you get me ? it has stopped people wanting lifts though which is the only good thing to come from it lol

My concern is will fitting a centre box shut it up or is the back box of such a design that its going to be like that whatever you do ?

A friend of my mums did once work for a company doing this, I remember him telling me how much of a nightmare it was to do the Lotus Carlton due to mount positioning.

Some genuine answers on this subject could be very useful to people as trackdays are becoming stricter and generally I'm well past having a REALLY noisy car, my daily driver is hardly a "barry car" but I am victimised for having that noisy exhaust which gets on my tits

[Lee]

Very roughly, the number of silencers will affect the volume and the rear box will effect the tone.

A bigger box will drop the tone (to a boomy moise like Jonlems) whereas a small rear box will normally be higher and raspy (like a cherrybomb lol). How boomy or raspy they are will depend on how many mid boxes you have.

This is a very rough explanation though, there are quite a few variables.

[Jonlem]

I know fook all about exhausts, well how to keep them quiet yet retaining enough flow to give the power.

Good example of that is someone I know of with a Sierra Cosworth who gained 50hp when he swapped from a good stainless exhaust to a 2k Hayward and Scott system, It is a 550+hp engine though and he was chasing big numbers without fitting a dustbin sized turbo.

[Mike]

Engine displacement plays a part in the exhaust note, as does MPi or Carb :thumb:

[Lee]

Good silencers shouldnt effect flow too much. Most of the difference can come from how your exhaust builder does the bends. Some will have a cheaper bender which actually reduces the ID of the pipe when its bent, rather than a mandrel to keep the ID the same.

You see, using a cav shifter isnt as pointless as people think :)

[Sturge]

A good quality mandrel bend shouldn't leave any reduction in the id, that's the point of the mandrel :)

[Ash]

resonance and frequancy - the metal the exhaust is made from will resonante at a certain point, dependant on thickness and quality. Also you have to remember that low notes (bass) have a longer wave length and travel further...

I'd imagen an exhaust acts alot like a (mind blank - speaker inside a speaker) system and is very directional with the high frequency's but is unable to control the lower notes, causing all the nasty noises.

- will try and find the name of the system and example...

[Fridge]

Most will have your normal manderel bender which actually reduces the ID of the pipe when its bent, rather than more expensive machines use heat and a coil whilst bending to keep the ID the same.


I thought the point of inserting a mandrel was to maintain the ID? Otherwise you're just bending it like any monkey could.

[Nova_Tek]

Which system would sound better?

When I say better I mean a system that is not raspy has a good continuous sound and is not loud as funk?


Define "better" ?

Some interesting points made. I too thought the mandrel bend machine allowed to bend pipes without reducing its dimensions.

With regards to the interior noise I think that probably has a lot to do with the resonance frequency of the floor pannels which probably amplify the exhasut note. Something like dynamat would help to reduce that noise.

I ask this because currently on my 1.2 I have a sportex back box fitted on an otherwise standard system. Its a decent volume untill you hit the 2.8k mark then its a bit raspy and loud. Im assuming this is due to the engines characteristics and I guess, although I may be wrong, the engines power & grunt is from that rev range hence the extra noise. Being a carb with a K&N I assume this also amplifies the noise.

Problem with this is that even with light driving people always assume theres either a chav in the car or he wants to race. This is something I want to get rid of when building my LET.

So a bigger backbox helps to keep the exhasut note in the lower frequencies and reduce raspyness and very loudness and in theory more silencers, as the name suggests, keeps it quieter.

Does the tailpipe, regardless of backbox design, have an influence on the noise made from the exhaust?

[Lee]

I thought the point of inserting a mandrel was to maintain the ID? Otherwise you're just bending it like any monkey could.

You are correct, I have edited my original post lol.

[Dod]

Whats the best layout for an exhaust system to optimise performance?

[BRoadGhost]

100 - 150BHP 2" ID
150 - 200BHP 2.25" ID
200 - 250BHP 2.5" ID
RickDraper 3"+

…as a rough guide ;)

[Dod]

What about silencers and bak box sizes, numbers etc?

[MK999]

Been looking into exhaust performance recently, basically it's a black art lol

You can look at a hugely impressive system like the swindon btcc setup and find it loses 5bhp at top end compared to a standard cast iron manifold (just an example, i'm sure the swindon exhausts were extensively tested and refined to the limits) The straightest and largest exhaust you can find can sometimes be the worst, and possibly the point i'm finding mentioned most is that the perfect exhaust for an engine on one setup, is absolute cack with the exact same engine running a different cam and induction system.

Only way to know for sure is to run it on the car on a dyno vs various setups, and use flow benches for all the design work, it gets pretty intense.

(this comes from a combination of, four-stroke performance tuning by A. Graham Bell, Tune to Win by Carroll Smith and modern engine tuning by the same Mr Bell for those interested, all pretty good reads if you can get your head round it, four-stroke performance tuning probably being the best of the lot for engine related stuff)

[BRoadGhost]

As previously mentioned what goes in and through the engine will play substantial part in what kind of sound wave travels through the system.

Assuming N/a High peak and revs the goal is longest possible, least deflected primaries complete with balance bars (1 - 4 & 2 - 3).

What comes after will shape back pressure; greater number of silencers generally means less noise but more torque.

I was / am tempted to go twin side exits once car's back on the road if it's too noisy…

[Sturge]

Does the tailpipe, regardless of backbox design, have an influence on the noise made from the exhaust?

can have next to no effect, or a massive one, depending on the style. if it stays the same diameter it'll have very little effect, if it opens out at all, it'll act like a trumpet. i had a 3" pipe going to a 5x2.5 oval and that was ridiculously loud, swapped the tailpipe stub for a 3" round and it was nice but sensible volume


Broad, 2.5" is good for over 300, gp.N and some wrc cars run (or did when i was making them) happily on 2.5

[spanishfly]

Best bet is find a good exhaust builder and let them decide what's needed once you have told them the requirements. The guy that made mine did a great job.

[Nova_Tek]

can have next to no effect, or a massive one, depending on the style. if it stays the same diameter it'll have very little effect, if it opens out at all, it'll act like a trumpet. i had a 3" pipe going to a 5x2.5 oval and that was ridiculously loud, swapped the tailpipe stub for a 3" round and it was nice but sensible volume


Broad, 2.5" is good for over 300, gp.N and some wrc cars run (or did when i was making them) happily on 2.5

Right, I can see the effect of the tailpipe now that you mention it. Not a fan of sausepan sized tailpipes, considering to go oval.

I know its a case of trial and error which is a pain as many of us will probably have the cash to do a full custom system but not afford various other systems to compare to and RR them, well me anyway.

I have been looking at systems and the Ashley stainless steel 2.5" kit which seems to be really affordable is probably what I will go for but I will probably take it to a specialist to get some good silencers and backboxes matched up.

How good is the LET manifold? Another consideration when it comes to exhausts is the manifold as some will effect torque etc. Can the LET manifold mate to a 2.5" system? Can the manifold do its job for 250bhp?

:thumb:

[Dod]

Doesnt a LET Manifold have a Turbo bolted to it though mate?

[Nova_Tek]

Yep