Exhaust design theory, guides and tips

[Xtatic]

Hi

I am in process of improving my fully custom exhaust for my s50e30. I want it to be as freeflowing as possible but i also want to retain reasonabe sound levels, no drone/rasp...
Also i want it to sound really good and tuned (perhaps the way exotics sound if possible, i do understand that most of it is engine sound, yes, but S50 can sound pretty damn good - see the videoclips of the Stromung catback on understeer.com)

I was wondering if anyone has some links to articles, guides or whatnot that would give me some knowledge on how to design the whole system to satisfy my needs

Thanks
Alex

[Mitch]

I would be interested as well. I would really like to get some hard data on exhaust design theory. There's more to it than guess and check. A lot more I suspect. Maybe not with cat-backs, but full exhaust system design. Maybe it's just because I'm an engineering undergrad, but I'm absolutely fascinated by the prospect of being able to work out a system (tube sizing, cat/resonator placing and sizing, etc) on paper that I know will be theoretically best or even taylored to my specific application.

-Mitch

[ohnoes]

A bit OT, but to the OP, why don't you list your S50E30 in the "Cars" section of your profile? That's a sweet car to brag about! :D

[Xtatic]

A bit OT, but to the OP, why don't you list your S50E30 in the "Cars" section of your profile? That's a sweet car to brag about! :D

heh, thanks! i don't post much on bf.c, mostly on my local bmw forum.

[xatlas0]

http://www.amazon.com/Scientific-Exhaust-Systems-Engineering-Performance/dp/0837603099/sr=8-48/qid=1158763531/ref=sr_1_48/102-7104388-5386529?ie=UTF8&s=books

[Itsablurr]

Hi

I am in process of improving my fully custom exhaust for my s50e30. I want it to be as freeflowing as possible but i also want to retain reasonabe sound levels, no drone/rasp...
Also i want it to sound really good and tuned (perhaps the way exotics sound if possible, i do understand that most of it is engine sound, yes, but S50 can sound pretty damn good - see the videoclips of the Stromung catback on understeer.com)

I was wondering if anyone has some links to articles, guides or whatnot that would give me some knowledge on how to design the whole system to satisfy my needs

Thanks
Alex

Backpressure, gas velocity (and heat retention/loss), exhaust pressure waves and patterns, scavenging, and gas turbulence... those are the primary issues to be concerned with.

I was going to type up a big, long post about exhaust design, but then realized I have a link that can do it for me.... read carefully- there is a LOT of good information in here.

http://www.burnsstainless.com/TechArticles/Theory/theory.html
http://www.burnsstainless.com/TechArticles/Construction/construction.html

[Xtatic]

good input guys, keep it coming

i was however more concerned about the sound, while keeping it decently free flowing.
I need this knowledge for my daily car, not for a dedicated race car.

[empty]

good input guys, keep it coming

i was however more concerned about the sound, while keeping it decently free flowing.
I need this knowledge for my daily car, not for a dedicated race car.


Then why even bother. Listen to a bunch of different systems on an identical and pick the one that sounds the best.

[Xtatic]

Then why even bother. Listen to a bunch of different systems on an identical and pick the one that sounds the best.

did you read my first post? where i say "fully custom exhaust"

[Itsablurr]

you're going to need to think more in terms of acoustical properties..... headers tend to "hollow" out the sound as compared to cast manifolds, for example..... larger bore pipes broaden the sound and soften it a bit at the same time.... smaller pipes sound more focused and not quite as "big".

What kind of muffler are you looking at? A media packed turbo? Welded chambered? A media packed turbo style muffler will give a bit of a deeper sound, cutting out some of the high frequencies. Throaty. A welded chambered muffler generally isn't as thick-sounding in the lower registeres, but has an aggressive mid-range frequency accent from the size of the case. The larger the case, the lower this resonant frequency range is.

Keep in mind, that this (exhaust) is only a coloration of the sound coming from the engine. Your primary sound is dictated by your combustion chamber shape, displacement, cam overlap, and dynamic compression ratio.

[promp3]

IN MY OPIONON,
its not really that complacated, are you going to make a single exhaust or keep it true dual? that will change sound alot I think.( and make things alot harder if you go dual)
hours and hours of testing and tuneing to make the perfect exhaust may only gain a couple hp as apposed to using blatintly wrong practices such as crush bent tubing, to small i.d or crap sound canceling/emisions parts could lower hp alot more. sound/volume is mostly dictated by what muffler and combonations of things are used in the exhaust. but the engine in the end has the most influence on what it will sound like. a 350z will never sound like a toyota camary though they bolth have a v6, no matter what mufflers you use.

I have heard so many diffrent theries whith exhaust, I dont even know what to beleve any more. I really don't know how you can atribute any gains to after market "cat-backs" other than a much better muffler, because there is not much piping that gets changed, and a majority of stock exhaust is still in place.
there is no way(I hope) for you to scientificly test,engineer and compare any diffrent designs you come up with.
so, I think you jsut have to use comon good knowage with exhaust.(mandrel bends,high flow cats,good mufflers no bottle necks or sharp angles and just keep it simple.
bottom line, just guess and hope for the best! and as far as diamiter you use, well I am all about bigger is better, its just a matter of how much bigger. the more air and fuel your engine eats the more it has to poop. if you do things to increase the amout of stuff you get into the combustion chamber, or increase the effecency, i.e high lift cams,intake,ports, .ect. the more exhaust gas there is to excape. especaly wen you take in to mind that it is expanding.

with a turbo engine, the theroy I have heard is the best exhasut is no exhaust, becasue you really don;t have to worry about velosity and scavanging so much becasue you are forcing it all out because the combustion chamber is forced air in under pressure plus the turbo its self is a restriction, so you want as little restiance as possable in your exhaust.

did you know the stock pipes are diffrent sizes before the cats? one is 2''id and the other is somtheing like 1.7''! after the resonators they bolth go to 2''
alot of people are using 2.5'' single and I did the math to calculate the area of the stock twin 2'' it came out that there was more space than 2.5'' with the stock system so I went with 3 i.d'' single and it seems to work well. I would like to have been able to dyno becasue when you are talking about like 5-10 hp you really cant feel it at all. 3'' does seem big for the applacation but I am happy with it.

btw the 3.2L m3 and 2.8L 328i have identicle size exhaust systems,(I've got bolth sitting in my gargage) the only diffrence I can think is in the mufflers.
so it cant be that importat to have exhaust piping exact other wise I would imagine they would have designed 2 diffrent systems for the diffrent engines.

I hope this helps I just wanted to share my views on exhaust. I don't like the way people with with a engineering perspective look at things they concentrate on exotic princapals and stuff when really you just need to make a damn pipe with no restrictions. you cant look at exhaust like headders. same reason I don't beleve in x-pipes