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Author Topic: Why Is An Exhaust Bigger And Wider At The 'Exhaust' End ?  (Read 8874 times)

Offline neilep

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Dear Shattered, Tired and Exhausted Peeps Of Wonderfun and Science Luff,

see this exhaust ?




Nice eh ?.......

But why does the ' exit ' holes need to be bigger than the other pipes (setting aside the catalytic converter)

Look , here's another :




having said the above......Is it specifically BECAUSE of the catalytic converter ?..if not, then is it just aesthetics or is there any real science behind it ?


Blow your top and REVeal the answer !*le groan*

Thanks


neil








 

Offline ukmicky

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Why Is An Exhaust Bigger And Wider At The 'Exhaust' End ?
« Reply #1 on: 02/04/2008 22:17:08 »
Looks and maybe to reduce back presure
 

Offline turnipsock

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Why Is An Exhaust Bigger And Wider At The 'Exhaust' End ?
« Reply #2 on: 03/04/2008 00:51:25 »
I think it may just be for show.

Two strokes use expansion pipes, where there is a lot of science going on. They tend to be thinner at the end.

« Last Edit: 03/04/2008 00:57:19 by turnipsock »
 

lyner

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Why Is An Exhaust Bigger And Wider At The 'Exhaust' End ?
« Reply #3 on: 03/04/2008 10:22:16 »
If you want to 'encourage' exhaust gases to leave the cyclinder,you can tune the exhaust by making a 'Helmholtz resonator'. A suitable size of fat section can do this for you but it means that the engine works well but over a narrow rev range. High performance engines are often associated with gearboxes with many ratios to keep the revs within this range..

The reason for the wide bit at the end is probably to allow the gases to move slower than when they come out of the exhaust ports. The lower speeds will generate lower frequencies which are softer to the ear. it also sounds 'throaty' which is associated with a bigger engine. (Virtual performance for boy racers?)
 

Offline turnipsock

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Why Is An Exhaust Bigger And Wider At The 'Exhaust' End ?
« Reply #4 on: 03/04/2008 12:01:53 »
 

Offline lightarrow

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Why Is An Exhaust Bigger And Wider At The 'Exhaust' End ?
« Reply #5 on: 03/04/2008 20:50:01 »

turnipsock, that's fantastic. It explains in one second what I've only tried to visualize for years. Thank you!
 

Offline neilep

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Why Is An Exhaust Bigger And Wider At The 'Exhaust' End ?
« Reply #6 on: 03/04/2008 21:03:19 »
If you want to 'encourage' exhaust gases to leave the cyclinder,you can tune the exhaust by making a 'Helmholtz resonator'. A suitable size of fat section can do this for you but it means that the engine works well but over a narrow rev range. High performance engines are often associated with gearboxes with many ratios to keep the revs within this range..

The reason for the wide bit at the end is probably to allow the gases to move slower than when they come out of the exhaust ports. The lower speeds will generate lower frequencies which are softer to the ear. it also sounds 'throaty' which is associated with a bigger engine. (Virtual performance for boy racers?)

THANK YOU SOPHIECENTAUR.....so...it's really down to aesthetics and Earsthetics !!..*sorry*
 

Offline neilep

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Why Is An Exhaust Bigger And Wider At The 'Exhaust' End ?
« Reply #7 on: 03/04/2008 21:05:39 »


VUNDERBAR !!....it displays in one second what will still take years for me to understand !.....*have pity upon us academic wannabes*
 

Offline Karen W.

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Why Is An Exhaust Bigger And Wider At The 'Exhaust' End ?
« Reply #8 on: 03/04/2008 21:08:05 »
 

Offline neilep

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Why Is An Exhaust Bigger And Wider At The 'Exhaust' End ?
« Reply #9 on: 03/04/2008 21:10:02 »
WARNING WARNING >>>>>BAD JOKE ALERT !!



With all these diagrams...I'm Exhausted !! ;D




 

Offline Karen W.

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Why Is An Exhaust Bigger And Wider At The 'Exhaust' End ?
« Reply #10 on: 03/04/2008 21:28:58 »
LOL LOL LOL LOL LOL.....Hee hee hee... that was very good! LOL! YAYYYYYYYYYYY!
 

Offline DoctorBeaver

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Why Is An Exhaust Bigger And Wider At The 'Exhaust' End ?
« Reply #11 on: 03/04/2008 21:29:51 »
"Piston broke"
"Yeah, so am I"

 :D
 

Offline ukmicky

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Why Is An Exhaust Bigger And Wider At The 'Exhaust' End ?
« Reply #12 on: 03/04/2008 23:25:53 »
It depends on the type of engine and its state of tune.  The system you have in your picture is designed for a car not a motorcycle and is a big bore system designed to reduce back pressure which will in turn increase performance bhp /torque of the car its placed on, especially if that car has had other modifications done to it increasing the flow of fuel and air into and out of the engine .This is usually at a cost as it can increase fuel consumption emissions and sound levels coming out . 

Back pressure basically restricts the flow of gases out of the engine and whilst engines require a car a certain amount most car manufacturers  main build criteria is not performance and build in more back pressure into the system than is required in order to improve emissions levels, fuel efficiency ,and sound levels at a cost to the performance of the car.
Basically most cars if not all will gain a power boost over the standard spec if you place an exhaust on your car that get the gases out quicker like the one in your picture.

My car for instance has a turbo and benefits greatly power wise with a aftermarket almost straight through exhaust because on full boost the exhaust gases coming are restricted to much by the standard exhaust but as the turbo does a good job with silencing the exhaust note the sound level has not increased to unacceptable levels.

Cats reduce the emissions of certain substances of newish cars at the cost of power and due to their restrictive nature some performance exhaust systems will remove the cat as an easy way to improve response and power ,this however  can then have repercussions if the MOT inspector is doing his job properly which many dont.
« Last Edit: 04/04/2008 23:19:29 by ukmicky »
 

lyner

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Why Is An Exhaust Bigger And Wider At The 'Exhaust' End ?
« Reply #13 on: 07/04/2008 12:48:05 »
Quote
The system you have in your picture is designed for a car not a motorcycle and is a big bore system designed to reduce back pressure
When a tuned system is at resonance, the low pressure downstream of the exhaust valve is in phase with the opening of the valve. On either side of this, of course, you can have high pressure and that will make things worse. Hence a 'peaky' performance and the need for a million gears.
There's an idiot who roars up our road in the middle of the night and I swear he changes up about ten times as he pulls away from the traffic lights.
« Last Edit: 07/04/2008 12:50:34 by sophiecentaur »
 

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Why Is An Exhaust Bigger And Wider At The 'Exhaust' End ?
« Reply #13 on: 07/04/2008 12:48:05 »

 

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