Jenko_Sport
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Registered: 25th May 06
Location: Stoke-on-Trent
User status: Offline
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Graph
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jazzer2k3
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Registered: 24th May 03
Location: **SHEFFIELD**
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Maths assignment  ?
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davey_jack
Member
Registered: 28th Jan 06
Location: Horley, Surrey Drives: Clio 182
User status: Offline
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quote:
Originally posted by Jenko_Sport
Davey Jack back pressure is what you get in your exhaust but if your getting rid of to much of your exhuast gases then you could loose power technically.
 There is nothing in your exhaust that creates a backward pressure towards the exhaust ports on the engine! I'm tired of hearing this sort of thing from people working at kwik fit etc, its rubbish!
It is possible that by using a over sized bore exhaust system you can destroy the vacuum that is created by exhaust gases leaving the engine.
However there is no such thing as "back pressure"
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Dave A
USER UNDER INVESTIGATION - DO NOT TRADE
Registered: 10th Dec 03
Location: County Durham
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exhaust design is a lot more complicated than people realise. most off the shelf exhausts are more for the look and sound rather than perfomance really.
what some people term as 'backpressure' does exist but its not strictly backpressure as such. Making an exhaust that is well designed isnt easy, to do it properly everything from inlet design, cylinder pressure, valve overlap values need to be taken into account (sometimes refered to as 'pulse tuning' )
the Idea behind 'backpressure' is this: as exhaust gasses exit the combustion cylinder into the exhaust posrt and into the manifold they are at a high speed. once the exhaust valve closes the gasses create a virtual vaccum behind them, this helps to suck the next load of waste gasses out and so on... if the exhaust bore is too big or too free flowing or the primaries of the manifold are too short or long then the amount of pressure pulling the next lot of waste gasses out of that cylinder is reduced therefore the gasses escape slower and the speed of the engine is reduced and the inlet gasses enter slower and so on....
there is a lot more to exhaust design than you might think......
hope that makes sense
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JayT
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Registered: 26th Jul 06
Location: Stoke On Trent
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god thats technical lol so in theory what the closest to a perfect exhaust system and backbox? i heard a 3" round tailpipe is the best, quote me if im wrong though!
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bieransri
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Registered: 13th Apr 06
Location: Kidsgrove, Staffordshire
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quote:
Originally posted by corsa boy Jay T
god thats technical lol so in theory what the closest to a perfect exhaust system and backbox? i heard a 3" round tailpipe is the best, quote me if im wrong though!
tailpipe wont affect things, its the shape/routing/bore of the manifold and pipework from manifold back, aswell as any restrictions such as silencers/cat's that would affect the flow etc
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JayT
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Registered: 26th Jul 06
Location: Stoke On Trent
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cheers for that bieran lol haha
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Dave A
USER UNDER INVESTIGATION - DO NOT TRADE
Registered: 10th Dec 03
Location: County Durham
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manifold is the most important part of the design. getting it perfect is very very difficult.
the exhaust I am makking for project Vsport will have a 4-1 stainless manifold, 2.25" bore with 1 large single rear box. idea behind the single large box is to keep gas speed as high as possible for as long as possible in the exhaust.
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JayT
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Registered: 26th Jul 06
Location: Stoke On Trent
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so if you were to take out the centre silencer and have a straight through centre section would this help speed up the gas speed!
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Dave A
USER UNDER INVESTIGATION - DO NOT TRADE
Registered: 10th Dec 03
Location: County Durham
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yes but must be done after considering if increased gas speed is needed. a higher revving engine say 7500 rpm+ will benefit more than a lower revving torquey engine from higher gas speed. most engines will suffer from a 4-1 and single silencer as they will run out of revs before seeing the benefit.
on a mildly tuned 1.6 the best setup is a 4-2-1 janspeed manifold and a well designed 1.75 / 2.00" exhaust with a twin box system and decat.
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davey_jack
Member
Registered: 28th Jan 06
Location: Horley, Surrey Drives: Clio 182
User status: Offline
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quote:
Originally posted by Dave A
exhaust design is a lot more complicated than people realise. most off the shelf exhausts are more for the look and sound rather than perfomance really.
what some people term as 'backpressure' does exist but its not strictly backpressure as such. Making an exhaust that is well designed isnt easy, to do it properly everything from inlet design, cylinder pressure, valve overlap values need to be taken into account (sometimes refered to as 'pulse tuning' )
the Idea behind 'backpressure' is this: as exhaust gasses exit the combustion cylinder into the exhaust posrt and into the manifold they are at a high speed. once the exhaust valve closes the gasses create a virtual vaccum behind them, this helps to suck the next load of waste gasses out and so on... if the exhaust bore is too big or too free flowing or the primaries of the manifold are too short or long then the amount of pressure pulling the next lot of waste gasses out of that cylinder is reduced therefore the gasses escape slower and the speed of the engine is reduced and the inlet gasses enter slower and so on....
there is a lot more to exhaust design than you might think......
hope that makes sense
Yeah thats pretty much what i was getting at mate.
It annoys me when people talk about "back pressure" because it does not exist! Why would anything applying a pressure against the exhaust gas flow ever improve performance!
I once heard a kwik fit employee say that if you ran a car with no exhaust section after the cat, the lack of back pressure would "fuck" the cat! ...f*cking twat
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