Duel downpipe .. a work of art!!

[RobMatthews]

I will let you know in the next few weeks what this type of system is like as i\'ve got a knightsports full twin pipe system to go on mine i\'ll do befor and after dyno runs and let you know

[interloper393]

Ooooo nice.. is this the system martin was selling?

[RobMatthews]

yes although it might be sold again by the time i get round to testing it:Nono!

[chrno]

Knightsports makes their downpipe dual so I don\'t think it\'s all talk.

[Speedworks]

double post

[Speedworks]

it\'s for sale on rhdjapan.com

it does make sense in a way because hot exhaust gasses need lots of romm to expand and to get out of the system so having a lot of space directly after the turbo\'s makes absolute sense.
Reducing  diameter towards the rear of the car will keep the speed in the exhaust system toget them out as the gasses cool  and need less room.

Ever though why group A rally cars just have a 70mm throughout exhaust system?

[MikeLMR]

Ever though why group A rally cars just have a 70mm throughout exhaust system?

because they are restricted on the engine capacity and have an inlet restrictor that limits mass flow to a set figure. No point going any larger than the bare minimum required.

[marco]

it does make sense in a way because hot exhaust gasses need lots of romm to expand and to get out of the system so having a lot of space directly after the turbo\'s makes absolute sense.

i agree, but i think what shawn and i are trying to say is that the cross sectional area of the two pipe is still less than that of a big single (using the feed one as an example) and it may also have other, negative effects such as increased pressure drop and increased heat transfer due to the larger surface area of the pipe work. the only way i can see this being negated is if the header arrangement at the top of the downpipe is more efficient, if using two pipe means that this part can be optimized then it may be worth the bother!

[Speedworks]

sure that\'s why, I have an 80mm Trust downpipe and will have dual 60mm runners from DP to catback with middle and rear muffler (dual 60mm).

It has more capaictiy than a single 70mm pipe and due to smaller diameters a lower resonance band, keeping the exhaust tone deep and low.

[Nik da Greek]

I may be being really dim here, but I thought that within certain limits a narrower bore which then widens further along means the gas flows faster? Which would surely be what you want, to clear the most restrictive part of the system as quickly as possible. That\'s why two-stroke expansion chambers start really narrow, and then ballon further along to utilise wave-pulse scavenging effects.
 
If this is true, wouldn\'t two narrow downpipes clear the turbo outlet of backpressure faster than one big one thats the same diameter all the way along? I\'ve probably got this entirely a!se-backwards, feel free to shoot me down in flames now!

[marco]

the difference is with a 2 stroke you want to get scavenging, and at certain revs use the exhaust pressure to slightly increase the charge in the cylinder. the difference here is that there is a turbocharger, and we want that to work as  optimally as we can. the amount of work that the turbo can extract from the exhaust if a function of the pressure difference across it- the greater the pressure difference, the greater the mass flow through it. so ideally we want the lowest possible pressure just after the turbo, which for us it atmospheric. so in an ideal world we would want no exhaust at all, but that wouldn\'t be really practical (or popular with the neighbors) so we need the minimize the pressure losses aft of the turbo. now for a given mass flow, you are correct in that the flow velocity will be greater in a smaller diameter tube, but that is because it has to flow faster to get the stuff through it (to maintain the mas flow) however pushing air/exhaust/water/whatever through a small pipe fat is less efficient than through a big pipe slow. think straws, you can breath through one thin small straw, but it takes less effort if you have a big fat straw. the extra effort is the same thing as the back pressure on the exhaust. so by reducing it we can theoretically gain a little from the turbo.
however there will come a point where increasing the diameter of the pipe (either exhaust or straw) will have negligible effects, as the back pressure is so low. the thing i am starting to wonder is just how close our exhaust systems are to this point!

marco

[interloper393]

bloody hell these exhausts are complecated things lol.. i thought it was just a bit of pipe :yes

[Speedworks]

mainly, most US and Japan exhaust use oversized bores. Even that big that exhaust gasses have so much space that the speed inside the tubing is reduced.
This means you get back pressure as well. Exhaust gasses jus float arround in the pipe


What you need is a venturi effect, lots of room when gasses exit the manifold/turbo to let expand the hot gasses , a narrow section and then free air.

I had a 76mm HKS DP on my previous FD, OEM cat and 70mm catback and exhaust gas speed was higher then ever meaning you have great flow so less backpressure on the turbo. no big end tip, just a 70mm straight pipe. Deep sound and a lot better response on the gas pedal

[marco]

sorry i am not following you with the narrow section- why would that help with flow? i grant you the speed would increase, but thats because you need to maintain mass flow through the system- i cant see how it would scavenge the exhaust in anyway.

i am not trying to be argumentative btw, it is just that the idea doesn\'t \'click\' in my head, and wondered if you could explain it a bit more.:Thumbs-up

[shaunwil]

I bought this a while back.

http://www.amazon.co.uk/Scientific-Exhaust-Systems-Engineering-Performance/dp/0837603099/ref=sr_11_1?ie=UTF8&qid=1203962092&sr=11-1

Interesting read, but more piston based. Bought it while I was researching the intake on the FB.

Shaun