Downpipes and physics

[ChristoT]

Is there a specific reason why downpipes are circular (apart from manufacturing costs)? Would there be any effect on performance from having a, say, oval-section downpipe?

[rustyrider]

No, as long as the cross sectional area remains the same or is larger.

[ChristoT]

That's what I was thinking, just wanted to double check.

[mickvp]

Just manufacturing likely. I would imagine it is much more difficult to bend an oval without crumpling it.

[ChristoT]

Thanks! I'm thinking about how my fabricated downpipe will clear the swingarm, and considering how best to do it.

[mr self destruct]

Exhaust pipes are circular for a number of reasons. Mainly ease of manufacturing because its far easier to bend a round pipe than an oval one, with its strength being the same in all directions it's less likely to collapse or kink. Another reason is that a circle has the smallest edge for a given surface area*, so any other shape will add to weight and material costs. Also, a circular design minimises turbulence and disruption to the gas flow.


*for example, for a surface area of 100sqcm, a circle will have a circumference of 35.45cm, a square has edges totalling 40cm (13% more), and a right isosceles triangle will have edges totalling 48.28cm (36% more).

[red98]

interesting ......important that the exhaust matches the exhaust port in the cylinder head for smooth gas flow 

[sadlonelygit]

if we want to get really technical, the best pipes have a taper from the exhaust port to @2/3 down the header (it depends on the stroke of the motor) to maximise cylinder scavenging.
a few years back hond@ used oval pipes on the rvf endurance machines, then discovered another 7hp when they fitted circular pipework.
fluid dynamics................fascinating

[Phil]

Exhaust pipes are circular for a number of reasons. Mainly ease of manufacturing because its far easier to bend a round pipe than an oval one, with its strength being the same in all directions it's less likely to collapse or kink. Another reason is that a circle has the smallest edge for a given surface area*, so any other shape will add to weight and material costs. Also, a circular design minimises turbulence and disruption to the gas flow.


*for example, for a surface area of 100sqcm, a circle will have a circumference of 35.45cm, a square has edges totalling 40cm (13% more), and a right isosceles triangle will have edges totalling 48.28cm (36% more).

That makes sense.

30+ years ago I was on a geography field trip in Devon and measured the speed of water down the river Dart by standing in the river with meter, in March. Although the gradient was less, the average speed of the water was quickest lower down the river where it was deepest with less drag off the river bed.
So guess a round pipe would get rid of the exhaust gases quicker as there would be less drag from the sides of the pipe. In an oval pipe, the centre of the pipe would be closer to the pipe side, so more drag.

[fireblake]

A few years back Micron were making Serpent headers. They spent a fortune on a machine that used water pressure to form the pipes. Within a few years they had gone bust. Micron no more

[fireblake]

http://www.micronexhaust.com/store/prodserpent.htm

[noggythenog]

So........im confused




Maybe i'm on the wrong thread.




Phil..on basis of your experiment...if it applies to exhausts then would decrease friction & therefore increase flow....or in exhaust terms be more free flowing exhaust.




But flow surely differs to pressure & i am led to believe that the pressure in an exhaust system is equally important, back pressure etc etc......






So would oval pipes also be bad for pressure or is that purely a case of if there is the same internal area then the pressure will be the same regardless of the shape?????








Just found something else out too......circular pipes can withstand large pressure differences between the inside and outside without experiencing any significant distortion......so your funny shaped pipes could get squished under heavy load.

[Dead Eye]

The term back-pressure is somewhat mis-understood I believe. The idea is that you don't want negative pressure such that gas flows IN to the exhaust ports. As the gases are released in high pressure pulses, you need to have a suitable system to prevent the negative pressure behind the high pressure pulse from becoming an issue - at least this is how I understand it and I could be wrong, but it makes sense in my head.

This explains Motads research into their "Serpent" pipes - they described a negative pressure effect on the inner curve of the first bend caused by the gases being squashed against the outer edge as their direction (vector) changes - by squashing the pipe at this area, it prevents the negative pressure area from developing in the first place as the gas fully expands in to the narrower pipe section... and I should probably stop rambling...

Back to the oval topic, would this shape (and others) not create varying pressure values when measured at different points on the cross-section of the pipe?

[midden]

if we want to get really technical, the best pipes have a taper from the exhaust port to @2/3 down the header (it depends on the stroke of the motor) to maximise cylinder scavenging.
a few years back hond@ used oval pipes on the rvf endurance machines, then discovered another 7hp when they fitted circular pipework.
fluid dynamics................fascinating

And the reason you're named sadlonelygit is? 

[midden]

As far as I know  you don't see many if any oval whirlpools at the bath plughole.  Or oval helter skelters.  round is easier to gain momentum, gas particles dont want to pick up speed on a straight only to be bashed to a near stop around druids bend