Why do exhaust pipes....

[Big Dave]

...have to be the same length?

In these days of computer dynamics they could fiddle the pipe diameters for constant back pressure?

Varying volumes mess it up?

[xwhatsit]

Is it just total volume, or does diameter affect things differently to length (e.g. like on a stroker)?

[BMWST?]

I know the length of the exhaust is a critical variable for tuning.there is also a relationship between each the cylinders exhaut pulses that has to be taken into account. I cant explain it any better than that

[colsan1]

On two strokes they dont.
Late model strokers are usually V twins so the
front and rear pipes will not be the same.
AFAIK on diesels(4 strokes) the length and dia of the pipe has and effect on the engine temp and gas flow.
I'm sure some one clever will be along soon to give you a proper answer

[Mikkel]

Fluid mechanics = black magic.

[Scouse]

I belive Ducati and Harley do this to some extent with different diameter header pipes

[Motu]

As for length - you just double it to catch the same harmonic.

[AllanB]

Ducati had/have a larger diameter on the rear header than the front to compensate for its reduced length. The press made a bit about this a few years ago - this implies volume.

I suspect in a perfect world all would be the same for max performance - but notice any set of aftermarket pipes on a American or Jap V-Twin and length does not appear to be of interest. They have different length pipes between cylinders, short pipes, extra long pipes etc.

Mufflers
I'm looking to muffs presently as I plan to make a set for the Hornet this winter. All the aftermarket stuff is pretty much the same regarding internals - a 44 -50mm perforated straight through pipe wrapped in sound absorbing fibre of a length around 450-500 mm long. Bolt a smaller outlet pipe in the end to make it quieter. All claim to be the best out there
There is however a big difference in quality of construction between brands.

[stig]

the total resistance/back pressure
- has a linear relationship to the length and,
- a non linear relationship to the velocity.

velocity is directly related to
- flow, how much air is being forced down it, rpm+throttle (roughly the same for each pipe in this example)
- and the pipe diameter.

as the flow increases in the smaller pipe the pressure losses increase greatly
compared to the larger diameter pipe where the losses increase proportionaly to the extra length.

short answer - they cant make a different diameter pipes behave the same over the whole rev range.

I'm not sure why it would matter if one cyl made slightly more or less power than the other though. in the real world the will anyway with diff compressions across the cly's etc.

Would guess that the ducatis and harleys mentioned are prob a case where it was close enough not to bother winding extra lengths of pipe around the bike.

[Yow Ling]

I fthey were all different lengths , everyone would moan about the piss poor quality control

[pritch]

There is however a big difference in quality of construction between brands.

Now there's a thing that is often overlooked hereabout. The big excitement generally being generated by a bargain price.

[AllanB]

Now there's a thing that is often overlooked hereabout. The big excitement generally being generated by a bargain price.

100% correct here.

A couple of the big name builders fit, welds etc are quite stunning - you can see that time has been spent on the design and build.

One of the American name brands than are 'keenly priced' look like recycled vacuum cleaner parts in comparison to the above.

[Blackshear]

100% correct here.

A couple of the big name builders fit, welds etc are quite stunning - you can see that time has been spent on the design and build.

One of the American name brands than are 'keenly priced' look like recycled vacuum cleaner parts in comparison to the above.

As if they were soldered by a blind person, or were welded suing a mild steel thinger-mabobby.

They need the perfect ripples... Of better yet, fused!