exhausts

[MR22ZZ]

see if I can do this with text diagrams...

It was my belief that there is a window in which the efficency of the exhaust is maximised due to air flow, brownian motion, heat dissipation etc

too narrow and you cannot move the air fast enough to free the combustion chamber and lots of eddies also form due to friction with the exhaust walls and due to the close proximity of the walls no clear channel can be formed

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too wide and you can get eddies forming in the middle of the channel because there is too much space and molecules that make up the exhaust gas become too dissosiated from each other so Brownian Motion takes effect and they just travel at randon slowing the exit velocity of the gas

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ideally you want to have it so you have the minimum amount of interaction of the molecules but still have them all following the same path in the most efficent manner.

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LEGEND:
-------- = Exhaust wall
==== = Smooth flowing exhaust gas
@@ = Eddies (turmulence)

[o_man_ra23]

Props to Stivo, OldCorollas and Jeffro... very interesting reads, and good technical theory/basic calculations. Also spoon, at higher RPM, the pulse time is shorter, and as such if you have the same basic exhaust, with the same gas velocity, then you will require shorter runners to be able to time that pulse, whereas for lower RPM torquey motors, you will need longer runners to time the pulse. This is not unlike the theory for radio frequency circuit boards... where you can use the high frequency of the signals to determine the lenght, width and distance between of tracks to make inductors, resistors and capacitors from a cut PCB. In general tho, the higher the frequency, the shorter the wave lenght, and so the shorter the peice of track you need to do the same job. Anyone who has done anything with RF PCBs would be able to make sense of extractor primary/secondary theory quite quickly. Also, the cost of mandrel bent exhausts compared with press bent exausts even with mass manufacture can become quite high, as they take more machine hours to build, and so you cant produce them as quickly. Because time is money, even if the materials are provided at the same cost, the machine labour time will increase, as will electricity usage. a 1c increase per car over 1,000,000 cars is $10,000 less in the companies back pocket, so the managing director misses out on his payrise, and starts firing designers... That is why they are made as cheap as possible.

Cheers, Owen

[oldcorollas]

This is not unlike the theory for radio frequency circuit boards... where you can use the high frequency of the signals to determine the lenght, width and distance between of tracks to make inductors, resistors and capacitors from a cut PCB. In general tho, the higher the frequency, the shorter the wave lenght, and so the shorter the peice of track you need to do the same job. Anyone who has done anything with RF PCBs would be able to make sense of extractor primary/secondary theory quite quickly.

thats exactly it....
the basis of the lengths of the pipes (and to some degree the width) is based on the frequencies at which the pressure pulses (and negative reflected waves) bounce around..

but it is far from simple.

as well as the basic pulse, which is effectively a sound wave!, there is also some effect fromt he inertia and momentum of the gas itself, but this effect can be smaller than the "sonic tuning".

you also MUST consider, the CHANGE in the speed of sound in the exhaust gas with density and temperature! this has some benefits and some negatives from exhaust building, but you MUST consider it when talking about theory of exhaust, sinc eit is predominantly based on the speed of sound, as well as the actual velocity and density of the gas...

[slapper]

With reference to cost would it really matter when this stuff is being mass produced and the price difference between many things when you buy so much is almost nil???- LeeRoy

that's precisely why they do care!
as someone else pointed out, 1c when you're production run is a million sounds like it might be worth the effort. I can assure you that it is actually worth the effort ...

In a past life, I worked in the automotive industry (including time spent in Chicago and Detroit - ooh, give me snow, give me snow ). In one particular instance, the people I worked for copped the thousands of dollars of added engineering costs to replace one chip with a cheaper part - in the order of $2 versus $1.50. Big automotive manufacturers will scrabble madly to save cents.

cheers,
Slapper