david r simpson wrote:
>
> With no malice towards the orginal author, I a not sure about having high
> temperature increasing the gas velocity. Isn't it true that for the same
> mass flow rate highter temperature will CAUSE higer velocites due to the
> greater VOLUME flowing through the exhaust. With the greater volume you
> will have more back pressure. Less back pressure is what you really want.
> The use of heat wrap is most likely for the protection of other equipment
> mounted near the exhaust?
The exhaust, from port to tailpipe, is a partially-closed system. At the
port, gas pressure is high, and atmospheric at the other hand. In a
completely closed system, the volume is fixed, so increases in
temperature result in increases in pressure. That's the thinking behind
insulating the exhaust system.
In a typical exhaust system, the slug of exhaust gas exiting a cylinder
is steadily cooling, and losing kinetic energy because of drag. While
inside the exhaust system, it behaves a lot like it does in a closed
system--it cools, the volume decreases, and therefore the pressure
required to drive it out of the exhaust system also decreases. Gas
pressure is directly related to gas speed, so the velocity also
decreases.
The slower the velocity, the more likely it is that the next slug of gas
in the engine firing order sees that previous slug of slower-moving gas
as an obstruction to flow, and has to give up kinetic energy to push it
along. As engine speed increases, so do the kinetic energy losses which
translate to increased pumping losses.
This is sort of where all the "power-robbing back pressure" advertising
jargon comes in... increasing flow in a partially-closed system requires
one or both of two things--increase pressure, or reduce drag. If those
wide-open glasspacks reduce drag (some do not, or not much) then the
flow increases. If the effective pressure of the gas slug is raised, the
velocity is raised, and the flow is raised (this is mostly true at the
gas speed at which piston engines operate--optimally, 275-350 ft/sec
velocity--and becomes less true were gas speeds to reach sub- and
trans-sonic rates, around 800-1000 ft/sec).
Wrapping the pipes (or having a flame-sprayed ceramic coating applied)
simply slows the temperature drop of the gas slug, so its overall
pressure is higher, and exit speed through the system is higher.
All this only has to do with gas mechanics--it doesn't address the other
issues of exhaust system design--pressure waves and sonics, which is
where pipe tuning comes into play.
Cheers.
--
My other Triumph runs, but....
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