Exhaust system stuff

[Marc Swanson]

> There's been some conversation regarding best exhaust system diameter for
> our turbo cars and I'll throw in what I've found.
>
> Although Corky Bell does say that the best exhaust for a turbo is no
> exhaust,


I corrected myself on that.  He does NOT state that point.  He instead states
that the best turbo exhaust is the LEAST exhaust possible.  Subtle difference.

> that doesn't suggest that the largest diameter exhaust system is
> therefore the best.  Freestream atmospheric pressure at turbo exit is one
> thing... a huge sewer pipe exhaust is quite another.  An exhaust pipe is an
> enclosed column of hot gas that is rapidly cooling, being pushed from the
> hot end of the pipe and has only one direction to go.  Given the wall
> friction (drag) and the gas that is becoming more dense (due to cooling) as
> it goes down the tube, the situation is not ideal.  Making the diameter of
> the system larger makes the problem worse until the diameter gets so large
> as to simulate freestream atmospheric volumes and that would likely have to
> be larger than 10" in diameter.


I was wondering the physics behind why bigger is not better.  He explains his
reasoning using exhaust gas velocity but the change in density after cooling
would seem to make sense.


> Bell states in his exhaust system chapter on page 132, " 'The larger the
> better' is not the case."  He also includes a table, 11-4 on page 133,
> where he shows the recommended exhaust system diameter for a given
> horsepower.  He shows that a 3" exhaust system is ideal for the exhaust
> flow of a 700hp engine.

yep, he shows a nice table calculating the ideal sized pipe for single and
dual exhuast systems.  For us "lowly" 300hp seekers 2.5" is supposed to be
just right.  Not what conventional wisdom has been but I'm beginning to buy
the physics behind it.


> Not since the factory raced these cars has that
> number been mentioned... and they essentially didn't have an exhaust
> system.
>
> My 2 1/2" system is good for 450hp so there is little danger that I'll out
> strip it's capability anytime soon.
>
> Yeah, several folks in our community run 3" exhaust systems and they swear
> by them... but no one has yet brought to the list dynoed differences
> between a 3" and 21/2" system.


True.  Haven't heard much comparos between 2.5 and 3" setups.  Gary
Kaklikian's report about his switch from the 4kq Scorpion exhaust system
(roughly 2 1/4") to a custom 2.5" setup gave him an extra 2psi.....

here is his initial report indicating more boost with the new exhaust:

http://www.audifans.com/cgi-bin/lwgate/QUATTRO/archives/quattro.000112/Subject/article-34.html


Later


-Marc-
87 4ktq
88 90q