Re: The Rapp on exhaust

[PDQSHIP@aol.com]

In a message dated 95-10-24 08:20:22 EDT, you write:

>
>  Our messages are getting out of sync. Please be patient! I appreciate you
>  sharing your expertise and real-world experince more than you know!
Didn't think we were out of synch......  Got all your messages.......
>  Actually, if you suck through a turbo you have the same effect, it's the
>  pressure differential before and after the turbo that creates the flow,
Hmmmmm.......  Not sure turbo theory agrees with you here, yes high pressure
backpressure will tend to "stall" the fan, but it's the high pressure at the
fan that creates flow, the low pressure is the low pressure at the wing of
the fan that affects the speed of the flow.......  To say there is a sucking
effect is not exactly right, if you put vaccuum on the back side of the
turbo, it would not operationally spin necessarily, its before the turbo that
makes the big difference....  You need to get that vaccuum idea out of your
head glen..........
>  so lowering the pressure after the turbo to create a "sucking" effect
>  would increase the pressure differential and increase efficiency and
>  boost level.
Not to the point that you think glen, that theory just doesn't apply with
atmospheric pressures.....  And if you look at the efficiency of a turbo
glen, it is given to you based on turbine spin and air output (volume and
density), not pressure differential
>In an ideal world you would want the exhaust gas from the
>  turbo to exit into a perfect vacuum.
Agreeed, but you don't have a perfect vacuum, and you can run the turbo open
to air (which in theory would be the best flow after the turbine you could
have real world) and your performance gains are not as great as you'd
think.....  You will get hi end flow maybe, but you will lose all sorts of
performance before 5krpm, you will have a slightly faster spinning turbo, yes
but it will also choke the engine at low rpm's.........   In fact, you are
better off chasing the downpipe to the end and then paying great attention to
the exhaust right before it enters the cat, cuz that is where the turbo is
tuned to the performance extent......  Sooooo, you want to wrap that
downpipe, or better yet make it biggr and wrap it to get the gains you are
after.....  the downpipe is by definition a header glen, run a car with no
headers or open headers, how much gain do you get?, the turbo theory is no
different......  Think of the turbo as an engine not a turbine........  You
are thinking way too much on the pressure differential theory glen, I know,
cuz I have looked at what happens in every part of that turbo, and
differential pressure is not even close to being significant for our
applications............
Your airplane wing analogy is a good
>  one, but, it doesn't matter if the wing is moving through the air, or if
>  the air is moving over a stationary wing.
And.......
>
>  I guess what I cannot get through my thick head:  WHY is maximizing flow
>  velocity after the turbo a good thing?

Cuz backpressure on the turbo is not good, so you want the highest velocity
of flow after the turbine, that comes with heat......  If you have gasses
entering the turbine at 100mph, unltimately you would want to flow them away
from the turbine at the same speed......   However you LOSE that velocity by
two things, the energy to move the turbine blades and the energy lost in
heat......  So retain the heat and you've simply reduced the velocity lost
...........  If you cooled the air after the turbo you get density not
velocity, and increased backpressure on the turbine.........


HTH

Scott