Why chase screams

[mlp]

Yes, I know.  I promised, no more scr***s, but its cold and snowing outside
the barn today and daydreaming on the list seems to have been a little slow
lately,  so I'll use a thinly disguised pseudonymous  analogy -- Why chase
screams?  You know "screams."  As in "On my god, what have I been missing
out on, why didn't I think of or do that sooner ..... yeehaaahhhh etc.,
etc."  Well maybe not that great a thrill, but still an improvement?  Maybe?

FWEIMBW,MOO (that's the "For What Ever It May Be Worth from the Mouths of
Others"), quote:
".....Okay, we've established that a reduction in backpressure means an
increase in power, but how do the normal upgrades reduce backpressure? It is
very easy to see how exhaust upgrades reduce backpressure. What is not so
clear is how an intake or an intercooler could have the same effect. The key
to understanding here is to understand how the turbos translate flow
restriction in the intake system into backpressure. Turbos create an
increase in pressure between their compressor intake and output. They create
a corresponding increase in backpressure on the exhaust side. These two
pressure differences are related. If the turbo needs to create a pressure
increase of 14 psi on the intake side, it will increase the backpressure on
the exhaust side by some multiple of that number. It may seem reasonable to
assume that 14 psi of boost pressure will produce 14 psi of backpressure,
but the different geometries of the turbo compressor and turbine sections
(and other factors) make this not the case. Corky Bell suggests that
backpressure on a properly designed turbo system should be limited to about
2.5 times the boost pressure. I believe this recommendation includes the
backpressure caused by the basic exhaust system, but it still suggests a
multiple greater than one for the backpressure caused directly by boost
pressure......"  
Some one else's words not mine, but I thought the really interesting part
was the ...... "It may seem reasonable to assume that 14 psi of boost
pressure will produce 14 psi of backpressure, but the different geometries
of the turbo compressor and turbine sections (and other factors) MAKE THIS
NOT the case."  Emphasis added.
And for the math challenged, i.e. that would of course be me, but hey I'm
twice as good with my shoes & socks off, you can take a look at
http://www.maxcooper.com/rx7/how-to/fuel_system/why.html .  Scroll down to
the bottom of the page and you can play with the model there.  PS I know
nothing about the assumptions made about the calculations run by the java,
or whatever it is, script that does the modeling work but the I at least
find the thinking interesting, and probably not just applicable to cars and
modifications etc. constructed in Colorado barns above tree line, or 9,000
vertical feet, which ever comes first.  
Mike P.



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