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Hey guys, I thought I would put my own post in to start this section off which should hopefully give insight and answer a lot of questions. I shamelessley copied this from Steve Panasuk off the Talon Digest - its not worth saying again if its already written well.
Turbocharging Physics
1)Dynamic power and torque range of the engine. A street-driven gasoline
engine like Eagle talon , that can rev to 7500 rpm almost certainly
requires a different turbo than a nearly constant-speed aircraft piston
engine that never sees over 2750 rpm (or for that matter, an auto-tran v-8
pickup truck)-even if they have identical maximum power. To increase
compressor flow range for engines with high dynamic range, it is common
practice to use compressior designs with a broad flow range-which tend to
have reduced peak efficiency-meaning the compressor flows somewhat less and
heats the air more at peak efficiency. And even with broad range turbos,
car companies and hot rodders usually have to decide between turbos
optimized for peak power versus turbos optimized for low-end torque. For
the broadest range, some of the newest turbo cars like the RX-7 route all
exhaust through one turbine at low rpm for radically fast turbo spool-up
and high torque at low engine speed, opening up a second "sequential"
turbo at consequent higher rpm for increased flow and reduced exhaust back
pressure. BEGI turbo cars frequently utilize Variable Area Turbine Nozzle
(VATN) turbos from Aerodyne which use variable geometry vanes in the
turbine instead of a fixed-size nozzle in order to focus exhuaust against
the turbine blades, providing low rpm boost (typically far more than even
a supercharger) yet opening up the vanes tho provide low restriction for
high top-end power.
2)Level of maximum boost. How much thermal and mechanical loading can the
engine stand? Do restrictions in the engine intake and exhaust sysem,
design or turbo compressor flow capacity limit maximum boost such that the
turbo just can't make more than a fixed amount of boost? Is the turbo
using a wastegate or pop-off valve to artificially limit boost (which can
potentially be increased by us willing to run a little closer to the hairy
edge in return for more power). What is the ratio of boost pressure in the
engine inlet ot exhaust pressure in the header upstream of the turbo (this
ratio explains why inlet boost pressure is only part of the story and why
all levels of boost are not equivalent)? Does the fuel system of the car
have the surplus capacity to provive sufficient fuel for increased levels
of boost, even if the engine can stand it? What is the effective
compression ratio at max boost, and what octane fuel is available?
3)Compressor inducer size and configuration (how big is the inlet "fan" on
the compressor; how much can it flow). Combined with the need to avoid
operating far above the compressor's peak effciency "island" or even
overspeeding, inducer size provides an upper practical limit to compressor
flow.
4)Compressor exducer size and configuration (how fast and hard does the
turbo "throw" the compressed air as it leaves the compressor housing).
Bigger diameter exducers tend to provide fast turbo response, but the need
to avoid a high moment of inertia (more rotating weight located further
outboard from the compressor shaft), coupled with minimum compressor
blade strenghth requirements, is a limiting factor.
Note:5 and 6 determine the output of the supercharging ection of the turbo.
5)Turbine inducer size. A small wheel turns faster for a given blast of
exhaust, but it has less torque to push the compressor to really hhigh
levels of boost.
6)Turbine exducer size. A bigger exducer is less restrictive and less
reponsive. Bi turbines operating at peak power can produce "crossover"
conditions, in which tremendous power and efficiency is produced as the
intake pressure exceed exhaust pressure upstream of the turbine.
7)Turbine Housing A/R ratio (nozzle Area versus leverage or Radius from
shaft centerline). A turbo's ability to achieve maximum boost may be
limited by the possibility that, at lower rpm, there is not enough
exhaust gas power to spin the turbo fast enough to achive maximum boost. A
small A/R increases exhaust pressure against the turbine for rapid builup
of low rpm boost. But at high rpm, high loading, exhaust backpressure
upstream of the turbo limits power by forcing the engine to work
increasingly hard to push exhaust gasses from the cylinder. A big A/R
provides less back pressure and therefore higher top power.
8)Mass and radius of the turbine and compressor wheels, which determines
the rotation inerta and therefore, acceleration capability-I.e.,
responsiveness. The moment increases as the square of the radius. Small,
light wheels are easily accelerated-which is why two or three turbos may be
more reponsive than a single big turbo. On the other hand, small turbos
may have further to accelerate to get from turbo cruise rpm to full boost
rpm, requiring higher rpm to achieve a given level of flow.
9)Engine exhaust flow energy capability. Low heat exhaust energy from
fthe engine, high heat loss in plumbing from engine to turbine, and
inefficient exhaust headers can negate the potential of a bigger turbine
for high-end horsepower, producing both poor horsepower and turbo lag.
Long headers or pipes that bleed off heat can hurt turbine performance. A
fast, hard run that really heats up the turbine can produce faster
response on the next run.
10)Exhaust system restriction downstream of turbo. Anything that restricts
flow out of the turbine (narrow exhaust, too-small catalysts or mufflers)
will hurt responsiveness and ultimate flow.
MuRiX
97 Eclipse GS HRC Stage II
And a whole lot of other mods...
89 Accord LSi - yes it's mine 
http://murix.home.icq.com/index.html
05 Mazda RX-8
06 Lotus Elise
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