Funny this subject would come up today.  I just finished reading this
web
page:
http://www.scuderiaciriani.com/rx7/turbos.html
A quote off this page:
"BTW, I wouldn't recommend anything smaller than .96 exhaust A/R
ratio."
When you read this page please remember that these guys are looking
for big
HP gains but some of the info applies.  It is easy to get the HP we
want
(about 250 hp at about 405 CFM [or 32 Lb/Min] at 6500rpm, about 7 to
8 psi
boost from Paul Yaw's numbers:
http://personal.riverusers.com/~yawpower/ac13bt.html ).  The hard
parts are:

1. Keeping the compressed air cool.

2. Not overspeeding the turbo.

3. And as we climb, keeping the compressed air cool and not
overspeeding the
turbo.

Solutions:
1. By looking at the chosen turbo's map, and staying within the map's
most
efficient range (high percentages) to keep the boost as cool as
possible
then using a big intercooler to cool it further.  Also, bu using the
least
amount of boost needed to the do the job.

2. Airplanes use big exhaust A/R's. I would start with a big A/R 1.3
(looser) and then go smaller (if need be) to bring the Max hp level
down to
the Max rpm.  Too small an A/R and you can overspeed the turbo.

3.  By using your max CFM (about 32 LB/min) draw a vertical line up
on your
map for your chosen turbo.  This will give you and idea of how high
you can
go without overspeeding your turbo by looking at the numbers (pressure
ratio) on the left side of the turbo map.  The highest point of the
line
within the mapped area's will give you the max pressure ratio (this
is the
max boost to keep sea level hp) before you overspeed the turbo or
where you
have to back off the boost, hp, or rpm to stay within the map.

Clear as mud ? : ).  We can not go wrong copying Paul Yaw or we can
try
using the GT 35 (GT 37 will give more Alt and hp) with an exhaust A/R
of 1.3
(starting point) so we do not have to use a waste gate (we limit
boost with
rpm and blow off valve or bigger exhaust A/R).  All turbine wheels in
GT
series are inconel.  Both GT's cost about $1200.  Expensive muffler,
huh!
Ernesto

Hans Conser wrote:

Think of it as a cheap 50% increase in power.

 
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