Chrissi said:

We have narrowed the turbo selection down to the Garrett GT35 84

trim 0.94

A/R and GT37 82mm, 52 trim, 0.54 A/R or GT37-TO4 housing 76mm, 52

trim, 0.70

A/R as being the closest to the TO4e-50-P trim.

The drawing of the GT35 is it is a bolt-on with a TO4 manifold

footprint.

The GT37 with TO4 housing is ambiguous as to whether the housing is

the

turbine or compressor section therefore the footprint is in

question.

I think the GT37 units are much closer to getting centered on the

turbine

maps based on what we are guessing so far about performance (Cliff

Manzke's

numbers)

The GT series is Garretts answer to the new performance market. They

are all

ball bearing. Not sure if all turbine wheels in series are inconel

or if that

is option.

My thoughts are this, although Greg's "no wastegate" approach worked

for him,

if instead you go the wastegate route and use an adjustable boost

controller,

although more expensive and complex to install, it adds an extra

margine of

safety to the system and at critical times may be one less thing to

monitor

and control. Going without means settling for "best guess" and I'm

just not

one who is comfortable with that ambiguity.
Any thoughts, ideas, criticisms or advice?
...Chrissi

-Rotary RX-6 wrote:

Chrissi, I thought that you were after an A/R ratio in excess of 1.0.

Isn't the A/R

ratio in most sports cars far less then 1.0 for acceleration purposes?

Aviation

turbos I believe are around 1.3. Please correct me if im wrong. but the

smaller the

ratio, the faster it will accelerate to increase velocity not volume.

????

This is such a great forum.

Disclaimer: I'm not an expert.

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

 
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