François Badoux wrote:

Hi Paul,



Some news and data from Mistral which, I hope, will be of interest:

snips.....

BTW Francois I am curious about your stock turbo experience in the Turbo
Arrow.
How many hours on it since it was overhauled?
What manifold pressure are you limiting it to?

Paul Lamar

François Badoux wrote:

Hi Paul,

We have 20-21 hours on the Arrow's overhauled turbo, so far so good.  We
limit MAP to 39'' on the Arrow, but we routinely go to 45'' on

the bench.

At Geneva's elevation of 1,400', we measured maximum turbo RPM

at 105,000

RPM with MAP at 44-45'' and the engine giving 230 BHP.

Having said this, we totally agree with your comment in the thread "Info
about stock turbo", it is inadequate for aircraft operations and it will
overspeed at altitude.  IMHO, the worst inadequacy is not on the turbine
side but on the compressor side:  too small an impeller for altitude
operations.  It also results in a compressor outlet temperature that is
abnormally high... probably a problem of low efficiency.  But,

you know, I

am no turbo specialist (...and sometimes I think it's a bit of a black
art... hence the name "Black Magic" for some automatic

wastegate, don't you

think?), I only know what our experience is.

Too bad, it is a neat, compact turbo installation, but we are

changing it.

Best regards,

Francois

Thank you Francois. That is the best information we have on the
stock turbo
so far. Now what is that rule of thumb on turbine RPM? Here it is in
the Macinnis book on page 105. 2% increase in RPM per 1000 feet. 20%
faster RPM for 11,400 feet or 126,000 RPM. That combined with a couple
hundred degrees hotter exhaust ought to do the job on the turbine.

230 HP at .5 BSFC is 115 pounds of fuel per hour or 1.9 pounds of fuel
per minute. At 14.7 to one stoechemetric (s?) that is 28 pounds of air
per minute. My guess is the pressure ratio is about 1.2 to 1.4.

Looks like we are well off in the right regions of the T3 "60"
trim compressor map at 1400 feet. The super 60 trim is about the best
we can do in the T3 size but not good enough. So it is going to take a
TO-4 compressor wheel. That is possible and is known as a hybrid in
Turbonetics speak.

A larger compressor wheel should slow the RPM and put the stock turbo
in a more efficient place on the compressor map. My guess is a TO-4
"40" trim if that is mechanically possible.  Perhaps it is also possible
by souping up the stock compressor housing.

I will attempt to measure the A/R ratio of the stock turbine
housing to get a better feel for just what this turbo is equivalent
to in Garret turbos.


Paul Lamar

Paul, That is interesting.  If you are interested in simply upgrading the
stock turbo the RX7store has several bolt-on options.  In fact, that is
really what I should be doing since it will be very hard to get anything
else to fit.  I would really like to open up that exhaust a bit.  Even
without a turbine wheel and a wide open waste-gate I am making far less h.p.
at a given M.P. than are Tracy and Rusty and Ed.  I think I could solve most
of that by putting on a bigger turbine housing.

Dave Leonard

I think it is a good approach if your turbo has not already failed.
Since you are going to spend big bucks to have your turbo overhauled
you need to think it over. It cost Mistral $650 in Florida as I recall.

Paul Lamar

Naah! I did mine for less than $400 in Ft Lauderdale .
Bulent

 
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