Paul,
I can't remember what the A/R ratio stands for, I think it is the ratio
of the inlet tube area, to the turbo area.
Can you clarify that point for those of us that don't quite understand.
George (down under)


"* Turbine A/R - Turbine performance [HP] is greatly affected by
changing the A/R of the housing.  Using a smaller A/R will increase the
exhaust gas velocity into the turbine wheel, causing the wheel to spin
faster at lower engine RPMs giving a quicker boost rise. This will also
tend to increase exhaust back pressure and reduce the max power at high
RPM. Conversely, using a larger A/R will lower exhaust gas velocity, and
delay boost rise, but the lower back pressure will give better high RPM
power."

Somebody on the Internet :)
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Basically what you have for a given size turbo charger is a fixed R or
radius. Changing A (area) changes the velocity of the exhaust gases
impinging on the turbine blades. Of course this puts back pressure on
the engine and the harder it must work to  keep up the flow.

Heating the air increases its pressure and therefore its velocity with a
given pipe size. That is why it is important to retain the heat in the
exhaust gas before feeding it to the turbine. Exhaust gas can approach
super sonic velocities depending the size of the exhaust pipe. The
combination of heat and high exhaust gas velocities add up to tremendous
potential energy in the exhaust. Close to 250 HP in a 200 mechanical net
HP rotary engine.

In the case of the rotary we have too much heat so we must be careful
not to run the turbine too fast as the blades are weakened by the high
heat and might fly off. Centrifugal force on the blade = M x V^2/R... M
is the mass of the blade.

The force imparted to the turbine blades per square foot  by the exhaust
gas is V^2 x .0026. The .0026 constant is a function of the gas density.
V of course is the velocity in MPH. V^2 means V squared.

Think of the scroll as a fire hose nozzle. The smaller the area of the
hole in the nozzle the faster the water comes out. If that nozzle is
pointed at a water wheel the faster the water hitting the wheel the
faster the wheel will rotate up to a point. The faster the wheel rotates
at a given torque the higher the HP. All turbines have peak torque when
they are not rotating.

Without rotation there is no HP generated. That is when the velocity of
the gas striking the blade is a maximum. As the turbine speeds up the
relative velocity of the gas striking the turbine blade is reduced until
the blade is moving at the speed of the gas. In that case there is no
torque so the HP becomes zero again. See the attached chart.


Paul Lamar ...No rotor no motor.

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"VNT technology ? variable nozzle technology ? will be as beneficial to
gasoline as diesel. The hot side of the turbocharger, the turbine, is
where we try to obstruct [extract] energy out of the hot exhaust flow.
At low engine speeds, where there is relatively little flow, little
temperature, we locally accelerate the flow and there is better kinetic
transfer of kinetic energy from the gas to the wheel. I like to compare
it to a garden hose: if you have the hose further away, you squeeze the
hose. VNT does miracles; it brings available torque down to almost idle
speed. In the old days, you had to rev up your engine ? gas or diesel ?
up to 2,000 rpm. Today you see full boost at about 1,500 rpm."

By Garrett?s Martin Verschoor

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Good article on understanding turbo charts.


Paul Lamar ...No rotor no motor.

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This is all great stuff... Turbos and how to select the right one has
always been a mystery to me... This Helps a lot.  I do have a couple of
questions though:

1.  When calculating the Pressure Ratio shouldn't we use Boost psi
divided by the atmospheric pressure you will see at cruise Altitude?
Same when calculating Rankin Temp?  I guess the calculations should be
made at SL and various altitudes to the best overall performance.

2.  What is the Volumetric Efficiency "VE" for a 13B?

3.  What is the CID for the 13B?

4.  When they talk about Absolute pressure "P" in the calculation for
"N". What do they mean?

Mike C.


1. yes.

2. A good side port rotary will give close to 100%. There are no intake
valves to get in the way. Despite the valves, an intake manifold tuned
four valve piston engine, will come close to 100%.  A tuned p-port
rotary, over a range of several thousand RPM, will be about 120%. Free
supercharging :)

3. A perfect vacuum is zero pressure as in outer space. Atmospheric
pressure at sea level is about 15 psi absolute. 15 psi boost is 30 psi
absolute.

Paul Lamar ...No rotor no motor.

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So if P is Atmospheric psi plus boost psi... Then does that mean that
when they describe "T" as absolute ambient Temperature in Rankin.  Would
it be the Inlet temp, in Rankin, as the air enters the Rotor after
compression? Which we would always limit to 140 deg F max... Right?

Mike C.

All correct. However the sensors read out in F.

BTW a PicAXE electric or electronic waste gate control is
the way to go as you can use several tidbits of information to open
the gate. Intake air temp., manifold pressure and data from the detonation
sensor although that is after the fact.

If you want to get really trick you can use a model airplane servo
and manifold pressure sensor to closely control the manifold pressure
over a wide range.