Lately I have seen a lot of turbo inducer sizes quoted on the forums,
the size of the compressor inducer will not tell you the performance
capabilities of the turbocharger.
To do this you will require more information on the compressor wheel
and the compressor housing. There are a number of factors in compressor
wheel design that can dramatically change the performance.
Compressor maps are the best way are the best way to determine a wheels
performance capabilities. You must also keep in mind that the map is
obtained using a specific compressor housing. Using a different housing
will change the map. You should look for a map that best meets your
performance goals and also meets the engines surge requirements. A good
design will do this and still protect your turbo from off throttle
surge or bark that will reduce the life of the turbo, (the BD Super B
is a good example)
The wheel trim (difference between inlet to outlet size) a larger
diameter wheel rotating at the same speed will produce a higher air
velocity that converts to higher pressure.
Also the length of the inducer veins can be different a larger shaft
diameter would require a larger compressor hub diameter so inducer vein
length would be shorter. (I have seen examples of 20lbs/min
differential with the same size inducer)
New casting methods and hardening processes allows manufacturers to
design thinner veined and taller compressor wheel designs without
sacrificing strength. This also makes extended tip designs possible on
high speed compressor wheels. In the past this design was used only on
very large slower speed turbochargers.
Keep in mind that if you have two wheels with the same inducer size
that perform at the same efficiency but one has a lager overall
diameter. Borg-Warner’s extended tip is a good example as it increases
diameter without changing much of the total mass, at the same rotating
speed the larger diameter wheel will produce a higher air velocity and
static pressure in the engine inlet plenum. This will result in more
air into the engine when the valves are opened.
And then there is the turbine.
Twin or compounding turbochargers.
There is a misconception that bigger is always better when it comes to
the turbochargers used for the first stage of compounding
turbochargers, this is not always true. The rule of thumb in
compounding is that the inducer size of the second stage turbo should
be the same size as the first stage outlet. If this is not done you
will risk over speeding the first stage. Say the first stage has an
outlet size of 3.1 ins as on the S400 and you are using a HX35 as the
second stage that has an inducer size of about 2.2 ins, the match will
not be very good. Yes you will produce boost in fact the first stage
pressure will be high( 35-40lbs) and the temperature will be also be
high. The reason this happens is a compressor wheel is an open rotating
compressor and not positive like a piston compressor. When the air
encounters a smaller diameter it will not compress to that size.
Instead the flow becomes restricted, this reduction in flow reduces
compressor wheel load and the turbocharger speeds up producing a higher
boost at a lower flow. The second stage wastegate now has a problem
controlling first stage air velocity and increased boost. Usually this
is means the wastegate has to be opened almost fully instead of
controlling overall boost levels. If the match was correct the overall
boost level could be controlled with the second stage wastegate. The
end result is running any turbo with the wastegate open all the time is
very inefficient. A wastegate is designed to control the max boost
level not to run open all the time.
When we built the BD compounding system we built so that it could be
controlled this way. The only other factor that will increase boost is
too much fueling. We developed our own wastgate that can be adjusted to
65lbs this has worked very well on 98-02 trucks. Our own truck produces
625HP and has run the quarter mile in 12,2 seconds and has been a daily
driver for over 2 years. People who have made the change from twins
using a larger first stage are surprised at the increased performance.
They also thought bigger was better
