Hey, Kevin, sorry to be slow but you asked for thoughts about retard
 timing for the turbo, and I want to do some more investigating (I care,
 because I'm planning a turbo, too), but I do have a couple thoughts for
 consideration.
     The engine has no clue how the manifold came to have the air
 pressure it has, and doesn't care (with caveats).  So, I agree that
 maximum boost should probably be set at 8 psi, but I think the timing
 retard may bear thinking about.  What mainly risks detonation on an
 engine (assuming no stupid levels of boost) are hot intake charge,
 excessive leaning under load, and excessive cylinder pressure when under
 high load/low RPM conditions.  They are interrelated.
    My gut reaction is aircraft are lucky in not having a lot of the
 problems autos face, and for which autos use huge amounts of retard.  We
 don't have to accelerate from low engine RPM at high load, and we often
 soon escape the hot ground level air temps.  A rotary in particular will
 almost always be at what an auto would consider very high RPM (generally
 calling for larger advance, not less) when it is under load.  And an
 aircraft can be manually set on the rich side when at greatest risk
 (take off), which helps cool the charge and has a slower burn rate,
 again helping to avoid the need for extreme retarding.  I'm assuming no
 one would install a turbo without an intercooler, and of course at
 altitude aircraft have a cooler environment for both initial intake
 charge and intercooler cooling.
     I guess bottom line it seems like a bit of retard is likely wise,
 particularly at high load/low altitude like take off, but probably more
 critical is keeping the boost limited during those conditions so the
 manifold pressure never gets too far above what a great breathing n.a.
 motor might see at sea level (and you hear no one calling for retard
 there).  Remember retarding carries its own problems, primarily that if
 the charge doesn't have time to fully burn, you're going to get a lot of
 extra heat in the engine and especially in the turbo/exhaust as it
 continues burning on the way out.  Seems to me that a retard factor
 should be applied, but perhaps only when the manifold pressure and/or
 temperature is elevated above what a n.a. motor might see.  Well, and
 maybe even at cruise if you have altitude compensated the manifold
 pressure (so it thinks it's back at ground level) but are wanting to
 lean it (some retard should be okay there, because a lean mixture will
 burn much more quickly anyway, but since the RPM is high and the load a
 bit lighter, I still wouldn't think too much).  Even where some retard
 is called for, because of the generally high RPM of the rotary, and
 quick-acting, non-valve actuated exhaust system, maybe not retard on as
 steep of a curve or much total as on a auto.  Just thinking out loud.
 Greg


Greg;

Thanks. I agree with you on most points- I have read where boosted engines
are retarded at high boost to counter detonation, with recommendations for
boost at 8 psi to have timing at 4 deg BTC. Megasquirt code for the turbo
rotary has a table with just such a setting.
I agree that aircraft engines operate in a different environment than cars,
but I would consider it a more severe environment. Yesterday in South
Florida at X51 the reported temp was 92, with our C172 seeing over 100 at
the end of runway 10 before takeoff. We used full power for takeoff and
held this setting until 2000 feet. Our density altitude was 1700 feet
making climbing a bit slow. We were doing practice ILS approaches at KTMB
with touch and goes, go around and do it again kind of scenario. The tower
required max rate of climb until 2000 feet for each takeoff. At max power
and full rich our CHTs were approaching 400 deg, and our CHTs were high as
well. The point is that for the first critical minutes of flight, we are
WOT, rich, hot, and running as hard as we can. It is only later than the
air cools and we get to pull back to cruise. As far as the intercooler, I
have seen many GA planes that dont have them because they are used for
normalizing not extra power, and don't kick in until higher and cooler air.
For my application I will incorporate an intercooler, but I dont expect it
to be of much use at the end of a 100 degree runway. The efficiency of any
cooler is dependent on the differential temp of the air outside and the air
inside, as well as the density of the cooling medium. Neither is optimum
here at the moment.

Contrast this to a car- a race car sees brutal conditions for 8 seconds
max. An automobile sees max power and boost for a few seconds to take off
from a stop light or while passing, again for a few seconds.

Like you I will keep doing more research, but I have concerns about it.
Depending on what I discover, I will likely tie my boost (i.e. wastegate
control) to an adjustment related to a knock sensor or intake temps, with a
manual adjustment. If I use an electronic automatic wastegate, the control
will be electronically adjusted, manually it could be adjusting a tension
via a vernier and cable like most planes have as a mixture control.  At
worst I could reduce the boost by a couple of psi and protect the engine at
the cost of a couple hundred feet of runway.

Kevin Alderman

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