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

A Detroit Diesel Series 60 powered truck on a chassis dyno will have a turbo
out to the intercooler temp up to 350*F. At 100* OAT, that is 250*
differential. Cooling helps.

Dale Davies




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