<< Chuck wrote:
<snip>
First, I

had a trailing plug fail , it broke the insulator ...

Generally, broken insulators are a good indication of detonation.  Do you

have a knock sensor on the engine or some other means of detecting this
condition?  It may be worthwhile to run a little richer under high load
conditions till you can be certain you don't have a problem.

Second, I noticed that on take-off at the low elevation fields, which was
most of them on this trip, the RPM would be at 5200 which is normal,

then it

would drop off after 3 or 4 seconds into the takeoff roll to maybe 4700.

Isn't this an indication of cavitation at start up? Once the plane gets

rolling, the prop is able to get a decent "bite" which loads the engine
down.

Regards,
  Peter Militch Cozy Mark IV #740

Chuck wrote:

No, I have no knock sensor. But it did happen after I ran a tank of 80/87
avgas and could have detonated from the lower octane. I have had no problems
running regular 87 unleaded auto fuel, but had to use avgas on this trip. Is
80/87 a lower octane than autogas? Jon Lauter suggested retarding the timing
on the trailing spark, but I am using Tracy's controller with trailing spark
fixed to fire the same time as leading (not adjustable). I am using stock
factory crank angle sensor timing. I will not use 80/87 anymore, hopefully
that was the cause.

Prop cavitation sounds like as good a theory as any but I wonder why I never
heard of this on a Lycoming installation. I also fly a Lyc. RV-4 with a
fixed pitch cruise prop and it never did this. Interesting!

Chuck Dunlap

Perhaps the geared rotary has a bit more prop torque at 2160 prop RPM  
and can power through the prop stall to reach 2400 prop RPM static.
>From the looks of your long tuned intake manifold that could be
quite possible.

On the other hand it sounds to me like your engine is breathing limited and 
the HP is near flat from 4700 RPM up. What is max RPM that you see in the air 
straight and level? 

With a free breathing  rotary and plenty of fuel HP would be steeply 
increasing in a more or less straight line from 4700 on up. Have you 
changed to the new throttle body?

Perhaps the mixture is a bit too lean for max HP.
Do you have a mixture monitor on there with an O2 sensor?
What is the mixture doing at 4700 and 5200 RPM? 

EGT's?

Paul Lamar
 
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