Joseph Berki wrote:

What are the limitations for mounting the 13B using the

side locations

and the top of the intermediate housing like Powersport does?  They
seem to be the only ones doing this.  This would open up the area
between the firewall and front of the motor (pusher app).

Joe Berki

It works fine but like all such schemes it puts the motor
mount tubes in conflict with the exhaust system (particularly
the turbo) and some
times the water pump plumbing. It is possible to get a
conventional exhaust system in there but the extremely hot
exhaust pipes come very
close to the motor mount tubes and the rubber bushings. Also
the lower rubber
bushings have very high loads on them so they are likely to
bottom out under
the 800 ft pound torque pulse and provide less than ideal
vibration isolation.

This is the reason the massive stock Mazda RX7 second gen
rubber mounts
have over an inch and a half of maximum travel. At full
throttle in low gear the stock rubber bushings are seeing
even higher torque pulses than we are seeing with a 2:1 PSRU.
The first gen RX7's had them wider apart at the front of the
engine so they saw less static load and lower torque pulses
so the bushings were smaller.

These are some of the reasons the Questair scheme puts the
rubber mounts closer to the engine/gear-box/prop CG
combination which happens to be out near the motor plate.
Moving them laterally outboard reduces the torque load and
provides plenty of room for the exhaust/turbo system.

Moving them outboard is not an option with a pusher
unfortunately as the tail needs to be highly pointed. This
can be compensated for, vibration wise, by using larger and
softer rubber bushings. This means the rubber bushing will
still have linear travel left over to absorb the torque pulse
after it deflects under the static loads.  Make a free body
diagram for both systems and compare.

Paul Lamar

I keep seeing reference made to 800 ft lbs torque pulses. Would not some
or most of this be absorbed by the mass of the motor/psru?

Chuck Dunlap

I don't know about most of it but some for sure.

Paul Lamar
 
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