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
 
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