What Motor and Transmission mounts to use, What motor and transmission mounts to use Options

[Ken Mikos]

Replacing motor and transmission mounts in a original 1973 1.7. Any suggestion on the type and where to purchase? Thanks,

Ken

[Dave_Darling]

For a track-only car? Solid all the way around. Keeps the drivetrain more positively located.

May cause loosening issues with some fasteners on the engine; I seem to recall some kind of issue with Webers being blamed on solid mounts?

--DD

[sithot]

I seem to recall some kind of issue with Webers being blamed on solid mounts?

--DD

Richard Parr does not recommend them for use with PMO's.
5. Solid Motor and Transmission Mounts

I've noticed a proliferation of advertisements for solid motor-transmission mounts for Porsche 911 race cars. I'm sure they sell well because they are CNC machined out of solid aluminum and they look "cool". They are also considered "hardcore". My opinion is that any practical advantage they possess is vastly outweighed by a myriad of disadvantages.

The basic problem with solid mounts in internal combustion reciprocating engines is that they introduce destructive harmonics into the body of the race car and also back into the engine. Everyone has heard the true stories of cracked sheet metal, loosened nuts and bolts, sheared-off exhaust systems and broken engine cases.

The excessive vibration often sticks the float needle valves in carburetors causing flooding and other times aerates the fuel in the float bowls causing a lean condition.

The intense vibration also can knock out components in EFI systems leading to the common full-lean-at-full-tilt-equals-fried-engine syndrome. The additional tooth- rattling vibration and noise increases driver fatigue and the possibility of a crash. And if there is a hard crash, the solid mounts increase the likelihood of catastrophic damage to the engine and transmission.

Given these well known facts, why would anyone use solid mounts? It is because they stop the engine-transaxle unit from twisting, thereby improving shifting. This condition is substantially improved by using the Porsche sport mounts and is totally cured by using a WEVO shifter along with the sport mounts. Frank Eibell in Florida has made and installed anti-torque bars on the transaxle to cure this problem. Jim Patrick in Arizona makes urethane dampened 914-6 conversion mounts. WEVO urethane engine-transmission mounts are available now.

An interesting theory is that solid mounts actually decrease horsepower by forcing the engine to absorb vibrations that would normally be absorbed by cushion mounts. For every power pulse in an internal combustion reciprocating engine there is a reactive pulse responding to inertia, operating in a counterrotational plane. Containing this reactive pulse with solid mounts diminishes the power pulse by adding secondary vibrations. Greg Edmunds utilized this theory in designing motor mounts for his .4cc x 3.75 HP model airplane engines. By trial and error, Greg was able to tune out the unwanted vibrations by adjusting the durometer of the rubber in his composite motor mounts. His engines put out more horsepower and the airplanes flew faster with the cushion mounts! Full size aircraft never have solid motor mounts


As a footnote, not even the viton valves will hold the fuel level in some cases where solid motor and transmission mounts are used. For race cars, we recommend the Clubsport mounts which can be stiffened by modification. WEVO recently developed urethane motor mounts which should be the best of both worlds.