Transmission mounts and shifting

[Nick Triesch]

I just read that transmission mounts can affect shifting. Could someone please explain how this can happen. I'm asking because I do not understand why. Thanks.

[Curt K]

Nick, this is my 2 cents -

Worn mounts allow the transmission to move. The movement of the shift shaft and clutch when shifting can be enough force to cause the transmission to move within the worn mounts, however slightly, and this small movement may be enough to cause some binding of the shift shaft and hard shifting. I hope this makes sense. Others with more knowledge will chime, I hope.

[proporsche]

Hi Nick..if you trany mount are soft engaging your gear will get harder --too much movement..it meant to be solid...Then again it take a long time for the mounts to go bad...
Ivan

[Nick Triesch]

Wow! Thanks! Did not know.

[chrismorse]

Hi nick,
To keep vibration and noise to an acceptible level in our cars, we have to support the conssiderable mass of the engine and transaxle on rubber - no rigid mounting, or you loose your fillings.
So, with the lump quietly moving about behind us, we also need to be able to quickly, accurately and smoothly work through the shift linkage to get the right gear.
Relative movement between the body(which contains the linkage) and the lump make gear selection increasingly difficult.
Cruising is no problemo - however, when you are really cornering, braking or accelerating hard, there ican be significant relative movement which can make getting the right gear difficult.
If the rubber mounts are worn or loose, this can be a significant shifting problem.l

At thunder hill, I was blasting my old Ferrari 308 around turn three up to the top of the hill, turn four, max braking, a fair amount of right oversteer. The lump was tweaked hard right and forward and despite many kind, forcefull, desperate attempted downshifts, I could not find, or engage second.

After consideration of the center of gravity of the lump and the relatively short fore/ aft spacing of the rubber mounts I installed some less compliant custom polyurethane mounts - problem solved, lump secure, reliable shifting under load.

The trade off, (there are always costs), vibration at idle, particularly in the seat, (which is vastly better than the sphincter puckering when you cannot find a gear you desperately need to catch the oversteer).

Not only would I do the poly again, I bought poly engine/trans mounts for Dads 74 coupe.
Chris

[Nick Triesch]

amazing!

[merlinfe]

I researched this topic for awhile. Here's the solution I came up with and it works well for me. YRMV: Rebel Racing solid Transmission mounts and Porsche Sport (Cabriolet) mounts for the engine cross bar. Much improved shifting feel. Negligible NVH that you get used to as time goes on.

The only downside I could see was in replacing the engine mounts my crossbar snapped its mounting point. I "upgraded" to the Rennline engine crossbar. It mounts differently that the stock '83 crossbar (its more like the long hood crossbar).

Your going to have to play around with this one a bit personally as you will be the final judge on what works for you, of course.

-Steve

[Jack Stands]

WEVO also sells some mounts that are a stiffer polyurethane (but not solid) and seem to help the shifting by minimizing movement. I have them on my 914-6 conversion as the effect is much more pronounced with the longer shift rods compared to a 911.

[Tyson Schmidt]

IMO, the engine mounts affect shifting more then the trans mounts.

The way the mounts are designed, the engine cradle assembly controls twist, and the trans mounts assembly controls the fore-aft movement.

The biggest issue that affects shifting is the gates not lining up with where you expect them. For example, shift while cornering hard, the drivetrain and chassis twist, putting the gate somewhere in between the gate you are attempting to shift into.

A little extra fore-aft movement is hardly noticeable, but when the gates dont line up, well....that's when it becomes possible to select the wrong gear, and potentially mechanically over-rev the motor.