What will happen to a car with a manual transmission if it rolls the wrong way in gear?

Question

EDIT: The linked question does not address any of the three subquestions listed below. This question is about the mechanical repercussions of a car rolling the wrong way, the linked question is a discussion on parking techniques for manual vehicles. Parking techniques are mentioned here only as motivation for the question.

A car's engine is turned off, and the engine, transmission, and wheels are all connected because the clutch is engaged and the transmission is in gear. The car is forced to roll in a direction opposite of the intended direction of the gearing, for example by being allowed to roll down a hill that is steep enough that gravity overcomes the compression of the engine.

1. How would this affect the engine?
2. How would this affect the transmission?
3. Do modern manual transmissions resist movement in the wrong direction in some way, or is the resistance entirely a function of the engine compression?

This question was motivated by often conflicting advice I hear about parking a vehicle with a manual transmission. Some advice recommends using the first or reverse gear depending on the direction of the incline, some recommends only using the gear with the highest ratio.

It seems to me that selecting a gear depending on the direction of the incline would be done with the safety of the internals of the vehicle in mind in the case of an absolute worst-case scenario, which got me wondering just how much damage rolling in the wrong direction could cause (ignoring the devastation a runaway vehicle rolling down a hill would likely already cause).

Answer 1

The short version: nothing would happen, everything is fine. Turning the wrong way for a short time is not causing any measurable wear or damage. Long version:

1.) How would this affect the engine?

Car in first, rolls backwards. The engine would turn backwards. Every moving part of an engine is typically only loaded in the "forward" direction, if you will. Valves, pistons, crankshaft, camshaft, and any ancillaries driven by direct engine movement (water pump, oil pump, supercharger, etc.) will move in the opposite direction that they normally do. This will not damage them in the short term. Generally, things loaded in the wrong direction don't like it as much as being loaded in the correct direction, but how far you would have to roll is likely unknown.

As Solar Mike mentioned in the comments, there exists an edge case where a sufficiently worn timing chain could jump a tooth.

Running the oil pump backwards is not ideal because they are typically only designed to push oil when run in the forward direction. A sufficiently huge mountain could lead to oil starvation by the bottom, per the spirit of your question.

2.)How would this affect the transmission?

Car in first, rolls backwards. The transmission would turn backwards. Again, every part is used to being loaded in one direction, but none are going to explode turning backwards 1 time. In the worst case (pave Evrest 2020), I don't think it matters which way you go. I don't know of any test or shadetree knowledge which compares "revolutions till failure" of forward vs backwards. I'd assume backwards would fail first just because failure modes in that state are not extensively tested (if at all).

An automatic could maybe run into some issues with fluid linkages, which I believe typically only push fluid one direction, but I don't know enough about this to give you an informed answer.

3.) Do modern manual transmissions resist movement in the wrong direction in some way, or is the resistance entirely a function of the engine compression?

In a highly instrumented lab, I'm sure you might find that it takes 2% more/less torque to turn the transmission in the direction of the gear you are in. Some kind of gear rotating mass mechanical engineering/physics problem. In your garage, you will find that turning the shaft either way while in gear is roughly the same amount of effort

The resistance is not just engine compression, its the entire drivetrain. Nothing moves unless acted on by a force - the engine, transmission, crankshaft, wheels, the dead weight of the rest of the car - All of these things do not want to move.