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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).

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    Leaving manual transmissions in gear with the engine off is a mug's game in any case. Get in the car, turn the starter, then get out of the car again and see what you just hit. It works even better if the engine fires up on the first compression stroke.
    – alephzero
    Commented Jun 20, 2019 at 20:02
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    When parked, I always leave the car in gear. If it's on a slope, I also set the parking brake and turn the wheels into the curb. Been driving manual-shift vehicles since 1964, and — at least so far, only 55 years — I have escaped all of the above horrors. YMMV.
    – DavidRecallsMonica
    Commented Jun 20, 2019 at 20:27
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    Oh, btw, on an older car with a worn timing chain, rotating it in the wrong direction can cause the chain to jump a tooth affecting the timing or worse.
    – Solar Mike
    Commented Jun 20, 2019 at 20:28
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    I think a big factor would be how fast the engine was spun backwards. If it was very slow, probably nothing would happen. If it was pretty fast, maybe 30+ rpm, you would build pressure in the intake, and potentially blow out filters etc. Fun fact, some rare cars have 2-stroke engines, which run equally well forward as backwards, and you can pop the clutch in reverse and drive as though you have five reverse gears and one forward gear.
    – the_storyteller
    Commented Jun 20, 2019 at 21:56
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    I'd like to see the pitch of this hill that could cause a car in gear to roll from a standstill. I think I'd rather find a better place to park if it's that steep. Parallel parking definitely a Double Black Diamond in this scenario. In any case, like @David - I'd sooner put my faith in turning and curbing the front wheels properly, than obsess over which direction of gear is more appropriate. I find this a bit of a silly hypotetical, and a duplication of mechanics.stackexchange.com/questions/39007/…
    – SteveRacer
    Commented Jun 21, 2019 at 3:43

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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.

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    Are you able to comment on how pressurization of components might play out? As the_storyteller pointed out, with the valves working in reverse, intake and exhaust become reversed. My guess here as to what could go wrong is that the throttle body's valve(s) might not like being subjected to reverse pressures, but I'm not sure how damaging that could be for modern designs.
    – Zach F.
    Commented Jun 21, 2019 at 18:05
  • An automatic's clutches would not even engage. Urban legend: Henry Ford insisted on putting a hydraulic pressure pump on both input and output shafts of early automatics; that assured lubrication for a long tow and would pressurize the system so you could select Drive, engage clutches/bands and push start it.
    – Harper - Reinstate Monica
    Commented Jun 21, 2019 at 18:06
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    @ZachF. most throttle plates I see are offset shaft, so more than 50% of the surface area is on one side of the shaft. If so, air from the >50% direction will simply push the throttle plate open.
    – Harper - Reinstate Monica
    Commented Jun 21, 2019 at 18:07

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