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Let's suppose I leave my car in gear when parking on a hill (besides using handbrake and turning the wheels towards/away from the curb, depending on the direction I'm parking in). In case that matters, let's suppose the motor is gasoline (but I am also interested in answers for diesel-motors).

By the two possible "gear directions" (is there a better term for this?) I mean the following:

  1. Gears in the same direction as gravity, i.e. if I park pointing downhill, then first gear, if uphill, then reverse.

  2. Gears against gravity, i.e. if I park pointing downhill, then reverse; otherwise, if uphill, then first gear.

I did some research, to find which one you should do, but found there is no definitive answer. This is what I found:

  • If the gears are "against gravity" (i.e. reverse downhill, first gear uphill), then if the car accidentally moves, then the motor can be damaged by moving "backwards". (If I understood correctly, the reason is that the drive belt only works in the forward direction, and thus valves will hit against pistons.)
  • If the gears are "in the same direction as gravity" (reverse uphill, first gear downhill), then if the car accidentally moves, the motor can jump start alone, and the car will move forward autonomously from that point.
  • Newer cars cannot jump start automatically in the above case (only older cars).

My questions are the following:

  • Are these conclusions correct?
  • Are there any other pros/cons for the two directions?
  • How are newer cars immune to being automatically jump started, if they move in the same direction as the engaged gear?
  • How can I check if my car is new enough to not be affected by this problem?

PS: I'm already aware of this other question, which has been closed as opinion based, but I think that mine (1) isn't a duplicate, and (2) is not opinion based, because I am asking more concrete questions.

PPS: I also know that there are arguments against leaving the car in gear at all, when parking (some of them also mentioned in the question above), but they are out of scope for this question.

25

You've got several things wrong, as you've made a lot of assumptions.

First, in an engine if the ignition isn't turned on (ie: if it doesn't have power), you cannot push start it. You can turn that engine all day long and it isn't going to start. Just won't happen. It takes fire to make it run, which in this case is a spark (we're not talking about diesel engines here, just gasoline). If you don't have the key on, or haven't gone through many different hurdles, it's not going to start. In this case, it doesn't matter if the car is newer or older ... well, I guess if it was a lot older it might make a difference, but nothing from probably the 1930's forward. Bottom line on this is, your car will not spontaneously just start because it is moved with it in gear. Period.

Second, the timing belt/chain works in either direction. It will turn the timing components whether the engine is going backwards or forwards. I will say, it probably isn't good to run your engine in reverse, but it won't kill it. The only reason it wouldn't really be good for it is because the oil pump only works in one direction. Instead of providing more oil to the engine components which need it, you're actually purging whatever oil is already in there. Sucking it dry, as it were.

As for the rest of it, I'm sure there are reasons some would say leave your transmission one way or the other. I'm not going to answer that because it is conjecture ... IOW: this is the pure opinion part of the question. Some will say it is best to do it one way, while another will tell you it's better to do it the other. Both probably have valid points. If it were me, I'd ensure it's left in gear, one way or the other. Sometimes the parking brake may just not be enough. The main thing to do when parking on a hill is to ensure your tires are turned the correct way at the curb: If pointing uphill, point your tires away from the curb; if pointing downhill, point your tires towards the curb. This will help ensure your if your vehicle does start to move, the curb will catch it. If there isn't a curb and you aren't sure of your parking brake ... don't park on a hill.

EDIT: I realized I didn't answer the other part of your question ... diesel engines ...

As far as a diesel engine goes, it runs off of compression. That is its spark as it were. Generically, all a diesel engine needs to run is the fuel and air, plus the motion of the engine. Theoretically, it could run without the ignition on, as long as it has some fuel in order to do so. A diesel engine can run off of pretty much any oil, including engine oil (yah, the stuff in its crankcase). That said, though, as long as a diesel engine is not leaking oil into its intake runners, it isn't going to run - there's nothing there to combust. Even on older diesel engines, the way you kill them is to turn off its fuel source. This could be through a mechanical means (ie: shut off valve actuated via a manual device, like a pull), or electromechanical valve (ie: shut off valve actuated via a solenoid). Either way, as long as these devices are in order, it will not start as the engine goes around. Again, not going to happen.

  • 5
    Most modern diesel engines are electronically fuel injected, so you wouldn't get one to start without the ignition being on. – GdD Jun 17 at 8:05
  • Turning the engine backwards will make the timing belt behave differently. There is a spring-loaded tensioner on the "normally-slack" side of the belt, and turning it backwards will compress that spring and allow the crank pulley to move further than the cam pulley before it tightens up. (Whether it changes enough to bend a valve is another matter.) Turning backwards, with the "normally-tight" part of the belt now slack, the likelihood increases that the belt could jump one or more notches and throw off the timing. – smitelli Jun 17 at 13:46
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    "Dieseling" isn't theoretical - that's why they have a name for it. As @GdD says above, won't happen now, but I remember as a kid that whenever the neighbor came home - he worked heavy construction - with his International Harvester truck and he turned it off and went inside the old heap would keep running for a minimum of 20 minutes. And the darn thing was loud! – davidbak Jun 17 at 16:18
  • It's also worth noting that you can start and run a whole lot of two stroke engines backwards. – Rachey Jun 18 at 12:56
  • Yes, I remember reading that in certain antique "bubble cars" there was no reverse gear ... you stopped, restarted the engine in reverse, and re-engaged first. Such a car may still hold the world record for the maximum speed of a production car being driven backwards :-) – nigel222 Jun 18 at 13:09
8

The gear ratios in most manual transmissions are such that first gear has a lower ratio than reverse (actually a higher numerical ratio is a better way to express this but I'll follow the convention of this discussion). Often the reverse ratio is about halfway between first and second gear. I guess the reasoning is to allow a bit faster speed when backing up a long way.

Using the lowest ratio gear is the important point so I've always used first gear for parking. The issue about the engine being run backwards is a good one but I would expect manufacturers to design for this. Perhaps a high-compression high valve-lift race motor may not be immune but I haven't had the privilege of driving one on the street :-). Even in that case the race motors I've seen up close the chain or timing belt has very little slop.

2

The reasoning behind leaving in gear is that first and reverse gear are both low ratio gears, this provides enough resistance against the car moving should the brakes fail. If you consider how engine braking works to slow a vehicle and how quickly, you will see that using low ratio gears is also an effective brake method to prevent movement from standstill.

  • This doesn't answer any of the OP's questions. They already know it's advisable to leave a car parked on a hill in low gear - the question is whether that gear should be forward or reverse. – Nuclear Wang Jun 17 at 13:05
  • I don't think you can apply all principles of engine braking here. It works because the pistons create force that turns the crank in one direction, and a sudden gear change makes the crank force fight against the inertia of the car. If the engine wins, the car slows down. If inertia wins, the motor revs up. But if the engine is off and producing no force, then the crank is "freewheeling" and the only thing fighting Inertia is the engine compression. This is enough to keep the car from moving on flat ground or a mild incline, but if the hill is steep enough, inertia from gravity might win. – Christopher Hunter Jun 17 at 18:35
  • And yes, I have experienced a vehicle that had a poor e-brake, and bad enough compression that on a very steep driveway it would slowly creep backwards when parked in gear. – Christopher Hunter Jun 17 at 18:41
  • @ChristopherHunter: Unpacking the point you're making, it sounds like it doesn't really matter what gear you leave the car in then. – Ellesedil Jun 17 at 22:27
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    @Ellesdil I thought about that quite a bit. It's true that a lower gear will provide somewhat more resistance, but only because it will cause the engine to have to turn more times for the same distance traveled. In practical terms though, a) the actual difference between gears may be so marginal as not to matter, and b) if the car is rolling enough for it to make a difference which gear was selected, you've pretty much already lost the fight. – Christopher Hunter Jun 17 at 23:19
1

The top (most voted) answer draws pretty good conclusions. But I would like to add regarding the Forward vs. Reverse argument; that (with the exception of older, or larger trucks) that reverse is almost always lower than First. They nearly always share the same shaft in the transmission. One exception that I can add with great certainty, is my 1964 Chev C20 pickup truck. It was purchased without the (optional) emergency brake. I know from experience that 1st gear will hold it on 40° incline (driveway) without the aid of a curb for several days, and not move.

Lastly; if the car/truck has low/poor compression. Neither gear will matter, and given that diesels almost always require higher compression than gas powered cars. Diesels will more likely carry an advantage over their gas powered counterpart(s).

0

I haven't driven a stick in years, but as I recall Reverse always seemed a little less certain/sticky than first. It was going against a spring and seemed very easy to pop out.

  • 1
    That probably has more to do with your shifter mechanism than the gear itself. – Ellesedil Jun 17 at 22:26
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    I agree, but it was that way in more than one of my manual cars. The reverse gear was spring-loaded a little to prevent you from going into it by accident--you had to push against the spring then down, releasing just took a slight push up and the spring would slam it back into neutral pretty quickly. – Bill K Jun 17 at 23:01

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