Best gear to park manual transmission in?


#1

Let’s say you have a manual transmission and 1st and reverse are basically the same exact gear ratio. Let’s say the vehicle is parked with the front facing downhill. Is it best to have it in 1st gear or reverse? The reason I ask this is that I used to always park my vehicles in the gear opposite the direction the car is facing. If the front is pointed downhill, I would park it in reverse. If the back was facing downhill, I would put it in 1st. I recently had an experience that seems to indicate that it would be best to park it in the gear for the direction the car would roll.

I had one of my trucks fully loaded with firewood. It was really loaded up, probably overloaded. I parked it on a hill and put it in reverse. It began to roll forward once I turned it off before I could put the parking brake on. I know the clutch wasn’t slipping as I could feel the pulse of the engine turning over due to the compression. The clutch is also basically new and it was also a lot harder to start after this happened. I put the parking brake on and chocked the wheels with firewood so it wouldn’t roll.

I have done a few more experiments with this truck in similar situations. It seems that the truck is less likely to roll and overpower the compression of the engine when the truck is in 1st when it would want to roll forward or reverse when it would want to roll backwards. This is a Ford 4.6L. Does it make any sense?


#2

Using reverse when heading downhill will cause the engine to spin backwards if the vehicle rolls. Not a good thing. Use first gear when heading downhill, reverse when heading uphill…


#3

My understanding (which I guess Caddyman will disagree with) is that you always use the lowest gear, which might be reverse or first depending on the car, no matter which way the hill goes.


#4

Back when timing belts didn’t have automatic tensioners (like my '75, '81 & '89 Hondas) turning the engine backwards could make the belt jump a tooth if it wasn’t tight.
The belt did jump AHEAD a tooth on my '75 Civic once, before I knew about it’s proper care and feeding.


#5

Yeah, the engine was hard to start after it was spun backwards but has been fine after this happened so I don’t think any real damage was done. This is a chain driven engine so I would hope it wouldn’t jump time or break because of this. I do not know if the Ford 4.6L is interference or not either.


#6

You should not depend on the engine to hold the vehicle on a hill…Perhaps the parking brake needs to be repaired or serviced…Parking “in gear” is just a backup measure…If the engine turns over once, it can continue to turn over…


#7

Keep it in the gear that’s the same direction that you’re rolling, like Caddyman says. Your engine is an interference engine, all Ford chain engines are. And should you jump time on this chain you’ll wish it would have been a timing belt. It would cost far less to repair.

Also, the parking brake shoes on your truck are in my opinion under-engineered. Parking brakes on these trucks are marginal when new, and years of wear combined with an overload could easily overcome the parking brake. Don’t rely on the engine to keep it from moving. Think about it…an average guy with a 1/2" drive ratchet can easily turn over an engine. Surely a load of wood can do more.


#8

Theoretically, an engine will brake better turned backwards because it is pumping air from the exhaust to the intake and there is no throttle in the exhaust to limit cylinder filling so the cylinder fills completely with air and the closed thottle on the intake traps air in the cylinder causing it to take a lot of energy to blow the air past the throttle.
Turned forward, the closed throttle limits cylinder fill and you are compressing a vacuum while the unthrottled exhaust does nothing to restrict the expulsion of air.
Some diesels use an exhaust throttle to give extra engine braking on downhills.


#9

Yes, but the danger in turning an engine backwards lies with the overhead camshafts and timing chains used to drive them. Turning an engine forwards will simply turn everything in the desired direction. Turning it backwards will pull the slack side of the chain tight, possibly compressing the hydraulically operated tensioner and guides. This could result in the engine jumping time when the starter is engaged and the chain rapidly snaps forward.


#10

B.L.E.#sthash.nED2csJF.dpuf I would have to think about that for a while.


#11

I agree that the parking brake is marginal on this truck. I had the entire rear brake system including the parking brake redone not long after I got it. Now I park the truck in low range when it is loaded to provide extra resistance if there is any slope at all. The obnoxious thing about this is I cannot leave the truck running when I have to get out and open a gate if it is loaded. I must shut it off and put it in gear as an extra safety margin. If there is much slope, I use low range.

Either way, it sounds like a very bad idea to allow an engine to turn backwards unless it uses timing gears. I will always make sure to leave it in the gear that would allow it to turn forwards. I could also see the snap of the slack being jerked out of the chain breaking the chain which would not be good.

This truck is a 1997 F-250 Light Duty which is the oddball one with the 7 lug wheels. I am pretty sure it is a 2 valve engine. Sure, it isn’t the most powerful but I don’t have to deal with the pesky cam phasers on some trucks from that era. It is mainly my firewood hauler and winter 4WD.


#12

@EllyEllis I discovered this effect while flying a small radio control model airplane powered by a small 0.26 cubic inch four stroke glow engine. I landed the plane, it was running normally and shut the engine off with the throttle trim control. I flipped the prop over by hand and was kind of surprised that the engine suddenly had absolutely no compression whatsoever. So, I was going to try to start it again so I opened the throttle a little and as soon as I did, the compression returned completely. I reclosed the throttle and the compression went away, opened the throttle and the compression returned. Well of course, if the throttle doesn’t allow any air into the cylinder during the intake stroke, there is vacuum in the cylinder and instead of compressing air, the vacuum actually sucks the piston back up to top dead center.
That’s also why four stroke engines idle so much more smoothly when the air fuel mixture is adjusted so the engine idles with the highest possible manifold vacuum, I.E. most closed throttle for that engine speed.

Of course, turn it backwards, the engine pumps air from the exhaust to the intake and since there is no throttle in the exhaust, you have full compression, and if the throttle is completely closed, the air coming out of the intake has no place to go, it’s like an airpump with the discharge blocked.


#13

I am in full agreement with @Caddyman on both points. I am reluctant, though have, to have the motor spin backwards and I never depend on the manual transmission to hold the vehicle. Wheel chocks should always be used. @cwatkin is absolutely right. Low range enhances a manual transmission, almost to the point of being fool proof on not too steep grades. My previous tractor, which was a manual and the dump truck I make deliveries in when loaded, both have insufficient motor drag for the transmission to be anything but a minor slow down when rolling. That’s why we use the bucket and the emence wet disks locked on the tractor and chocks on the dump truck, even with the automatics.

I live on the side of a mountain and nothing is better then throwing as much (and as big) as you can under the wheels and parking against trees. I would consider not trusting anything on really steep terrain, especially not a cable actuated parking brake and low gear with out parking it in front of a last resort barrier.

Bottom line for me is, it’s all about the weight you must hold vs the size of the motor. A standard transmission Vette IMHO is a no brainer, while a loaded 3/4 ton is always a problem. Drove too many busses filled with kids not to be worry wart.

Besides, if you are loaded with firewood, throw a couple pieces ( or just one under the nearest if temporary) under the wheels and don’t worry !!
If you are cutting your own wood, it’s easy to cut wheel wedges that work GREAT. If you are really worried, buy a pair of contoured wedges professionals use and hang them near a wheel on the back for quick use.


#14

Regardless of the technology used to operate the valves, it’s a bad idea to turn any engine in the opposite direction than it was designed to operate, at all but the very slowest of speeds. Depending on the slope of the cam lobes, you might be doing damage to them or other parts of the valve train when putting force on the parts in a way that was not intended. Depending on how much it’s “worn in”, you might be stressing bearings, etc. as well.


#15

I often place firewood wedges under the wheels when parked on a slope for an extended period of time. I have now also started using the low range and a gear that would cause the engine to turn in the normal direction each time I park it. I have been cutting firewood A LOT lately to make sure my stash is maxed out before winter and have been using the truck quite a bit.

I am sure this engine is plenty worn in. The truck has 260K plus miles on it although the engine may or may not be a junkyard replacement. It still burns zero oil between changes which isn’t bad for a vehicle of this age.

I have seen some crazy things with two strokes. I once had a chainsaw that had some type of easy start spring mechanism. The spring broke and the part cost as much as the chainsaw. I just filled the mechanism with epoxy so it was solid. It would start and the chain would run in reverse about half the time after I did this. I would simply turn it off and give it another pull until the chain was running in the normal direction.

I also once had a cheap Poulan 2 stroke trimmer. I swear they call these thing Poulan because all you do is “pull on” the starter rope. I hated the trimmer. It was hard to start and the string feed was always messing up. The carb had fixed jets and was WAY TOO LEAN but I guess it was this way to satisfy the EPA. Well, one day it was super hot outside but I decided to trim and mow my yard. I mean it was like 108 or 110 degrees outside. I finished up trimming and turned the trimmer off. It continued to pick up speed and was obviously dieseling and igniting without the spark plug operating. It sounded strange and the RPMs just continued to climb. There was finally a loud pop and the thing rattled to a stop. I figured it had thrown a rod so I took the engine apart to find the rod was just fine. It had tried to run itself in reverse and switch directions so it had sheared the flywheel key. I looked at buying a new one only to find out that the flywheel key was integral to the flywheel which cost almost as much as the entire unit new. I hated this thing anyway and used this as an excuse to buy a real trimmer made by Stihl.