# Why does downshifting keep a car from gaining speed down a hill?

Why does downshifting keep you from going fast while going down a hill? I can understand slowing down by downshifting; where the engine takes the car’s kinetic energy and uses it to speed the engine up - thus slowing the car down. But why does keeping your car in 2nd gear keep your car from flying down a steep hill? I thought that perhaps the combustion engine might form a vacuum; but that’s impossible due to the fact that ignition still takes place and the expanding gas would actually make the car go faster… Help me understand this…

Call me stupid, but I don’t quite understand what you are asking. To me the two things sound like the same thing (slowing down, and not going fast on the hill). I believe that the important thing is the number of revolutions that a car in second can make it each second. Going down the hill only means that there is more downward force going down the hill.

If I keep my car in second gear and go down a steep hill, my car won’t go faster than 25 mph. Why doesn’t it just keep going faster and faster by making the engine go faster and faster? Why does the engine only want to go so fast and no faster? (thus keeping my speed down)

The engine and transmission are holding the car back. It is the same as slowing down by taking your foot off the gas pedal. As long as you don’t give it gas it will hold you back. There is the little thing called gravity though. You will go faster as the weight of the car overcomes the engine and transmission. Just use your brakes.Or you could shift to first.

I believe it is because going in to a lower gear (or just staying in second in this case) means that the car has to use so many revs to keep its speed. And even though the hill will have some pull on the car because the car is TRYING to go forward (due to gravity) it won’t be sucked down the hill as fast as if it was in nuetral gear where there is no resistance. This is because the wheels are attached to the car, and the car cannot produce enough revs to keep it’s speed up, so it is actually slowing the car down. Whereas, in high gears the car is going faster due to the lower number of revs needed to remain at a higher speed. Also, the force of the car going down the hill in a high gear would probably be faster than the pull of gravity. Hmm… I hope that makes sense, and if it doesn’t just explain where it doesn’t make sense and I’ll try to explain it again. That is what I believe is happening.

Actually, when coasting downhill in second the car wants to go faster than the engine is running.

Say you’re driving along flat ground at 30 mph. You’ve got a V-8 engine. For illustration purposes, each cylinder fires 20 times a minute. If you speed up the number of cycles a cylinder completes incleases, take your foot off the accelerator, the number decreases.

While you are idling, each cylinder fires maybe 10 times.

Coasting downhill with the engine running slows you down because the engine wants to idle (10 strokes a minute) and the car wants to move faster than the engine allows, forcing the pistons through their cycles. The vacuum formed by the pistons trying to advance in their cylinders quicker than air can fill the vacated space in the cylinders causes them to slow, inturn slowing the drive shaft, in turn slowing your tires and your rate of descent.

Wow, that was way more elegent that what I said. Nicely said.

Quote: “The vacuum formed by the pistons trying to advance in their cylinders quicker than air can fill the vacated space in the cylinders causes them to slow, inturn slowing the drive shaft, in turn slowing your tires and your rate of descent.”

But wouldn’t the ignition of the gasoline/air mixture negate any vacuum that was formed?

I think compression is what you meant, Chief. The engine wants to idle and the drivetrain is trying to push its RPM up. Without you adding a little throttle to the endeavor, the valves are not going to pick up the pace and the resultant backpressure provides the engine brake (with a stick - an auto will, usually, upshift).

Yeah what beatle said. It helps to not only know what you’re talking about, but to have the vocbulary to talk about it. I was doomed to three years of woodworking vice a couple years of car maintenance in high school. I still haven’t overcome the stigma.

joemill – yes the air/fuel mixture does compensate for the vacuum, but not as quickly as if you were forcing the mixture into the cylinder via the injector. The mothion of the piston is actually pulling the mixture into the vacuum, thereby slowing the car.

Now if anyone wants to talk about shoe polish…

Hasn’t anyone here opened a manual tranny? Second is a much smaller gear & it must rotate much much faster to do the same amount of work, so it slows things down. Probably like that.

Anyway, it’s a dumb idea to do that. Use the brakes.

Actually, handy, it isn’t dumb. Brakes will run hot and fade on very steep hills. Using the engine as a braking device can actually slow the car down considerably, which may come in handy when one is keen on actually making that hairpin not 100 meters away.

1. Those rising pistons have to compress the air in the cylinder. This is wonderful for acceleration if the air being compressed is composed of the ideal air-fuel mixture (and, in fact, works best if there is a LOT of it in there to compress), i.e., when that spark plug fires, the expanding gases shove the piston back from whence it came with accelerating effect. But with your foot off the gas, you get a much leaner and thinner air-fuel mixture, one that, although it does burn, and does generate enough expanding-gas pressure to keep the engine spinning, just barely does that much, and as the engine speeds up (or attempts to do so), falls short of even that (therefore, equilibrium at a slow engine speed, aka “idle” speed). So heavy car tries to roll faster and faster down a hill. Engine is asked to spin faster and faster, but as it does so, its internal combustion processes do less and less to offset the resistance to compressing the thin, lean air-fuel mixture, and again you hit an equilibrium – a faster engine speed equilibrium than the idle you get when you are standing still, but an equilibrium nonetheless.

2. Having explained my understanding of all that, I must ask a question myself: how about those weird Cadillac 4-6-8 engines? Remember those? They ran on 4 cylinders under light loads, 6 under medium load, and 8 under full load, presumably to save gas. OK, I understand how the computer + coil could disable spark to the unused cylinders, but how the heck did they disable the intake process, the compression process, and the exhaust of (in this case) unburnt air-fuel mixture? Intake and exhaust manifolds don’t strike me as likely candidates for responsiveness to automotive computers, and you’d think the hassle of compressing gas in 4 cylinders would bring the working 4 to their metaphorical knees, not to mention the worrisome aspects of wastefully sending unburned gasoline vapor down a hot exhaust pipe – if it doesn’t generate a big ugly fireball, it would at best fail to save gas, wouldn’t it?

AHunter3: I don’t know anything about those Cadilac engines, but it seems like they’d work ok if they had electronic fuel injection. The computer could just turn off the injector to the unused cylinder. Maybe they also had a separate, computer controlled valve (not running on the camshaft) that could open the cylinder so it didn’t put a compression load on the part of the engine that was actually running.

Another thing to remember are the gear ratios.

All the friction losses are normally overcome by the output of the crankshaft toward the gearbox .

Lets say the gearbox had a ratio in 3rd of 5:1 where 5 is the engine rev and 1 is the gearbox output.The torque of the engine stays the same but the gearbox output torque increases not by a factor of 5 as seems at first obvious but by the square of the ratios - ie 25 times.

Now do this in reverse, that is , the drive is coming from the wheels back into the gearbox. To increase the braking effect in that 5:1 gearbox all you do is increase the engine braking force by 1/25 of that required.

The lower the gear the greater is the gearbox ratio which in turn multiplies the engine braking effort.Just remember though that it is done by the square of the ratio.So if we changed to 2nd gear and the ratio was 10:1 then the braking torque of the engine increases 100:1

The engine braking torque remains the same but now you can see why a lower gear would slow you down more.

Just wanted to clarify on Coldfire:

Coldfire is absolutely right. If you’re going down a hill, there’s no better way to kill your brakes quickly than by
riding the brake the whole way down. Please downshift. You will also have much better control and balance of your car.

The clarification is that some people assume that on a flat straightaway you should downshift into a corner to slow down the car. (BTW, I assume Coldfire was just talking about hills.) This is not true. Use the brakes to slow down into the corner, then downshift straight into the lowest gear you need for acceleration, and then accelarate through the corner. Don’t go 5-4-3-2. Just go 5-2 after braking. or 5-3. Whatever ya need. Some people continue braking a little through the first portion of the curve, some shift right at the beginning. Do whatever feels best for you. The basic rule is: slow in, fast out.

Here’s why, as far as I can tell: When you have the car in gear, the powered wheels do not rotate freely. They only rotate when power is delivered to them by the engine (try rolling a parked car thats in gear to see what I mean). When in neutral, no engine power is delivered to the wheels, and they are allowed to rotate freely, thus allowing the car to gather speed when rolling down a hill.