I just had a discussion with a friend of mine and we couldn't come to a compromise.

The question is: What happens if you drive down a steep road in your everyday manual car - second gear engaged for example-, and don't shift up, even if revs are going up to the redline or even towards the limiter? Does the latter actually work if you don't push the accelerator and can you damage engine or transmission? Will the engine overheat?

In contrast, how does an automatic transmission handle this situation, and how does it handle a downhill drive in general? Since we all know, we should drive down a slope in the lowest gear possible to preserve the brakes from overheating - does an AT do that, and is there a kind of "gyro" checking the car's angle, possibly forcing the transmission to change gears in a different way (=later)?

To all the experts: Please discuss!

Given that the rev limiter doesn't mechanically brake the engine/transmission, I imagine it'll just go past the redline.

However, as fuel will be cut due to zero throttle, I doubt it would damage anything.

That's all just guesswork though.

If there is an engaged clutch between the engine and gearbox (i.e. in a manual transmission, or auto with lock up), then the engine will continue to reciprocate faster and faster. A rev limiter may ensure a lack of fuel but it won't stop valve float (which even without combustion will still have an effect on engine braking), or your con rods from snapping from the increased stress due to the ever faster moving pistons applying larger forces to them. I don't see heat being a problem. If you kept the rpm very high and the conrods didn't break, then there is the possibility of the the valve springs fatiguing and failing, or without enough bounce, colliding with the pistons.

With just a torque converter, there is no direct connection between the engine and wheels, but the fluid will only allow so much slip, so you'll get the same problem, only it'll happen at a slightly (10%?) higher speed.

If your in second in a manual then the friction from the motor etc, etc, etc, etc, etc, fancy physics no's, etc, etc, more fancy physics no's ........

Basicly, rev any auto motor high enough and it will explode spectacually somewhere from 5000 rpm up.

Quote from smove :

In contrast, how does an automatic transmission handle this situation, and how does it handle a downhill drive in general? Since we all know, we should drive down a slope in the lowest gear possible to preserve the brakes from overheating - does an AT do that, and is there a kind of "gyro" checking the car's angle, possibly forcing the transmission to change gears in a different way (=later)?

Automatic transmissions don't handle that situation automatically. At least the ones I've driven. They usually have a way to lock the transmission into not using the higher gears, but you have to activate it manually, when you need it. So for instance, it would tell the transmission to only use (say) 1st and 2nd gears. It's marked by an L or a number such as 1 or 3. Depends on the specific car I think as to how it's set up. In the case above where it can use 2 gears, it would be marked with a '2'

EDIT: the van outside is PRND3L. Meaning: Park, reverse, neutral, drive, 3 gears, only 1st gear.

Quote from Racer X NZ :

Basicly, rev any auto motor high enough and it will explode spectacually somewhere from 5000 rpm up.

Please stop talking.

Quote from Crashgate3 :

Given that the rev limiter doesn't mechanically brake the engine/transmission, I imagine it'll just go past the redline.

However, as fuel will be cut due to zero throttle, I doubt it would damage anything.

That's all just guesswork though.

modern cars already do cut the fuel to pretty much zero when using the engine brake.

Quote from Crashgate3 :

However, as fuel will be cut due to zero throttle, I doubt it would damage anything.

If you leave it in the same gear, and the road is steep enough and there's enough room to just become faster and faster... at some point your revs would go way past the limit, and the engine would break. It doesn't have to run on fuel to break, all it needs is too much revs.

Only one way to find out. Take your mom's car, find a steep hill and let'er rip!

I can't believe I'm going to dignify this post with a response but since I'm anally retentive...

Quote from Racer X NZ :

it will explode spectacually somewhere from 5000 rpm up.

Truck engines can be toast at half that rate.

i like it up the bum

E: Lies, hacked!!!

I did this in a 1l 3 cylinder Corsa hirecar going down a bridge... Nothing much happened and it reved over 8k before i changed when limiter is at 6400...


Didnt sound too healthy but only damage was glowing manifold melting the sleeve off exhaust sensor wire...

Quote :

In contrast, how does an automatic transmission handle this situation, and how does it handle a downhill drive in general? Since we all know, we should drive down a slope in the lowest gear possible to preserve the brakes from overheating - does an AT do that, and is there a kind of "gyro" checking the car's angle, possibly forcing the transmission to change gears in a different way (=later)?

Throttle position, engine load, gear selection and speed determine how an automatic transmission handles the situation.

Usually D or Drive don't have a one way clutch engaged so the transmission will just freewheel and not increase RPM as the coasting speed increases. Manually selecting gears 1, 2, 3 or whatever usually allows the transmission to apply more clutchpacks which in turn enable a one way clutch which doesn't allow the transmission to free wheel and will increase RPM and create engine breaking as coast speed increases.

Lots of car enthusiasts on this forum...and a lot of them don't have basic knowledge...makes me scratch my head.

Quote from Bob Smith :

I can't believe I'm going to dignify this post with a response but since I'm anally retentive...

Truck engines can be toast at half that rate.

And an S2000 motor ran rev to 8000 RPM under normal operating conditions without damage. Many stock, road-legal motorbike engines make peak horsepower at around 13000 RPM and continue to rev all the way to a 16000 RPM limiter.

Like you, I'm baffled by how people can make such unfounded claims about ICE operation without the slightest clue of what parts actually fail internally when an engine is over-reved and why.

Quote from Homeless_Drunk :

Lots of car enthusiasts on this forum...and a lot of them don't have basic knowledge...makes me scratch my head.

Well, "enthusiast" doesn't automatically mean "expert".

I for example am an enthusiast for cars and in this way interested in how they look, sound and drive, but unfortunately I'm not a mechanic or mechatronic.

And that's why I was asking - thanks for all the answers so far.

If I'm interested in something, I try to learn about them. If I liked cars, and just knew what they looked like or what they sounded like, then I'd consider myself a bit of a thicko.

It's hardly complicated really. Just need to read a few books.

Quote from tristancliffe :

If I'm interested in something, I try to learn about them. If I liked cars, and just knew what they looked like or what they sounded like, then I'd consider myself a bit of a thicko.

It's hardly complicated really. Just need to read a few books.

Same here. Aviation, computers, bikes, engines, and to a lesser extent, cars. If I was interested in a topic, I also wanted to understand it fairly well, and that meant a lot of reading.

One of the things I like about my trusty old automatic Subaru. Plenty of options when it comes to gears. A lot of cars these days just have something like PRNDL, but I've got PRND321. And since I've only got 4 forward gears, I've BASICALLY got control over all of them, lol.