Understanding Downhill Shifting?

[Jeremy556]

There is. It's called a rev limiter. Every modern day vehicle has this programming in the ecm. However, it's not a good idea to just let it sit there and bounce off the rev limiter...hit the brakes and bring it down. Typically a lower gear will hold the vehicle at the proper speed and proper revs and you will never get close to the rev limiter.

Rev limiter can only control fuel and ignition to keep you from over revving with the throttle. When the engine is driven by the wheels, fuel and ignition don't have any limiting ability

 

[Jetboy]

Just curious… Where did you get this?… :noidea: especially the part that Hondas are not allowed on certain roads? I've never seen a single road that says Hondas are not allowed… ( or any other specific brand of vehicle for that matter) I have an 08 Honda and I use the transmission to downshift on steep descents and it works perfectly…

Vehicle Restrications - Mount Washington Auto Road, Gorham NH


They can't hold 1st gear. It's not a functional capability of the transmission. I don't know mechanically why they can't. On the bright side Honda sponsors a van for Honda owners to ride in because their vehicles are not allowed. I believe the policy is the result of a few very near misses. I'm not honda bashing. I ride a honda motorcycle and have had a handful of accords and an Acura TL that all were very reliable vehicles. They're just not built for towing a trailer down steep mountain passes.

 

[Bodie the Dog]

Why I downshift

At one end of the spectrum you'll have people who tell you that brakes are cheaper to repair than transmissions and that you should NEVER downshift on descending grades.

At the other end are those who are paranoid about overheating their brakes and will downshift to the lowest gear possible in order to avoid having to use them, even on short stretches.

As is usually the case with such matters, there's a happy medium -- and where that medium lies on the spectrum is entirely up to you.

Ok, I'm not a wrencher, so take this with a grain of salt, but I downshift and use 4x4 high sometimes when other people wouldn't.

I believe that more than the brakes and transmission are involved. The forces applied to various parts of your truck as you're driving down a steep hill (how many tons do our t4r's weigh?) can be considerable. Other areas see that stress, I reckon, like motor mounts. By using engine braking, you're spreading out those forces through the entire vehicle, instead of just on the brakes and associated components.

Ok, those of you who really know this stuff, feel free to correct me if this is wrong, lulz.

 

[Jetboy]

Ok, I'm not a wrencher, so take this with a grain of salt, but I downshift and use 4x4 high sometimes when other people wouldn't.

I believe that more than the brakes and transmission are involved. The forces applied to various parts of your truck as you're driving down a steep hill (how many tons do our t4r's weigh?) can be considerable. Other areas see that stress, I reckon, like motor mounts. By using engine braking, you're spreading out those forces through the entire vehicle, instead of just on the brakes and associated components.

Ok, those of you who really know this stuff, feel free to correct me if this is wrong, lulz.

You're converting mechanical energy to heat no matter how you do it. It's just a question of where and how. Engine braking uses the entire drivetrain and spreads the heat out to more friction surfaces and primarily it's transferring the energy to the air that is passing through the engine. The brakes will convert all the energy in a fairly small area with limited cooling. They can't keep up on long downhill grades.

It does add some wear to the driveline - ujoints, transfer case, transmission and engine - but... when was the last toyota automatic you've heard of having any problems? They basically last forever. Most automatics will automatically downshift on their own. The 4runner will in many scenarios too. But not as much as you'd want on long steep grades - especially with a trailer.

Keeping your brakes cool is also a safety issue. They can overheat and you can lose braking control. 4Runners have huge brakes for their size/weight. It can be done though. And more likely you can overheat and warp your brake rotors. Plus the brake pads and such. Over the life of the vehicle I think the math works out pretty heavily in favor of engine braking.

 

[northcoast]

I'd watch the RPMs... you can get the cat. con. hot enough to throw a dash light. I've actually done it twice before, but not in the 4runner.

 

[MaddoxPics]

Awesome, thanks for the repor. Definitely going to be interesting with the snow melt this year. I've be itching to get back Emigrant, we'll see what happens but it maybe September before anyone can get in there.

Yes, that's an amazing area. I was hoping to backpack with my family up near Kaiser Pass. Short trip, but with the snow up there that changed plans. I thought waiting until the end of June would be fine, but looks like early August is our best bet now.

 

[e60ral]

If you need to brake just make sure you aren't riding them, brake firmly then get off the brakes

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[pcrdude]

The rev limiter will cut the spark, but not limit overspeed due to mechanical parts. If I shift a manual transmission into second at 100 mph, things are gonna break.

 

[e60ral]

Is it even possible to overrev an automatic in low gear, wouldn't it just shift up? And even if not, since the TC isn't locked up, wouldn't it just eventually cook the transmission?

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[cbreze]

In 50 years of driving I've always used the transmission to brake the vehicle when going down longer downgrades. Was taught this way and have never screwed any trannys up doing so and have perhaps saved lots of brake wear over the years.
I recall when vehicles had no tachs and you just used the engine noise and had a feel for it as far as keeping it within a proper range. In winter time around here I couldn't imagine not using the tranny to brake. Driving is alot just plain old common sense.

 

[world vs venkman]

I live in the Appalachians in WNC, and I'm guessing I don't face the same grades that you do. That said, I can vouch for the 4R being much better at engine braking than my previous Honda was (which, coincidentally, I ditched because the transmission was going out).

I've only had my 2015 4R for a few months, but the adaptive transmission seems to be working really nicely in terms of keeping me from having to ride the brakes--I rarely need to do any manual switching.

 

[TEujunga]

In 50 years of driving I've always used the transmission to brake the vehicle when going down longer downgrades. Was taught this way and have never screwed any trannys up doing so and have perhaps saved lots of brake wear over the years.
I recall when vehicles had no tachs and you just used the engine noise and had a feel for it as far as keeping it within a proper range. In winter time around here I couldn't imagine not using the tranny to brake. Driving is alot just plain old common sense.

I agree, but common sense is becoming scarcer and scarcer in this increasingly technology-oriented society where every problem has to have a metric, binary (yes/no) solution. Sorry, I'll quit philosophizing...

 

[SATSTRAIL]

Is it even possible to overrev an automatic in low gear, wouldn't it just shift up? And even if not, since the TC isn't locked up, wouldn't it just eventually cook the transmission?

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This exactly. And this was the point I was getting at. Any modern day automatic transmission, the 4runner included, with the programmed shift logic will never allow the engine to mechanically over rev. Case in point, if you are traveling at too high of a speed the programmed shift logic will not allow you to downshift to a gear that will cause mechanical damage to the engine due to over rev. Try down shifting into first gear while doing 60 to 70 mph, it won't happen. And whatever gear position the programming does allow you to downshift to you wont simply be able to continue coasting to the point that it causes a mechanical over rev. Over revving the engine is not a concern on modern day automatics. The electronics simply do not allow it. If you were to manually select first gear and accelerate from that point and then coast down the steepest hill you can find you will never over rev the engine mechanically. It just won't happen. The engine braking will prevent the vehicle from reaching the speed that will cause the over rev.

You cannot compare modern-day automatic to manual transmission. Completely different.

 

[jrob]

This exactly. And this was the point I was getting at. Any modern day automatic transmission, the 4runner included, with the programmed shift logic will never allow the engine to mechanically over rev. Case in point, if you are traveling at too high of a speed the programmed shift logic will not allow you to downshift to a gear that will cause mechanical damage to the engine due to over rev. Try down shifting into first gear while doing 60 to 70 mph, it won't happen. And whatever gear position the programming does allow you to downshift to you wont simply be able to continue coasting to the point that it causes a mechanical over rev. Over revving the engine is not a concern on modern day automatics. The electronics simply do not allow it. If you were to manually select first gear and accelerate from that point and then coast down the steepest hill you can find you will never over rev the engine mechanically. It just won't happen. The engine braking will prevent the vehicle from reaching the speed that will cause the over rev.
You cannot compare modern-day automatic to manual transmission. Completely different.

Guess you missed this:

Rev limiter can only control fuel and ignition to keep you from over revving with the throttle. When the engine is driven by the wheels, fuel and ignition don't have any limiting ability

Mechanical force will absolutely over come an ignition cut.
Electronics < Physics

OP-Engine braking is a thing because brake failure is a thing. You use the engine to assist in braking, because once the brakes cook, the engine alone is not enough. Hence why runaway truck ramps are a thing. Use 2nd-3rd gear, when your RPMs bump up to the 3500-4k range, apply gentle progressing to forceful input on the brake pedal until you are back down to 2k-2500 range (or speed limit-ish) then release. Braking input should be on or off, don't ride them. You can pick out an experienced mountain driver by how little their brake lights come on.

Happy driving.

 

[philsey]

Guess you missed this:



Mechanical force will absolutely over come an ignition cut.
Electronics < Physics

OP-Engine braking is a thing because brake failure is a thing. You use the engine to assist in braking, because once the brakes cook, the engine alone is not enough. Hence why runaway truck ramps are a thing. Use 2nd-3rd gear, when your RPMs bump up to the 3500-4k range, apply gentle progressing to forceful input on the brake pedal until you are back down to 2k-2500 range (or speed limit-ish) then release. Braking input should be on or off, don't ride them. You can pick out an experienced mountain driver by how little their brake lights come on.

Happy driving.

There ya go. ^This, exactly. I was taught this eons ago. Engine braking, then brakes on and off, keeps you in check, allows brakes to cool.

 

[RPS99SHO]

I use the same principles taught in CDL training. Max engine braking when possible. If brake application is necessary frequently, brake firmly to 5-10mph under safe speed, then coast to allow brakes to cool. Rinse and repeat.

 

[SATSTRAIL]

Guess you missed this:



Mechanical force will absolutely over come an ignition cut.
Electronics < Physics

OP-Engine braking is a thing because brake failure is a thing. You use the engine to assist in braking, because once the brakes cook, the engine alone is not enough. Hence why runaway truck ramps are a thing. Use 2nd-3rd gear, when your RPMs bump up to the 3500-4k range, apply gentle progressing to forceful input on the brake pedal until you are back down to 2k-2500 range (or speed limit-ish) then release. Braking input should be on or off, don't ride them. You can pick out an experienced mountain driver by how little their brake lights come on.

Happy driving.

I understand what you're saying and I agree that engine braking is the way to go. My point was that the electronics on the vehicle will not allow you to downshift to a gear that will cause engine damage even while coasting. Therefore you will not be able to mechanically overrev your engine. If you're doing 70 miles an hour on the freeway you will not be able to downshift to first or second gear.