Tried "B" For The First Time

I've found that using the CC to regulate speed works better and won't whine out your engine. If you need to brake, you'll need to re-engage the CC. If your really whining out your engine, your going way too fast to be in B mode.
I only use B on steep down hills with hair pins to save my brakes. The decent down into South Lake Tahoe is a prime example of steep curvy down hill where you could smell the burning brake shoes of flat-landers. They're lucky that the stop lamp intersection at the bottom was replaced with a round-about, 5 years ago. There was always horrific accidents in that intersection, because people got their brakes too hot on the decent and couldn't stop. I actually saw tire skid marks going straight through up and over the round-about, the first year it was installed. Drivers licenses here seems to be a right, not a privilege.

 

It may be noisy and disconcerting, but isn't a problem, so don't worry about it. The ECUs control engine RPM and won't allow it to overspeed. And without any fuel, it is under less stress than when running at similar RPM to climb a hill.

This is really meant for longer hills and vertical drops where regen will fill the battery well before the bottom of the hill, leaving you using brake pads long enough to overheat the brakes. There are some mountain roads where you can overheat and damage the brakes. That is why the Pikes Peak road has a mandatory brake check station, where an attendant uses an IR thermometer to check brake temperatures, and orders vehicles exceeding some limit to park in a cooling-off zone. We witnessed one while checking the adjacent gift shop, and it smelled horrid. A member here found another road, where he suffered heat damage twice before learning how to use B mode.

For hills short or shallow enough that the battery will not fill up, you don't really need to use B at all. It is more energy efficient to use CC or the brake pedal to maximize regeneration instead.

There is no "too fast to be in B mode." The ECUs set the engine RPM electrically, and won't allow it to become excessive. This isn't like a traditional transmission where a poor gear choice can mechanically force an engine overspeed condition.

 

You're misunderstanding what B mode is for. It's actually there to LIMIT regen so the battery doesn't overcharge on steep declines(think mountains, not hills). It uses the engine to bleed away energy instead of using the brakes. Even though the engine is revving, that doesn't necessarily mean combustion is going on. The engine is actually absorbing energy instead of creating it. Brake regen will still happen, but it will happen at a lower rate with a greater safety buffer so that it's less likely to get to max charge too quickly. It's only meant to be used during periods where you expect to constantly be on the brakes(say you're on the downward side of the Alleghanies or Appalachians).

 

Sorry; I'm not going to be the guinea pig to test that hypothesis; I pulse my brakes to control my speed and reduce engine revs. Unlike the flat-landers that ride their brakes all the way down the mountain.

 

The 'hypothesis' is well-enough tested, there'd be little more to learn from you testing it again anyway, so you're off the hook.

If you don't have an OBD-II scan tool that can show you engine RPM, it's easy to hook one up. If you ever see the car use an RPM above the programmed max (5300 for gen 3, I don't have the gen 5 number offhand), then you can come back and show us we've been wrong for 20 years.

So, while there isn't a "too fast to be in B mode, you'll overrev your engine", there is a "too fast to be in B mode and expect it to hold your speed". Because the car will only rev the engine to a certain RPM, B mode is never able to dissipate more power than what the engine dissipates at that RPM.

The power that gravity is pouring into the car depends on three things: how steep is the grade, how heavy is your load, and how high is your speed.

For any given grade and load, there's a certain speed below which B mode can hold your speed and even slow you down. At exactly that speed, B mode will use its maximum engine RPM and just manage to hold your speed, and if you're traveling faster than that, B will stay rock-steady at its maximum engine RPM but it won't be enough to hold your road speed, so the car will gradually roll faster and you'll have to help with the brakes from time to time.

A steeper grade or heavier load will lower that corresponding road speed.

For more on calculating just how much power the engine braking can dissipate, you can see this thread (which uses gen 3 numbers):

How much engine braking a gen 3 is capable of | PriusChat

Whenever your descent grade ✕ load ✕ speed works out to more power than that, you're going to have to also use the brakes some.

On the question of how to use the brakes, intuition at first glance might go either way: snub hard with the brakes from time to time then release them (extremely high heating alternating with cooling) or brake off the same overall power more steadily?

Better than resting on intuition, the U. Mich. Transportation Research Institute studied it, and definitely came out in favor of the repeated snub-then-release approach.

In any case, whatever amount of power B mode can dissipate for you is that much less left over for you to dissipate with the brakes. And you can always reduce the amount of power you're making the brakes handle, by taking the descent at a lower speed. (Or on a less-steep route, or with a lighter load, but your speed is the main thing you get to choose when you're already there.)

And if you choose the speed that's low enough for B mode to dissipate all the power, that saves you needing the brakes at all.

The only downside is you might be impatient, or have drivers behind you who don't like what that speed turns out to be.

 

IDK; I got speed creep on both my Prius C and Prime and chicken out around 45-50mph - I know when engine RPM sounds too high.

You can go ahead and test it on your own car - As I stated, not a guinea pig or test rat.

 

I had a Jetta TDI manual, as I recall that held speed better than B-mode.
We're talking apples to oranges because of the manual transmission and higher diesel motor compression ratios. I could look at my tachometer to see if I'm red lining. It wasn't quick, but there wasn't a mountain it couldn't climb; even after swapping in smaller fuel injectors. Motor-heads would swap-in larger injectors to get off the line faster, so I got the slightly smaller injectors out of an automatic for free, helping him change-out those injectors. That added 10 mpg to my driving style.
Sold it before the VW diesel gate scandal as diesel fuel prices went through the roof. There wasn't a competitive advantage vs gasoline anymore. I put up with the more expensive maintenance because I was doing well over 50K miles a year as a field technician. There was cheaper and more reliable options out there.

 

It's all about grade ✕ load ✕ speed. There's a little hill—nothing mountainous—coming in to town here, and the speed limit is 25 mph. Cruise is able to hold my speed rock-steady at 25 if it's just me in the car. It can't if I have a passenger, and it wouldn't be able to hold, say, 30, even with just me.

... as many have, many enough to leave no continuing need to "test it" further.

"I know when engine RPM sounds too high" merits some comment; pretty much every driver brings an intuition about what "sounds too high"—but intuition is trainable, and a tachometer (coupled with knowing the RPM specs for the engine) is how to train it. (It also can need to be trained again for different vehicles and engines.)

I remember knowing someone in college who had never driven, bought their first car after graduation, and flatly ruled out ever using 3rd on a freeway entrance, saying the same thing, "I know that sounds too high." Even just spending a little time learning the engine sound at different RPMs would have helped, but that car didn't have a tach.

I kinda think any car that (a) comes with a manual tranny, and (b) is going to be somebody's first car, really ought to come with a tach.

 

I did it hundreds of times over the 15 years I drove a Gen3. The route to my parents' house, where I served many shifts of farm work and elder care, included a 7% grade of almost 2000 vertical feet. Our other travels took us over many more long steep hills elsewhere, some much taller.

I had a ScanGauge installed the whole time, showing engine RPM, and carefully watched how high it would go. Even when sounding like a enormous Hoover vacuum, jet engine, or screaming banshee, the displayed RPM never exceeded or even matched Toyota's advertised limit. I can personally verify much of the behavior Chapman describes both here and in the other thread he linked. Including the parts about reaching the ICE's regen braking limit on steep hills that needed even more drag, resulting in the car speed continuing to creep higher until using the standard brake pedal to apply the friction brakes. But he is more detailed and has better diplomatic skills than me.

You are a relative newbie here. The forum's old timers had all this well tested and figured out long before you arrived, there are plenty of old threads about it.