AT compression braking

I actually feel the opposite to OP. It drives me crazy that my truck won't coast downhill without slowing down dramatically. DFCO be damned, but I usually have to give my throttle a tiny push to maintain a set speed downhill on road.

 

It had never done it before in the new truck, so it surprised me. I always use S Mode on inclines.

 

Better with ECT on too.

 

I’m starting to think you were heavily overloaded in payload and/or a trailer. How heavy were you?

 

That’s what I was asking, OP never answered the?

 

Ya we aren’t getting the full picture. Lol

 

Y'all missed it. 6k. So... quite overloaded and dangerous with stock brakes.

 

Going back to the original question, “Anyone know why the compression braking with the auto trans is almost non-existent?”… I don’t feel that that the engine braking is non-existent nor do I think it’s different between the automatic and manual transmission.

However, I am not surprised that a F350 has more engine braking because presumably it has a much larger engine with more pumping losses when the throttle is closed. Its’s an apples to oranges comparison.

A better comparison would be to another mid sized pickup with a v6.

BTW, while I am familiar with the term “engine braking” I had never heard the term “compression braking”. I looked it up and learned that it is a form of engine braking that involves a special device (eg a j brake) to effectively turn the engine into an air compressor. AFAIK, Tacos aren’t capable of that. We must reply on pumping losses and other internal friction within the deivetrain.

 

Thanks, I did!
And you’re right, 6k is a heavy mutha. Overloaded if that weight isn’t considering a trailer w brakes. Not sure why someone is even comparing a loaded 1 ton or larger to a midsized truck in this department especially

 

I did some thinking about the Atkinson cycle’s effect on engine braking and yes I think it would reduce engine braking because its goal is to deliver exhaust into the exhaust system at atmospheric pressure. This is the opposite goal of a compression braking device.

However, I’m unsure that the Atkinson cycle is enabled by the ECU during engine braking. It wouldn’t make much sense, because there is presumably no fuel being delivered to the engine.

 

Ah I did indeed miss it. I’m not thinking 6k is outside the capability of a taco though. It’s certainly above GVWR of 5600lbs, but I’m guessing Toyota built in a 20%+ safety factor. Brakes failing is a huge safety liability. I’m not an automotive engineer though.

Some related brake info here.

https://m.youtube.com/watch?v=Ql9eYh31kTw

 

I disagree in general... While 5600lbs is GVWR, I would not run a stock truck at or near that weight. I am below that on my 2nd gen and have upgraded brakes, suspension and power. I did the same upgrades on my 1st gen with the same load (lightweight camper). I can't imagine trying to run a stock truck over GVWR and expect it to safely stop - let alone going down passes in Colorado.

 

I didn’t mean I think it is advisable to carry 6000lbs. I meant that I wouldn’t expect a brake failure unless there were other factors involved.

 

Circling back to the brake failure… OP can you elaborate on your brake failure? What did your driving experience and what was the result post-mortem analysis?

 

Meanwhile I'm going down to S2 at stop lights in city driving. 35s on stock gearing

 

My brakes did not "cook," as in, they did not appear to overheat by sight or smell. They definitely warped, and they were warped before I went on that trip but not as bad. Even after that trip, I could only tell they were warped after they heated up. Casual stops did not produce any vibration.

Last I read about it, Jake Brake is a manufacturer of "engine brakes," where the exhaust valve stem collapses, which is different from "exhaust brakes," which was just a butterfly valve downstream of the exhaust manifold. We always called downshifting "compression braking." Maybe things have changed since I learned about that stuff years ago. Doesn't really matter in this convo, tho.

I don't know that comparing my Taco at 6k pounds (~4.5k typical curb weight) to my 350 at 9k pounds (~7.5k typical curb weight) is all that different. The 350 GVWR is 10.5k iirc. If pounds per liter of displacement were the main factor, the Taco comes to 1714lbs per liter and the 350 gets 1452lbs per liter. Is that a significant difference? I dunno. I would think that efficiency was way down at the bottom of the list of priorities for the 350 engine, where it was obviously pretty high for the Taco. Both have 6 gears and the same size tires, but the 350 has 4.30 gears. Taco is 3.73. I'm sure that makes a difference.

All that aside, I have panic stopped both trucks at that the above weights, and the Taco stops far more quickly. That said, I have never warped the rotors on the 350. In my experience, there is no real panic stop in that truck. You can push the pedal as hard as you want, but it doesn't stop any faster after a surprisingly early point. The Taco will activate ABS on the front wheels on dry pavement. The 350 will not lock up the rears even. There is probably some tradeoff in the different priorities of the braking system of each truck.

Interesting that so many people experience their Taco downshifting on its own in these circumstances. Mine only seemed to do it at moments it wasn't necessary.

I do think I need to try letting the engine rev higher. I just came down out of the smoky mtn switchbacks in my 350 and 2100rpm in 2nd kept it under control with no brakes. I spend way more time driving that truck then my Taco, so that's where I'm coming from.

Regarding the weight, I'm not happy about it. I have talked to plenty of people here that are well over 6k, but I really wanted to stay lean(er). One was 7k+ and plenty were heavier then me with far more miles and heavier use. Even so, the Taco feels like a track car compared to that shuttle bus. That 6k is with me, a full tank, 10gal of reserve gas and 14gal of water. That's the heaviest it ever is, not the baseline. I still wish it was well under 5600.

 

One clarification… 6k with or without you and passengers?

 

Regardless of the exact mechanism, the goal of a compression braking device is to release the fully compressed air directly into the exhaust system and thus effectively eliminate the power stroke. This results in the energy used to compress the air to be complexly lost to heat. You are basically making the engine into an air compressor.

On the other hand, the atkinson cycle is trying to do the exact opposite. It is trying to release the the air into the exhaust system at atmospheric pressure such that no energy is lost to compression heat.

Make sense?

 

That is with me only (as far as people), and that trip out west was with me only, so the truck would've been around 6k. The rough math got me to 5600. I was surprised to see the landfill and a Cat scale both give me 6k. Stuff adds up quickly. My truck is definitely not an "expo" build. It actually feels lighter on its feet at this weight, and with this suspension, then it did at stock weight, braking included.

 

fully loaded mine sits around 6780. No issues with braking. 6 years later and im still on the original pads that will probably last another 3+