AT compression braking

Anyone know why the compression braking with the auto trans is almost non-existent? My theory is because of the high degree of valve overlap with the simulated Atkinson cycle at light load. Coming down long descents with significant weight out west destroyed my rotors, and maybe my drums. Maybe a tune could be made to minimize valve overlap and lockup the torque converter by pushing the ECT button.

My f350 gasser has awesome compression braking at surprisingly low rpm...it also gets 11-13mpg.

 

Don't be scared to let that engine sing!! I was hitting 5k down the passes in CO with about 1,000lbs of cargo and barely touched the brakes. That said, having come from a 1st gen Tundra, I put Dynamic Friction rotors and Z36 pads on the truck the second day of ownership.

What is "significant weight" to you?

 

Lack of reciprocating and rotating mass, your F350 has a huge engine and a much heavier transmission and axle too.

These trucks have a small lightweight engine, they don't offer the braking ability of an old Dodge 225 slant six for example. Even though the Dodge engine was of similar displacement (3.7 liter for the metric folks) the crankshaft, connecting rods and pistons of the Dodge engine weighed significantly more thus were more effective.

Example:

Tacoma:
Crankshaft weighs around 45 pounds
Connecting rod weighs around 600 grams

Slant Six:
Crankshaft weighs around 75 pounds
Connecting rod weighs around 750 grams

 

Use S-mode to get more engine braking.

 

I was letting it wind out, just wasn't doing what I wanted. I weighed in at 6k pounds on the curb at that time. I did replace the front brakes with Z36 stuff. I have yet to tackle a descent like that with them, but they definitely grab better once they get warm. The stock brakes seems to be more responsive during light use, but fell apart with any sustained heat.

 

Maybe that's got something to do with it, but I don't think it's apples to apples. Your slant six did not have the crazy amount of valve timing variability the Tacoma does (or any variability at all). Something like 90* intake and 70* exhaust, iirc, and it's looking for max efficiency under light load/coasting, which I would think means max overlap. The opposite of an engine brake.

 

Good idea, I'll give it a try but I thought that just changed shift points and throttle relationship?

 

ECT changes the shift points.

S-mode sets the highest gear the transmission will shift into. So S 4 will be limited to 4th grear. It works well for descending hills.

 

S mode and down shift like you would in a manual.

 

Manually downshift.

 

Electric throttle also is cause for some of the lack of downshift feel if you’re comparing to an older vehicle.

I never found Tacoma to be lacking. I always just slap Toyotas into s4 mode and s3 on serious grades.

 

Are you thinking of ECT? S-mode’s primary purpose is engine braking. It forces the transmission to use a lower gear which both increases engine rpm and decreases the mechanical advantage the ground has on the engine. Read the s-mode section in the owners manual.

 

I should've said in the OP, I was manually downshifting using s-mode in these instances. I thought that would be assumed in a convo about compression braking with an AT, my bad.

 

Yeah I was thinking of ECT. I also was downshifting with s mode, though I don't remember what rpm I was holding. Probably not 6k. Probably around 4k.

 

Exactly How much weight?
If you have a trailer, you should have trailer brakes. Yeah, you can downshift and that helps, but if you’re overheating brakes to the point of cooking them, you sound overloaded. You should be able to handle braking alone, and use engine braking to aid/relieve.

 

Yes, S-mode and trailer brakes are a must or perhaps you need to look at a 1/2 or 3/4 ton.

 

I noticed on a recent trip through Colorado and Utah that the transmission learned to down shift without going into S-mode. It kind of surprised me when it happened. My '06 used to do it all the time on steep inclines. Apply the brakes steadily several times and it down shifted on its own.

 

Yes, the AI will do this. We live in a hilly city and even with just me driving with no passengers or load it will downshift on it's own. It's programmed to do that. Sometimes it can be annoying. However, when the truck is fully loaded with 4 passengers and a full load in the box going through steep mountain passes I always use S-mode and enable ECT. I never rely on the AI. I like to be in controll, not the computer.

 

The 3.5 is certainly not like my 4.7 v8 when it comes to that. I just chalked it up to a smaller displacement engine, but what you mention above probably affects it too. I like how the transmission reacts when going down a steep hill in 6th...soon as the brake is applied it downshifts to 4th.

 

The reason its likely not assumed is many of us have no issues with engine braking on descents. I drive the passes frequently here in CO and have zero issues with it, manually downshifting in S-Mode. I'll even occasionally engine brake to a traffic light on a flat road...