Down Shifting

[Crusin350]

Is it bad to downshift an automatic?

sometimes when im getting off the interstate I will turn off the overdrive to help slow my truck down. I do it around 40 mph so it it isn't so hard on the drive train, it is a pretty soft shift. but i got thinking maybe its not good for it... Anyone else do this ever, or know if its bad for my truck????

 

[Got Diesel]

I hope it's not bad for it. I sure don't think that it is. I will do the same thing when cruising around 40 or 45, this way it won't keep dpwnshifting when I go toget back on it.

 

[HCAbowman]

I do that also...I dont see how that can hurt anything-

 

[JohnBoyToo]

Have been doing something like that on all 3 Ex's....
I just put it in tow haul and tap/ride the brake as needed...

saves my brakes....

 

[Crusin350]

Glad im not the only one... Probably Ok to do it but didnt know?

 

[Hoss 350]

Is it bad to downshift an automatic?

sometimes when im getting off the interstate I will turn off the overdrive to help slow my truck down. I do it around 40 mph so it it isn't so hard on the drive train, it is a pretty soft shift. but i got thinking maybe its not good for it... Anyone else do this ever, or know if its bad for my truck????

You're doing fine. This is what the transmission was made to do. THink about it... You pull at wide open throttle towing huge weight, (full horsepower transmitted through transmission, say 275 HP? Multiply by gearing and weight of the trailer, and the torque is in the thousands of footpounds). Now, figure 25 braking horsepower (at most) out of your engine on compression, multiply by gearing and weight, and your torque foot pounds are probably only in the hundreds.

Torque wrecks trannies. Compression braking is very low torque in comparison to other activities you may be using your truck for. Downshifting is only dangerous because you can overspeed your engine (governor or no) if you grab too low a gear at too high a speed. Barring that, downhift away, my friend!

 

[Crusin350]

Wow..... Im glad to hear that, didnt really understan it until I read that last part.

Barring that, downhift away, my friend!

Thanks Hoss!

 

[94f450sd]

on the track i have to downshift to slow down enough to make the corner at the end.i gotta really squeez the brakes and downshift.if not i will end up in the sand trap that is 1/4 mile from the finish line.

 

[Hoss 350]

Wow..... Im glad to hear that, didnt really understan it until I read that last part.

Really quite simple, it is just a matter of horsepower. I probably overcomplicated it. Just for your knowledge, let me try to explain what I meant a little more simply.

You have a 10K load on your truck, going 60 MPH in Drive. You floor the throttle. The transmission downshifts under an input horsepower of 275 (from the engine). Granted, the engine defuels at shifts (I think) to reduce the ponies put through the tranny at shifts, but it is still likely over 200 horsepower. There is a LOT of stress on your tranny during this downshift, but it is something you probably would feel is within the normal operating parameters of the tranny and not feel too worried about it.

Or... You have a 10K load on your truck, going 60 MPH in drive, you let off the throttle and downshift (as soon as you slow down enough so the RPMs at downshift will not be over redline) to second. YOur engine produces about 25 horsepower (SWAG, but probably close) of braking (or retarding, negative) horsepower. There is 8 times less stress in the tranny at this point than there was on the WOT downshift (probably less, if the TC disconnects, but lets say it doesn't). Additionally, the higher RPMs and no load keep the fluid circulating quickly through the tranny cooler and through the tranny, and the low load situation doesn't produce much heat, so downshifting and coasting is a good way to cool your tranny off...

None of this is scientific, but it is pretty close to the way it works. Hope that helps...

 

[Crusin350]

Yeah, thats alot easier to understand. But now after reading the first one again it makes more sense too. lol I guess I just didnt have my mind on the right track the first time.