downshifting automatic tranny

[KingKevin]

Alright so I have a gen 1 solara with an automatic transmission. When coming to a stop I sometimes downshift to 2 andthen L to have some engine braking to conserve my brakes. Question is is this good for the car? Is it bad for the transmission? This probably applies to gen 2s as well which is why im posting it here. Somebody told me that it was good for the engine to do this... but I don't want to prematurely ruin my tranny, especially since I like to step on it every once in a while

[Black Bob]

Engine braking won't overall harm your engine or tranny as long as there both in good working condition in the first place. Engine braking creates extra minute wear in both briefly but that is the equivalent of normally revving your engine while parked. As far as engine braking to prolong the life of your pads and rotors is concerned, that's splitting hairs--engine braking 40% of the time and normal braking 60% of the time is not gonna show any noticeable difference from a person that brakes 100% of the time. Overall it's just plain hard to gage brake wear from just solely two different methods because too many other factors are involved in a typical commute--passenger weights, avoiding roadkill, avoiding becoming roadkill, jerks cutting you off, etc. etc. In other words, engine braking is mainly habit and deson't give you any noticeable advantage or disadvantage. The only thing you should never do is rev the engine while engine braking--bad for both the engine and tranny. Truckers engine brake because they tow anywhere from zero to forty tons on average and their brakes heat up pretty damn bad if they use them 100% in braking, keeping their brakes cool by engine braking is not a habit, it's their life-saver.

[1gSE]

^^ with that said...brakes are routine maintenance parts that are meant to be replaced when worn down....its cheaper and easier to replace pads/rotors than it is to replace an engine/transmission.....and with that said...I rev match, brake hard, grind gears, miss shifts etc etc. Cars are meant to be driven

[dinoxor]

also I read that engine braking is just gonna use up more gas since in term your downshifting is resulting in unnecessarily high rev.
I would rather use my normal brakes because it would be easier to replace and like 1gse said, it was meant to be replaced.

plus after purchasing one brake pads, my auto part store will always give me a free pair upon returning the old pads due to their lifetime warranty, even if it's just worn down from normal wear and tear

[ThurzNite]

also I read that engine braking is just gonna use up more gas since in term your downshifting is resulting in unnecessarily high rev.

I'd like to challenge that. Engine braking is a method of hypermiling because there is no injection of fuel. This's true of modern engines. I don't have a FSM but I'm sure it's in there.

I doubt solara automatic tranny drivers are trying to rev-match when downshift engine braking, so I doubt you're referring to that. Correct me if I'm wrong.

Dr J

[Mole]

i agree, no gas pedal pressed, downshifted, slowing down should not be using any gas.

[dinoxor]

ah then I stand corrected
so does that mean downshifting to slow down is recommended?

[1gSE]

also I read that engine braking is just gonna use up more gas since in term your downshifting is resulting in unnecessarily high rev.

I'd like to challenge that. Engine braking is a method of hypermiling because there is no injection of fuel. This's true of modern engines. I don't have a FSM but I'm sure it's in there.
Dr J

^^^ This. Thats why my wideband shows 22.x on the gauge when I'm off the throttle

[ThurzNite]

As far as conserving brakes and the labor/cost of replacing brake components vs everything else... I drive 5sp, and I downshift to 3rd, and sometimes to 2nd. I've had to replace my brakes, but clutch seems to be holding up. In my previous cars, cost of replacing brakes has always been higher than cost of servicing clutch.

But I don't know about wear on engine. I have no way to measure that.

I also don't know about automatic transmissions. I have limited knowledge in that area, so someone correct me if I'm wrong. The gears in auto are controlled by solenoids, which control valves that change the gears. Both solenoids and valves are points of stress and can fail. If you don't rev-match, the difference in hydraulic pressures can damage the solenoids and valves. This's highly simplified explanation, so I'll let an expert chime in.

Which leaves me to believe that engine braking is fine, but it's the rev-matching that causes stress in the drivetrain.

Dr J

[JonB]

Instead of asking us if it is ok to downshift your auto, you should be asking yourself why you are trying to conserve your brakes.

[KingKevin]

Not necessarily trying to conserve my brakes. It give me better stopping distance and I was told it was good for the engine.

[ThurzNite]

Not necessarily trying to conserve my brakes. It give me better stopping distance and I was told it was good for the engine.

If it shortens stopping distance, I wonder why magazines and reviews don't downshift when they're doing their 60mph -> 0mph tests?
Dr J

[crispone]

Not necessarily trying to conserve my brakes. It give me better stopping distance and I was told it was good for the engine.

False. Engine braking is NOT "GOOD" for your engine, tranny or other aspects of your "drive train". It MAY indeed reduce the incremental brake wear that is expended if you DON'T engine brake, but fundamentally, the engine, is NOT a brake. Using it to create drive train resistance to decelerate the car is placing the loads of momentum onto the entire system, CV joints, transmission components, engine components and has it's non-value add (and even outright negative) effect. Wear on brake pads and systems alleviates transfer of forward momentum forces through all the rest of the drive train. The "buck" stops AT the wheel's point of rotation, no transfer and distributed parasitic load loss.

Don't get me wrong, there IS a time and place to utilize engine braking for a NUMBER of reasons. Performance, balance, load distribution under decel that creates drive-train based "drag" on rear (in rear-drive cars) to stabilize decel on poor traction surfaces, etc, etc... so engine braking should not be dismissed altogether, rather understood and used accordingly. However, fundamentally, you should shave speed with the brakes as your primary deceleration source, and EVEN when engine braking, there should be a combination of BOTH utilized for optimum effect.

As far as "damage" from downshifting an A/T... so long as you don't do it from excessively high RPM points, the car is designed to handle it without major risk.

NOTE: As a point of reference, in HYBRID or electric vehicles, the deceleration is positively transferred through the drive train deliberately to load the electric motor in the drive system and "capture" the energy through regenerative braking principles which recharge the battery by causing the electric motor to now act like a "generator" or "alternator". Both good for decel aid along with the brakes AND functional in a value-add way. Not so with a non-electric system, where the mechanical transfer back of momentum through the system is being dissipated as HEAT through various units. Heat is NEVER a good element to introduce INTO the motor and/or tranny or drive train unnecessarily.

(Energy is never "lost", just transferred to another form... right?)


http://csep10.phys.utk.edu/astr161/lect ... 3laws.html


^^^ Newton's three law's for reference...




-crisp

[KingKevin]

@crisp Yes i agree with your points. Lol im in AP physics i know Newtons laws etc... I never use just engine braking, i combine it with the brakes and i usually use just brakes, but occasionally I downshift when coming at a stop for example at the bottom of a hill or something of that nature. ALSO the reason i was told it was "good" for the engine was that it bathes the engine in oil with limited impurities (aka gasoline). What you have said tho makes sense and i take all that into account. Thanks for the response

[crispone]

@crisp Yes i agree with your points. Lol im in AP physics i know Newtons laws etc... I never use just engine braking, i combine it with the brakes and i usually use just brakes, but occasionally I downshift when coming at a stop for example at the bottom of a hill or something of that nature. ALSO the reason i was told it was "good" for the engine was that it bathes the engine in oil with limited impurities (aka gasoline). What you have said tho makes sense and i take all that into account. Thanks for the response

Interesting basis for the "good for motor" logic. "Bathes" the motor. This would seem to be logical, but it is heavily dependent on the LOAD that you are incurring along with this "bathing" action. IF the "engine braking" is under moderate and less aggressive deceleration, then theoretically, the oil SHOULD be lubricating at a "higher rate" and more plentiful dispersion and flow than decelerating with brakes only and engine "dropping" out of the equation to idle. If it is a HARSH transfer of load from high RPM or on/off throttle, that could potentially out-weigh the additional lubrication's benefit factor. At the same time, the engine "dropping" back to idle SHOULD still be lubricating effectively (and hence, "bathing") the motor with oil but WITHOUT the "back-lash" load of the momentum, weight of the vehicle, drive-line load transfer and potentially harsh transfers of load from "on/off throttle" forces, etc...

I think many cases can be made for these different thought processes, but at the end of the day, I'd have to weigh my bias on where and how the heat was being generated and whether it was detrimental or not as a degree in comparison from scenario to scenario.


Fun to expound on the considerations, nonetheless!



-crisp