Downshift in automatic?

[shu5892001]

Hi guys,

The last couple of days I tried going easy on my brakes and using manual downshifts to slow down, but my car is an automatic... so I am just wondering if it's really bad to down shift in automatic transmissions?

Thanks,
Shu

[STeeLy]

Hi guys,

The last couple of days I tried going easy on my brakes and using manual downshifts to slow down, but my car is an automatic... so I am just wondering if it's really bad to down shift in automatic transmissions?

Thanks,
Shu

I do it all the time on the highway and have never had any trouble going from 4th - 2nd.

The car feels fine. Aside from you having to change your transmission fluid a little earlier than usual, it'll be ok.

[Harbour Rat]

Same as with a manual transmission you have to be sensible and not stick in 2nd at 100 km/h and bend the valves. Other than that you should be fine.

[stock3]

Automatic transmissions have clutch packs that will wear out prematurely, even changing the ATF more often will not prevent extra wear that you will be doing by manually downshifting. So while you're saving brakes, the transmission (which costs thousands) will take a beating. This is not the same as in a manual transmission, where the wear will occur on the clutch itself, and if you properly rev match, virtually no wear will occur. On an automatic slushbox you can't rev match and clutch packs are part of the transmission, so when they go, the transmission pretty much goes as well.

Ask yourself this question, why isn't the transmission programmed to downshift on higher RPM's straight from the factory? Why is it that it only downshifts, when coasting down, only around 1500 rpm or below? The reason is 1) clutch pack wear and 2) Torque converter prevents from proper rev matching.

DSG transmissions, for example, are programmed from the factory (usually in sport mode) to downshift at higher RPM's when slowing down, that's because they have regular clutches that are controlled by actuators so that the computer can do a proper rev match.

Regular automatics with torque converter in high end cars, like Lexus IS-F, lock the torque converter in 2nd gear and onwards, so essentially it acts like a regular clutch, these cars as well are programmed form the factory to downshift.

But our Mazdas have nothing but a regular slushbox, so while downshifting in order to accelerate faster is ok, downshifting just to engine brake is unnecessary wear.

It's your choice

[taz4432]

Same as with a manual transmission you have to be sensible and not stick in 2nd at 100 km/h and bend the valves. Other than that you should be fine.

You won't bend valves doing 100km/h in second. That's near redline (assuming that the 5-speed automatic's ratio is similar/the same as the 5/6-speed manual) and I'd like you to find me a manumatic that will let you - using manual mode - put the car in a gear that'll blow/damage the engine. Yes, you can shift through the 'PRNDL' and make it go into a gear that'll blow things up but using the manumatic feature you won't be able to.

[taz4432]

Automatic transmissions have clutch packs that will wear out prematurely, even changing the ATF more often will not prevent extra wear that you will be doing by manually downshifting. So while you're saving brakes, the transmission (which costs thousands) will take a beating. This is not the same as in a manual transmission, where the wear will occur on the clutch itself, and if you properly rev match, virtually no wear will occur. On an automatic slushbox you can't rev match and clutch packs are part of the transmission, so when they go, the transmission pretty much goes as well.

Ask yourself this question, why isn't the transmission programmed to downshift on higher RPM's straight from the factory? Why is it that it only downshifts, when coasting down, only around 1500 rpm or below? The reason is 1) clutch pack wear and 2) Torque converter prevents from proper rev matching.

DSG transmissions, for example, are programmed from the factory (usually in sport mode) to downshift at higher RPM's when slowing down, that's because they have regular clutches that are controlled by actuators so that the computer can do a proper rev match.

Regular automatics with torque converter in high end cars, like Lexus IS-F, lock the torque converter in 2nd gear and onwards, so essentially it acts like a regular clutch, these cars as well are programmed form the factory to downshift.

But our Mazdas have nothing but a regular slushbox, so while downshifting in order to accelerate faster is ok, downshifting just to engine brake is unnecessary wear.

It's your choice

Great explanation. Definitely much better than I'd be able to give - I'd get too technical.







Edit: Ooops..meant to add this in with my other post...my bad!

[shu5892001]

Automatic transmissions have clutch packs that will wear out prematurely, even changing the ATF more often will not prevent extra wear that you will be doing by manually downshifting. So while you're saving brakes, the transmission (which costs thousands) will take a beating. This is not the same as in a manual transmission, where the wear will occur on the clutch itself, and if you properly rev match, virtually no wear will occur. On an automatic slushbox you can't rev match and clutch packs are part of the transmission, so when they go, the transmission pretty much goes as well.

Ask yourself this question, why isn't the transmission programmed to downshift on higher RPM's straight from the factory? Why is it that it only downshifts, when coasting down, only around 1500 rpm or below? The reason is 1) clutch pack wear and 2) Torque converter prevents from proper rev matching.

DSG transmissions, for example, are programmed from the factory (usually in sport mode) to downshift at higher RPM's when slowing down, that's because they have regular clutches that are controlled by actuators so that the computer can do a proper rev match.

Regular automatics with torque converter in high end cars, like Lexus IS-F, lock the torque converter in 2nd gear and onwards, so essentially it acts like a regular clutch, these cars as well are programmed form the factory to downshift.

But our Mazdas have nothing but a regular slushbox, so while downshifting in order to accelerate faster is ok, downshifting just to engine brake is unnecessary wear.

It's your choice

Thanks for the detailed explanation! I guess I will stop using engine brakes lol

[TheMAN]

the tranny downshifts itself automatically (that's why it's an automatic!) when you do things such as slow down, turn, then accelerate... if you're not downshifting into high rpms (anything past 4000 will be "high rpm"), it won't really hurt it in the long run

[Harbour Rat]

You won't bend valves doing 100km/h in second. That's near redline (assuming that the 5-speed automatic's ratio is similar/the same as the 5/6-speed manual) and I'd like you to find me a manumatic that will let you - using manual mode - put the car in a gear that'll blow/damage the engine. Yes, you can shift through the 'PRNDL' and make it go into a gear that'll blow things up but using the manumatic feature you won't be able to.

I was just making the general point you need to be sensible downshifting an auto, just as you would a manual. I'm sure you're right and downshifting a 5 speed Mazda3 into 2nd at 100 km/h wouldn't over rev the engine. I'm sure I've run mine up to ~100 km/h in 2nd a few times on acceleration and not exceeded redline. I should have used a more ridiculous speed like 150 km/h for my example.

As for manumatics not dropping into a gear which would over rev the engine, that is a good thing but I still would still recommend being sensible and not intentionally trying to force the highest possible rpm on downshifts. If you're racing it makes sense to keep the engine wound up but even on the track you don't want to downshift into a gear which puts you at redline when you let out the clutch - you want to hit the fat part of the power band, for example 5,200-5,500 rpm for my 2.0 L. But the concern here seems to be with longevity in daily street driving, not trimming a couple hundredths off lap times at the track so the performance talk is not terribly relevant.

So, my point is still this: It is fine to manually downshift an automatic but, as with a manual, you need to exercise some common sense. Modern computer controlled/assisted slushboxes may try to protect you from over revving the engine if you get sloppy but constantly pushing it to the limits allowed by the computer will shorten life expectancy compared to operating the transmission in a more moderate, middle of the road sort of style. Basically, if you know enough to be concerned enough to ask if you may do any damage by downshifting manually then you are probably the sort of person who will do their best not to abuse the vehicle so you've already got the important part covered.

[taz4432]

I was just making the general point you need to be sensible downshifting an auto, just as you would a manual. I'm sure you're right and downshifting a 5 speed Mazda3 into 2nd at 100 km/h wouldn't over rev the engine. I'm sure I've run mine up to ~100 km/h in 2nd a few times on acceleration and not exceeded redline. I should have used a more ridiculous speed like 150 km/h for my example.

As for manumatics not dropping into a gear which would over rev the engine, that is a good thing but I still would still recommend being sensible and not intentionally trying to force the highest possible rpm on downshifts. If you're racing it makes sense to keep the engine wound up but even on the track you don't want to downshift into a gear which puts you at redline when you let out the clutch - you want to hit the fat part of the power band, for example 5,200-5,500 rpm for my 2.0 L. But the concern here seems to be with longevity in daily street driving, not trimming a couple hundredths off lap times at the track so the performance talk is not terribly relevant.

So, my point is still this: It is fine to manually downshift an automatic but, as with a manual, you need to exercise some common sense. Modern computer controlled/assisted slushboxes may try to protect you from over revving the engine if you get sloppy but constantly pushing it to the limits allowed by the computer will shorten life expectancy compared to operating the transmission in a more moderate, middle of the road sort of style. Basically, if you know enough to be concerned enough to ask if you may do any damage by downshifting manually then you are probably the sort of person who will do their best not to abuse the vehicle so you've already got the important part covered.

No, I definitely agree with what you have to say and me pointing out the 100km/h thing was just the OCD/Perfectionist me knowing that it was wrong, rather than it being me trying to invalidate your argument. You make some very good points and anyone reading who doesn't already do what you've mentioned would be wise to start doing so now.

[Harbour Rat]

Its all cool, I didn't take it as any kind of attack or anything. Even if I did, this is just the internet so who cares? After all, bits and bytes aren't sticks and stones. Hell, they aren't even words unless we choose to see them as such and even then, like I say, who cares?

[TheMAN]

Its all cool, I didn't take it as any kind of attack or anything. Even if I did, this is just the internet so who cares? After all, bits and bytes aren't sticks and stones. Hell, they aren't even words unless we choose to see them as such and even then, like I say, who cares?

I wish there were more people like you... too many sensitive ones on the intarweb :thumbsup

[taz4432]

Its all cool, I didn't take it as any kind of attack or anything. Even if I did, this is just the internet so who cares? After all, bits and bytes aren't sticks and stones. Hell, they aren't even words unless we choose to see them as such and even then, like I say, who cares?

Mmm...I didn't think you did but it was just to clarify because I've seen people get 'set off' by far less. People often forget how easy it is to turn off the computer and go do something else instead of sitting and flaming each other.

[n00bMeiSter]

I don't get why people are always soo obsessed with "going easy on brakes" or "saving their brakes" and using their engine to slow the vehicle down.
I may be wrong on this, but aren't new brake pads a heck of a lot cheaper than a new clutch (or transmission)?
(I'm talking cheaper in terms of buying the part and getting it installed)

[taz4432]

I don't get why people are always soo obsessed with "going easy on brakes" or "saving their brakes" and using their engine to slow the vehicle down.
I may be wrong on this, but aren't new brake pads a heck of a lot cheaper than a new clutch (or transmission)?
(I'm talking cheaper in terms of buying the part and getting it installed)

Assuming a manual transmission, if one shifts properly then there really shouldn't be any extra wear on the clutch/transmission. I like rev-match downshifting to slow down or heel-toeing if I need to slow down fast. Not only is it good practice, it's fun, it's satisfying to do it right, saves brake life (point of contention) and I'm always ready to accelerate if the light changes or I need to make a quick manouevre. Try doing anything quickly at 20km/h in 4th gear - you might as well get out and push.

Edit - Brake pads wear out far quicker than a clutch. A clutch driven by someone who knows what they're doing can easily last for and over 200k. Brake pads get nowhere near that under normal use and a new set of brake pads (for all 4 wheels) costs me not a ton less than a new clutch assembly.

[TheMAN]

engine braking is essential for panic stops or for those hilly areas, where there will be lots of repeated brakings and you don't want to overheat your brakes!
if your brakes haven't locked up, then engine braking will help slow you down even further in panic situations