Downshifting automatics, what happens?

[DadGum_Teen]

I know with manual transmission, downshifting won't hurt anything if you rev-match, and your not over-revving your engine. And I know with an automatic, the kick down linkage will automatically downshift when necessary. But what happens if you floor it and downshift your automatic to a lower gear that ends up around 3500-4000rpm? Can any damage be done? Short term or long term? I have a 727 torqueflight with the factory valve body. And just to throw it out there, what if you were to downshift from 2nd to 1st and end up at a whopping 5500rpm while flooring it? And no, I don't do that, haha. Thanks for any answers. And I hope this thread will also help others wondering the same thing. I have had trouble finding information on google.

 

[MopaR&D]

I drive my A-904-equipped '70 Duster like that all the time. To be honest I think my first gear is wearing out a bit as a full-throttle manual 1st-2nd shift takes longer than it used to but as long as you don't do it for every shift you should be fine. Just make sure you press on the gas before downshifting; don't let off or or downshift and then floor it, unless you're going at a slower speed or RPMs.

 

[toolmanmike]

I was at a Dodge dealer today and we were talking about transmission component rpm (as Scot was installing a planetary unit in one). If the planetary ratio is 3:1 and the engine rpm is 6000 there are parts flying around in there at 18,000 rpm. How do they stay together? toolman

 

[DadGum_Teen]

Good question toolmanmike. Someone will probably post a better answer later, but from the way I see it, the gears that are flying around at 18,000 rpm can withstand it because they are so much smaller in diameter. Take my RC car for example. I have a 1/18th scale buggy with a stupid fast motor and battery. If anyone else messes with rc, its a 8000kv with a 3cell lipo. The motor can spin 88,800rpm's at full charge. And it can easily hold up because the diameter of the shaft is only the size of a pen tip. make sense?

 

[bOb shingler]

nothing will happen to the trans but the motor will have a problem if it's not put to gether with parts to pull up from the rpm that you just down shifted at. lets say that you are in second gear at 5000rpms. the motor will need to buzz alot higher, about 7000rpms to pull your hotrod faster. ok lets say you just down shift from 5000rpms from second to first and don't mash the go pedal the engine will buzz to about 7000rpms just to slow you down. the trans can handle it just find it the motor that needs to have the right parts in it to stay together.

 

[Max340]

I was at a Dodge dealer today and we were talking about transmission component rpm (as Scot was installing a planetary unit in one). If the planetary ratio is 3:1 and the engine rpm is 6000 there are parts flying around in there at 18,000 rpm. How do they stay together? toolman

My thought is that your math is backwards. The planetary, except for the OD unit, would be a reduction ratio to allow mechanical advantage. 3 RPM input = 1 RPM output. 6000RPM engine = 2000RPM output shaft.

 

[toolmanmike]

My thought is that your math is backwards. The planetary, except for the OD unit, would be a reduction ratio to allow mechanical advantage. 3 RPM input = 1 RPM output. 6000RPM engine = 2000RPM output shaft.

You're correct! I was thinking bass ackwards. And an increase in "rotational stiffness" too. Thanks, toolman


http://en.wikipedia.org/wiki/Epicyclic_gearing