semi downshift?
08-12-07, 09:08 PM
#1
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semi downshift?
whats the little semi downshift feeling that the car gives you when you are cruising at 60mph and then say u want to pass someone so u give it gas to get up to 80mph? The tach needle suddenly kicks into life and the engine makes a bit more noise like it is making a mini downshift, but if i press depress the OD button it makes a more distinct downshift.
whats the mini downshift and why does it happen? hmm i guess the best way to describe it is like the engine is trying to swtich on more cylinders but our cars don't have that technology...
whats the mini downshift and why does it happen? hmm i guess the best way to describe it is like the engine is trying to swtich on more cylinders but our cars don't have that technology...
08-12-07, 09:23 PM
#2
I know my SC400 will double kick down between 45-60mph if I floor it.
The double kick down is when the transmission goes from 4th (cruising) to 3rd (because you floored it) to 2nd (because even that wasn't enough for what you were asking of the car).
The transmission is wired to go sequentially through the gears. It is incapable of completely bypassing 3rd and shifting directly from 4th to 2nd.
Perhaps the "semi downshift" sensation that you are feeling is occuring when the car is bypassing 3rd gear as quickly as it can.
When you disengage overdrive, you will be in 3rd to start. Now, when you floor it, the car only has to kick down once. This may be the more "distinct downshift" that you feel with O/D off.
I get a double kick down at 20mph too. Approach 20mph very modestly. You will feel the car shift into third. The RPM's will be real low. Now, floor it. You will go from 3rd to 2nd to 1st; but, the tranny has to scroll through the gears sequentially by design making for the "semi downshift" while 2nd is being bypassed.
I could have totally misread your post and be entirely off base here. But, this is the best guess I can offer.
The double kick down is when the transmission goes from 4th (cruising) to 3rd (because you floored it) to 2nd (because even that wasn't enough for what you were asking of the car).
The transmission is wired to go sequentially through the gears. It is incapable of completely bypassing 3rd and shifting directly from 4th to 2nd.
Perhaps the "semi downshift" sensation that you are feeling is occuring when the car is bypassing 3rd gear as quickly as it can.
When you disengage overdrive, you will be in 3rd to start. Now, when you floor it, the car only has to kick down once. This may be the more "distinct downshift" that you feel with O/D off.
I get a double kick down at 20mph too. Approach 20mph very modestly. You will feel the car shift into third. The RPM's will be real low. Now, floor it. You will go from 3rd to 2nd to 1st; but, the tranny has to scroll through the gears sequentially by design making for the "semi downshift" while 2nd is being bypassed.
I could have totally misread your post and be entirely off base here. But, this is the best guess I can offer.
08-13-07, 08:53 AM
#3
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Yeah thats on the button. The 4 speed auto is 3 gears and O/D(4th gear) so cruising at 60MPH you are in O/D(4th gear). If you press the gas a tad bit more it will down shift to 3rd. With the O/D(4th gear) off you are always in 3rd, therfore you get that extra umph, because its downshifting to 2nd when you step on the gas hard.
08-13-07, 08:58 AM
#5
Lexus Test Driver
i understand what ttttp is saying. I experienced it too in my SC. Try going 50mph, you'll be in 4th gear. Then gently push the gas down (but not enough to downshift to third), you'll feel a slight kick down and the revs will jump to about 2k and the car will accelerate quicker, but during this time it's STILL in 4th gear. After you feel this initial kickdown, if you push the O/D button and turn it OFF, you'll feel a more distinct kick as the car downshifts to third and revs jump even higher closer to 3k. It's just how the transmission operates, to save you that extra downshift in case you need to accelerate moderately but not too slow or too fast.
Note: This is all with ECT PWR OFF. If you turn ECT PWR ON, the initial kickdown mentioned earlier will actually be the more aggressive downshift!
Note: This is all with ECT PWR OFF. If you turn ECT PWR ON, the initial kickdown mentioned earlier will actually be the more aggressive downshift!
08-13-07, 09:00 AM
#6
i understand what ttttp is saying. I experienced it too in my SC. Try going 50mph, you'll be in 4th gear. Then gently push the gas down (but not enough to downshift to third), you'll feel a slight kick down and the revs will jump to about 2k and the car will accelerate quicker, but during this time it's STILL in 4th gear. After you feel this initial kickdown, if you push the O/D button and turn it OFF, you'll feel a more distinct kick as the car downshifts to third and revs jump even higher closer to 3k. It's just how the transmission operates, to save you that extra downshift in case you need to accelerate moderately but not too slow or too fast.
Note: This is all with ECT PWR OFF. If you turn ECT PWR ON, the initial kickdown mentioned earlier will actually be the more aggressive downshift!
Note: This is all with ECT PWR OFF. If you turn ECT PWR ON, the initial kickdown mentioned earlier will actually be the more aggressive downshift!
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08-13-07, 11:35 AM
#10
Zombie Slayer
The torque converter engages the driveline once the transmission reaches a certain RPM range. I believe our SC4's have a TC with a 1800-2000rpm stall speed (or the rpm figure where the driveline is engaged). This explains why you notice the "jump" to 2000rpms. When pushing the accelerator, the rpms rise to the stall speed of the converter at which time you now have "lock-up" or a connection between the engine/transmission/rear-diff.
08-13-07, 06:54 PM
#13
Pole Position
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thanks for all the input.
the auto tranny was a very genius invention. i drove a stick shift before so I didn't quite understand how an auto works.
the auto tranny was a very genius invention. i drove a stick shift before so I didn't quite understand how an auto works.
08-14-07, 07:50 PM
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I know the link has been posted before , as a post from another CL member is how i heard of it.... But for those who arent familiar with it. Theres alot of useful info on this topic , and alot of other topics as well....
http://auto.howstuffworks.com/torque-converter.htm