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  Reply # 1686578 12-Dec-2016 15:47
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Bung:

 

Fred99:

 

 

 

Switching OD off causes that increase in RPM because you've immediately only got hydraulic coupling complete with inherent inefficiencies / slip.

 

Sure there's an output from the trans before it goes to the diff, but it's all in one FWD transaxle unit whether that shaft is turning slower or faster than the input is kind of irrelevant, final drive ratio through the diff and size of the wheels determines RPM at xx km/h, it made more sense technically to use the term when "OD" was a over-geared unit separate from the main gearbox back in the olden days.  An "extra" you'd pay some pounds for on a jaguar etc.

 

I don't think we're arguing about anything here are we?  I just tend to think that "Overdrive" is/was used as the term for what they did with automatics, 'cause even though it was almost meaningless relative to the origins of the term, they could print "OD" on the button, people who didn't know what a torque converter is kind of knew what the effect was.

 

 

I think we are arguing. As far as the OD switch is concerned on an auto it locks out the top gear. Even though 4th in my trans is 1:1 it is still described as OD. If I am cruising at say 80km/h there may be inefficiencies in the torque converter but there won't be much slip. If I switch OD off I'll be in 3rd. Torque converter lock up clutches are more recent than OD switches

 

 

 

 

I think it depends on model (and perhaps maker), but I suspect you may have it wrong.

 

The Nissan I mentioned ('97) has an "OD" button.  It only has 3 mechanical gear ratios in the trans.  The increase in engine RPM with OD "off" in top gear isn't because it drops a gear, but because the direct drive through the clutch is off and it reverts to hydraulic drive through the torque converter which is "effectively" a lower ratio than 1:1.  It feels like 4 gears, but it's really only 3.

 

There's always slip in a torque converter.  Put it in gear with your foot on the brakes so it doesn't move, press the accelerator and the engine will rev but you won't go anywhere.

 

I think Toyotas etc of that era are the same.




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  Reply # 1686604 12-Dec-2016 17:00
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jonathan18:

 

Have you had a good look at the Jazz? 

 

 

CVT? 2005ish model?

 

You are embarassed to be in a car? who gives a .... in a $5k car? Jonathon are you a Jafa too? It passes with age :-).

 

Honestly I dont mind the old demio shape.


 
 
 
 


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  Reply # 1686610 12-Dec-2016 17:13
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TeaLeaf:

 

jonathan18:

 

Have you had a good look at the Jazz? 

 

 

CVT? 2005ish model?

 

 

 

 

Avoid 02-06 Jazz with CVT.

 

Even if they've got FSH, I'd still be very wary.

 

Google is your friend.


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  Reply # 1686628 12-Dec-2016 18:30
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Fred99:

 

The Nissan I mentioned ('97) has an "OD" button.  It only has 3 mechanical gear ratios in the trans.  The increase in engine RPM with OD "off" in top gear isn't because it drops a gear, but because the direct drive through the clutch is off and it reverts to hydraulic drive through the torque converter which is "effectively" a lower ratio than 1:1.  It feels like 4 gears, but it's really only 3.

 

There's always slip in a torque converter.  Put it in gear with your foot on the brakes so it doesn't move, press the accelerator and the engine will rev but you won't go anywhere.

 

I think Toyotas etc of that era are the same.

 

 

I'm not a mechanic but I think you have that wrong.  A torque converter should not slip except when the car is stationary or moving very slowly - otherwise a huge amount of power would be wasted, and heat generated, in the torque converter.  From my observations with such transmissions, the gear ratio (ie speed vs engine revs) is always constant for any gear (except when stopped/crawling), ie there is no slip.  If there was slip, you'd see revs increase instantly as you increased power without a gear change happening.  The "OD" button in most transmisisons - again from my experience - prevents the highest gear from being used, so if you press the OD button when in top gear, you'll get an immediate rev increase as the transmission changes down.




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  Reply # 1686652 12-Dec-2016 19:32
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Im gonna say that sounds logical as torque Autos are Jerky.

 

So Tiptronics are pretty safe and not $5k to replace like the DSG? sheesh.


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  Reply # 1686653 12-Dec-2016 19:34
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TeaLeaf:

 

Im gonna say that sounds logical as torque Autos are Jerky.

 

So Tiptronics are pretty safe and not $5k to replace like the DSG? sheesh.

 

 

Tiptronic isn't a different system, it's just a way of manually controlling an auto transmission.  It'll still be the same torque convertor or CVT as a non-tiptronic




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  Reply # 1686668 12-Dec-2016 19:58
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Darn most cars are tiptronic now, but you dont hear a lot problems about them like cvt or DSG???


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  Reply # 1686672 12-Dec-2016 20:17
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TeaLeaf:

 

Darn most cars are tiptronic now, but you dont hear a lot problems about them like cvt or DSG???

 

 

See above.  Tiptronic isn't a type of transmission, it's a way of controlling a transmission.  You can get "tiptronic" CVT which has a series of artificial ratios which you can select manually.

 

A couple of good articles below if you're still confused:

 

http://www.telegraph.co.uk/cars/advice/which-sort-of-automatic-gearbox-is-best/

 

http://www.caradvice.com.au/278721/transmissions-explained-manual-v-automatic-v-dual-clutch-v-cvt-v-others/




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  Reply # 1686675 12-Dec-2016 20:19
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hey that first link i posted haha.

 

yeah i get it, i know it doesnt describe anything other than shifting but it doesnt use clutches grinding together etc.

 

can a normal torque box have a tiptronic?


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  Reply # 1686677 12-Dec-2016 20:27
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shk292:

 

Fred99:

 

The Nissan I mentioned ('97) has an "OD" button.  It only has 3 mechanical gear ratios in the trans.  The increase in engine RPM with OD "off" in top gear isn't because it drops a gear, but because the direct drive through the clutch is off and it reverts to hydraulic drive through the torque converter which is "effectively" a lower ratio than 1:1.  It feels like 4 gears, but it's really only 3.

 

There's always slip in a torque converter.  Put it in gear with your foot on the brakes so it doesn't move, press the accelerator and the engine will rev but you won't go anywhere.

 

I think Toyotas etc of that era are the same.

 

 

I'm not a mechanic but I think you have that wrong.  A torque converter should not slip except when the car is stationary or moving very slowly - otherwise a huge amount of power would be wasted, and heat generated, in the torque converter. 

 

 

I don't have that wrong, and a huge amount of power is wasted and a lot of heat generated, hence separate heat exchanger for oil-cooling for slushbox autos by way of either separate radiator, heat exchanger for the auto trans fluid built into the main coolant radiator, or compact separate fluid to fluid heat exchanger mounted on the side of modern slushbox autos (looks a bit like an oil filter, but plumbed to engine coolant system).

 

That's why they use lock-up clutches since the '90s - and in the case of recent Mazda SkyActiv, lock-up in all forward gears, as the energy waste from the torque converter is substantial.  Now Mazda have to deal with all the extra crap (wear particles and heat) from the slip on the clutches engaging and disengaging much more as that's happening with every shift in every ratio, so much larger and more expensive torque converter with bigger clutches, trans oil filtration and very very important service intervals.  So far they seem to be okay - and IMO a good alternative for a fuel efficient auto for around town commuting, where I don't like DSG much, and don't trust CVTs based on their shocking history.

 

It's also why DSG and CVT became common - as they're much more efficient than conventional slush-box. (in the case of DSG then even faster acceleration times than manual transmission because the shifts are slicker/better timed than a human can achieve, more economical than manual because computers are smarter than humans at crunching numbers to work out the best time to shift ratio)

 

 




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  Reply # 1686679 12-Dec-2016 20:30
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TeaLeaf:

 

hey that first link i posted haha.

 

yeah i get it, i know it doesnt describe anything other than shifting but it doesnt use clutches grinding together etc.

 

can a normal torque box have a tiptronic?

 

 

yeah so tiptronics are just a manual way to change gears in a torque gearbox, up and down, and have safety features ie, your too slow, your red lining and will adjust gear regardless, so why bother i think. good old autos are a big deal. we arent talking Mclarens here, its a Mazda Axela or demio rofl.


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  Reply # 1686697 12-Dec-2016 20:39
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Fred99:

 

That's why they use lock-up clutches since the '90s - and in the case of recent Mazda SkyActiv, lock-up in all forward gears, as the energy waste from the torque converter is substantial.  Now Mazda have to deal with all the extra crap (wear particles and heat) from the slip on the clutches engaging and disengaging much more as that's happening with every shift in every ratio, so much larger and more expensive torque converter with bigger clutches, trans oil filtration and very very important service intervals.  

 

I would have thought that engaging and disengaging a clutch on each gear change is that same as what a DSG does? Or even a traditional manual?


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  Reply # 1686700 12-Dec-2016 20:42
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Fred99:

 

I don't have that wrong, and a huge amount of power is wasted and a lot of heat generated, hence separate heat exchanger for oil-cooling for slushbox autos by way of either separate radiator, heat exchanger for the auto trans fluid built into the main coolant radiator, or compact separate fluid to fluid heat exchanger mounted on the side of modern slushbox autos (looks a bit like an oil filter, but plumbed to engine coolant system).

 

That's why they use lock-up clutches since the '90s - and in the case of recent Mazda SkyActiv, lock-up in all forward gears, as the energy waste from the torque converter is substantial.  Now Mazda have to deal with all the extra crap (wear particles and heat) from the slip on the clutches engaging and disengaging much more as that's happening with every shift in every ratio, so much larger and more expensive torque converter with bigger clutches, trans oil filtration and very very important service intervals.  So far they seem to be okay - and IMO a good alternative for a fuel efficient auto for around town commuting, where I don't like DSG much, and don't trust CVTs based on their shocking history.

 

It's also why DSG and CVT became common - as they're much more efficient than conventional slush-box. (in the case of DSG then even faster acceleration times than manual transmission because the shifts are slicker/better timed than a human can achieve, more economical than manual because computers are smarter than humans at crunching numbers to work out the best time to shift ratio)

 

 

 

 

I stand corrected - and I have just found out that the six-speed auto box in my car has lock-up in all but first, which explains how it behaves

 

It's a shame that CVT isn't more reliable because it should be the perfect system.


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  Reply # 1686720 12-Dec-2016 21:04
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alasta:

 

Fred99:

 

That's why they use lock-up clutches since the '90s - and in the case of recent Mazda SkyActiv, lock-up in all forward gears, as the energy waste from the torque converter is substantial.  Now Mazda have to deal with all the extra crap (wear particles and heat) from the slip on the clutches engaging and disengaging much more as that's happening with every shift in every ratio, so much larger and more expensive torque converter with bigger clutches, trans oil filtration and very very important service intervals.  

 

I would have thought that engaging and disengaging a clutch on each gear change is that same as what a DSG does? Or even a traditional manual?

 

]

 

 

 

Kind of but not really - as it's giving 1:1 direct drive with no slip when the clutch is engaged and using the torque converter as a primary clutch to give smooth shifts between ratios, the torque converter clutch engaging as the crank and gearbox input shafts are getting near synchronised.  But they still use standard autos with complex drives, planetary gears etc.

 

They are (IMO) a reasonable compromise for better smoothness than a typical DSG, and almost as efficient / economical.


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  Reply # 1686730 12-Dec-2016 21:33
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shk292:

 

 

 

It's a shame that CVT isn't more reliable because it should be the perfect system.

 

 

 

 

Yeah - but it's that horrible idea that power should be transferred by friction between components, when to get variable ratio the contact area has to be small, thus there's always got to be some slippage.

 

Nissan have managed to whack their "extroid" CVTs in some higher powered cars (but for performance they still use a DSG in the GTR)

 

 Extroid works like this schematic :

 

 

That's a lot of power being transferred by friction with small contact area.  IMO it's a horrible idea - but it works.

 

Electric (with maximum torque available from 0 rpm up) solves the problem.


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