why is manual faster than auto ?

[Blip]

Ok my questions is why does a well-driven manual car accelerate better than the equivilant auto ?

My understanding is that with a manual the clutch disc has to be separated then repositioned on the flywheel at every gear change, but with an automatic car, the torque converter means the drive train is always connected to the engine.

So shouldn't this result in better acceleration for auto car ?
Or is the fluid inside torque converters not very efficient in transferring the spinning motion ?

I'm trying to learn about technical aspects of cars so any help would be appreciated

[gucom]

Or is the fluid inside torque converters not very efficient in transferring the spinning motion ?

thats the thing exactly...its very thick oil or something like that, but it always results in a power loss

[Wutputt]

The main reason is idd the efficiency of an automatic, it's worse compared to a manual. This is caused primarily by the torque converter. When it slips (and it does this almost the whole time when you accelerate) it has a low efficiency. Secondly an automatic needs a hydraulic circuit (and cooling) to be operated, which uses power.

On the other hand the initial acceleration (first second) of an automatic can be higher than an equivalent manual. The reason for this is the fact the torque converter has a stator which raises the torque converted to the transmission itself. At high slip this torque multiplication can reach values of 2.5, but it drops vastly when the slip within the torque converter is reduced.

But overall this torque multiplication can't overrule the efficiency losses of an automatic, so the total acceleration times will be slower.

Sometimes the automatic also has 1 gear less than the manual, in which case the gear ratios of the automatic will be longer, which is a drawback for fast acceleration times.

[ViperASR]

wow, thats really interesting, i never knew why manual was faster than auto, now i do. Some of you guys really seem to know what you are talking about. I think that eventually manual will become non existent because everyone will want to drive the fast and smooth shifting autos. Look at alot of todays faster cars, some dont even have a manual as an option.

[Wutputt]

Originally Posted by thamar But with VW it's new double clutch system, even the fastest shifting person gets beaten in the upshifts. Not in the downshifts, there the system is slow.

That's not an automatic transmission, but an automated manual transmission, though with two clutches.

Due to this split the gearbox in two separated gearboxes concept, it's fast when changing between odd and even gears. Upshifts, downshifts it doesn't matter, though downshifts may be a bit slower under certain conditions. It is idd slow when you want to change gears between two even or two odd gears, cause then it can't take advantage of its double clutch system.

[gbg]

Originally Posted by ViperASR wow, thats really interesting, i never knew why manual was faster than auto, now i do. Some of you guys really seem to know what you are talking about. I think that eventually manual will become non existent because everyone will want to drive the fast and smooth shifting autos. Look at alot of todays faster cars, some dont even have a manual as an option.

I would be not happy at all to drive a fast and smooth shifting auto Ferrari

The automatic gearbox is cool for everyday driving, but it does sucks when you want some performance; I'd rather have an auto-manual gearbox (i.e. tiptronic, autostick, etc.), they at least give you some feeling when you floor it

[Wutputt]

^^
Tiptronic and autostick still are conventional automatic transmissions, only with some gadgetry added

The more performance like transmissions your after - I guess - are automated manual transmissions, like Ferraris 'F1', BMW's SMG, Mercedes' Sequentronic, Alfa's Selespeed, Maserati's Cambiocorsa, Lambo's E-gear, and the double clutched ones like Porsches PDK and VW/Audi's DSG, etc.

[|Nuno|]

It's been said pretty much everything, but I just wanted to add that weight is another factor why manuals are faster.

Automatic transmission are heavier than manuals, which will obviously harm performance.

Just look at the 911 Turbo S:


Weight:

Unladen weight (DIN) 1,590 kg (Manual) / 1,630 kg (Tiptronic)

Unladen weight (EC directive) 1,665 kg (Manual) / 1,705 kg (Tiptronic)



Performance: Manual gearbox Tiptronic S


Top speed 305 km/h 298 km/h

0-100 Km/h 4.2 secs 4.9 secs

0-160 Km/h 9.3 secs 10.4 secs

Flexibility 80-120 km/h 4.8 s 5.5 s

In this particualr case, the torque curve up to 3500 rpm's is actually better on the Tiptronic car (see Wutputt's post for the reason), but still the manual is a lot faster.

But the Tiptronic used by Porsche is rubish, so with modern Autos the performance will be better, but still inferior to manuals.

[Blip]

Originally Posted by Wutputt The main reason is idd the efficiency of an automatic, it's worse compared to a manual. This is caused primarily by the torque converter. When it slips (and it does this almost the whole time when you accelerate) it has a low efficiency. Secondly an automatic needs a hydraulic circuit (and cooling) to be operated, which uses power. On the other hand the initial acceleration (first second) of an automatic can be higher than an equivalent manual. The reason for this is the fact the torque converter has a stator which raises the torque converted to the transmission itself. At high slip this torque multiplication can reach values of 2.5, but it drops vastly when the slip within the torque converter is reduced. But overall this torque multiplication can't overrule the efficiency losses of an automatic, so the total acceleration times will be slower. Sometimes the automatic also has 1 gear less than the manual, in which case the gear ratios of the automatic will be longer, which is a drawback for fast acceleration times.

I'm a little unclear on slippage.
It occurs when the impeller is spinning faster than the turbine, and it is bad because of loss of power. But at the same time it is good when there is high slippage since more torque is produced (2.5x) ? So why cannot we have high slippage all the time, can we force the transmission to always be slower than the engine so that we get more torque

[Wutputt]

Originally Posted by Blip I'm a little unclear on slippage. It occurs when the impeller is spinning faster than the turbine, and it is bad because of loss of power. But at the same time it is good when there is high slippage since more torque is produced (2.5x) ? So why cannot we have high slippage all the time, can we force the transmission to always be slower than the engine so that we get more torque

A the torque converter always wants to reduce the slippage and there are some good reasons for it. Slippage is not something you want if you want to get good performance figures.

If the torque converter would constantly be in high slippage (needed to have a significant torque increase) all the time, your engine would be constantly running at high revs without the car getting that fast forward, but with a really bad fuel economy. Secondly a lot of slippage means a lot of power losses, so your automatic transmission would have a really bad efficiency. Great power losses also mean a lot heat generation, which can cause failures.

What happens when you put your foot down in a car with an automatic is the engine rpm will raise suddenly. The torque converter just can't follow this sudden raise and slips (the positive side of this slip is rhe fact it's very comfortable). Then the torque converter counteracts this slip (turbine wants to turn at the same rpm as the impeller) and the car start accelerating. Due to the stator between impeller and turbine the slow acceleration caused by the slip is improved. But the stator isn't a miracle that creates torque out of nothing it just helps improving the otherwise sluggish acceleration.

[sameerrao]

The reasons put forward for the superiority of the manual has been well stated by the others above.

I think that a normal driver under normal conditions will be quicker in an automatic - given the same car with the same number of gears (for auto and manual). Unless of course you are willing to be brutal with the clutch.

Also today's torqueir cars disguise any weight penalties or torque convertor losses pretty well.

But anyway faster or not .. manuals are 10X more fun and involving.

[Blip]

Thanks a lot everyone for your excellent explanations.
I knew i could count on the experts here at JW.

[T-Bird]

OK the main reason Manuals are faster than Automatics are the fact that manuals from the factory are designed to shift smoothly so they reduce power from the engine as it shifts through various different control devices. If you can change the shift timing and firmness (line pressure) it is possible to have a faster Automatic then Manual but there is a possibility that things may start to fail but if your using a proven transmission they can take a beating. The Slipping that occurs in the converter isn't the problem under normal conditions and if the fluid in the car was too thin it would do it's job and if it's thick like motor oil you have some major problems. An automatic will always make more Torque when matched against the same exact engine as a Manual so they can really push hard and if you manual shift an Automatic like professional Drag Cars do you can out run most Manuals and get way more consistent times.

[AlienDB7]

Originally Posted by Blip I'm a little unclear on slippage. It occurs when the impeller is spinning faster than the turbine, and it is bad because of loss of power. But at the same time it is good when there is high slippage since more torque is produced (2.5x) ? So why cannot we have high slippage all the time, can we force the transmission to always be slower than the engine so that we get more torque

Slippage is BAD on an automatic tranny. Heat is generated when slippage occurs, and as we all (or should?) know, excessive heat is one of the main causes of tranny failure.

You probably heard of the auto tranny failures on hondas, from what I've read, it's mostly caused by constant slippage when the tranny is trying to generate enough torque to compensate for the low-torque VTEC system.

[caneswell]

Originally Posted by ViperASR wow, thats really interesting, i never knew why manual was faster than auto, now i do. Some of you guys really seem to know what you are talking about. I think that eventually manual will become non existent because everyone will want to drive the fast and smooth shifting autos. Look at alot of todays faster cars, some dont even have a manual as an option.

I hope not, autos are horrible! I would never buy one. Flappy paddle gearboxes are also pretty bad. Most simply don't work, and even with the good ones its more enjoyable to drive a manual.