[Mark e90]

Im just wondering what it is that makes automatic cars slower than manual cars ? I know that now with some of the new double clutch boxes ( dsg etc ) that this isn't the case but I'm talking about just the standard type auto boxes. is it because the gear changes are slower but acceleration in between gears is the same or does the auto box use up power from the engine reducing the overall power ?

I am looking for a BMW at the moment and have noticed its quite a significant difference in 0-60 times for example 325i Manual - 6.9 and 325i auto is 7.6. Thats almost a full second slower to 60 and the 325i manual almost does the same 0-60 as the 330i auto think there is 0.2 seconds difference !

Originally i had my heart set on a manual but they seem pretty rare even here in the UK so i have started looking at autos now aswell as i really need to be getting a car soon.

Still struggling to find the right one though !

[derryck]

Automatics have traditionally been slower due to a higher percentage of parasitic drive train loss so they're putting less power to the ground. However in recent years 'automatics' have surpassed manual transmission performance. In general these days it's very difficult to match the performance of a good automatic.

[Bamaben]

Dct is quicker than manual but regular auto has more moving parts which causes the parasitic loss of power.

[Fundguy1]

All close but missing the final point:
All autos, dct or not, transfer less power to the rear wheels than manuals including DCT. About 5% +/- less
DCT are faster slightly now because they shift almost instantaneously vs traditional autos, and faster than manuals ever could.
A good driver with a manual who can 2 step or powers hit can often make up the difference or even beat a DCT. Test times by BMW or car mags aren't conducted this way and in most cases you don't want to do this during normal driving.
If it's a linear loss, the difference in power would be more pronounced with higher outputs. So if it's a 15 hp at the rear difference with 300 hp, it old Bo a 30 hp difference with 600hp. This is a guess as I didn't look up to see if it's linear.
Finally, autos are typically used at drag strips because they have more consistency which is the name of the game at the strip.

[Antetokounmpo]

Originally Posted by Fundguy1

All close but missing the final point:
All autos, dct or not, transfer less power to the rear wheels than manuals including DCT. About 5% +/- less
DCT are faster slightly now because they shift almost instantaneously vs traditional autos, and faster than manuals ever could.
A good driver with a manual who can 2 step or powers hit can often make up the difference or even beat a DCT. Test times by BMW or car mags aren't conducted this way and in most cases you don't want to do this during normal driving.
If it's a linear loss, the difference in power would be more pronounced with higher outputs. So if it's a 15 hp at the rear difference with 300 hp, it old Bo a 30 hp difference with 600hp. This is a guess as I didn't look up to see if it's linear.
Finally, autos are typically used at drag strips because they have more consistency which is the named of the game.

1. A dual clutch has much less parasitic loss than a torque converter automatic. It's basically the same as a manual.
2. I don't know of any possible way you could shift a manual in 0.040 seconds. (Time it takes for the PDK to shift.)
3. By definition, if the loss is a percentage, it has to be linear.

[Welcome to NBA Jam]

Percentage does not always mean linear. Exponential decay, for example, is non-linear

[Fundguy1]

Originally Posted by Welcome to NBA Jam

Percentage does not always mean linear. Exponential decay, for example, is non-linear

I agree, which is why I foot noted it Maybe it's a lower percentage at higher power levels.

[Volasko]

A long time ago my father explained to me what a torque converter is (what is used in a STANDARD auto tranny) and I never forgot it. This is a VERY generic description but it helps. Imagine two fans in a thick viscous fluid (like oil), one fan is connected to the engine and the other fan is connected to the wheels. Those two fans are never connected (well they are in overdrive mode, but only during consistent cruising speeds) so there will always be power losses under acceleration. This is why you can come to a complete stop without stalling the motor. A manual tranny is a direct connection between the engine and the wheels (once the clutch is engaged of course) so there are no power losses (ignore standard bearing friction etc etc). This is why auto transmissions are often called slush boxes and in the past have been consistently slower (and use more gas) than MT
.

[blown07]

The main difference with a 6AT is the mass of the torque converter which will have a certain amount of drivetrain loss at an RPM + the tranny itself is heavier. The 6AT has a better 3.42 ratio which helps to counter some of this. At stock levels I imagine that this is still not enough to overcome the 6MT's advantage but at higher power levels the 6AT is faster/more reliable imo due to the better gearing, faster shifting and holding boost between shifts. The DTC has the best of both but not sure about reliability at higher HP numbers.

[Antetokounmpo]

Originally Posted by Welcome to NBA Jam

Percentage does not always mean linear. Exponential decay, for example, is non-linear

Originally Posted by Fundguy1

I agree, which is why I foot noted it Maybe it's a lower percentage at higher power levels.

Exponential decay is not a percentage, it's a differential equation. He clearly said about 5%, this means the loss will always be linear to the horsepower. Now that doesn't really matter because the 5% isn't correct anyways.

[blown07]

Originally Posted by Volasko

A long time ago my father explained to me what a torque converter is (what is used in a STANDARD auto tranny) and I never forgot it. This is a VERY generic description but it helps. Imagine two fans in a thick viscous fluid (like oil), one fan is connected to the engine and the other fan is connected to the wheels. Those two fans are never connected (well they are in overdrive mode, but only during consistent cruising speeds) so there will always be power losses under acceleration. This is why you can come to a complete stop without stalling the motor. A manual tranny is a direct connection between the engine and the wheels (once the clutch is engaged of course) so there are no power losses (ignore standard bearing friction etc etc). This is why auto transmissions are often called slush boxes and in the past have been consistently slower (and use more gas) than MT
.

The 6AT locks up a fraction of a second after leaving a stop so it is essentially direct drive after that.

[Welcome to NBA Jam]

Originally Posted by bk335

Exponential decay is not a percentage. It's a differential equation. He clearly said about 5%, this mean the loss will always be linear to the horsepower. Now that doesn't really matter because the 5% isn't correct anyways.

Exponential smoothing models, which are based on an exponential decaying percentage weight, are percentages of previous values of data over time. A reference value of 100 doesn't change it.

[Antetokounmpo]

Originally Posted by Fundguy1

I agree, which is why I foot noted it Maybe it's a lower percentage at higher power levels.

Originally Posted by Welcome to NBA Jam

Exponential smoothing models, which are based on an exponential decaying percentage weight, are percentages of previous values of data over time. A reference value of 100 doesn't change it.

Again, we aren't taking about decay here, we are talking about a fixed percentage based off a value. Since the percentage is fixed, it will always remain linear to the value. I don't understand why you'd bring up exponential decay in the first place, it's a fixed percentage. Not to mention, the 5% claim isn't even remotely correct.

[Fundguy1]

5% difference between manual and auto is typical. A manual usually leaches 15% how from crank to rear wheels. Autos typically 20%. This has been the standard since the 60s. A different torque converter will change this and get back some of this as well as shift faster, but it's not as smooth and is more jolting and jerking a shift which is why manufacturers don't use these. I didn't know if the percentage declined slightly as power increased or not. It was a question not a statement.

[hassmaschine]

you know, does anyone really care about .1 seconds 0-60 or whatever the faster shift times gain you? this is even if it really is faster, which is doubtful (maybe the DTC). It's not like you have two cars with the same mods at the drag strip, one with MT and one with AT to test..

Even if it's faster, I doubt it's significant, and it sure is a hell of a lot more boring.

and if we're talking the N/A cars, the manuals are certainly going to be faster regardless of shift times. you only have to shift once in a 0-60 race, twice in the 1/4 mile - that doesn't add up to much, not enough to make up the loss in WHP.

[Fundguy1]

Multiply Base Horsepower by the Appropriate Percentage - Automatic transmissions are said to lose 20% of the power from the engine to the wheels, while manual transmissions lose 15%. To determine rear whp, multiply the base horsepower by 0.80 if you have an automatic transmission and 0.85 for a manual transmission. Example: 250 hp x 0.80 = 200 rear whp.

From here:
http://m.carsdirect.com/car-maintena...eel-horsepower

[Fundguy1]

Originally Posted by hassmaschine

you know, does anyone really care about .1 seconds 0-60 or whatever the faster shift times gain you? this is even if it really is faster, which is doubtful (maybe the DTC). It's not like you have two cars with the same mods at the drag strip, one with MT and one with AT to test..

Even if it's faster, I doubt it's significant, and it sure is a hell of a lot more boring.

and if we're talking the N/A cars, the manuals are certainly going to be faster regardless of shift times. you only have to shift once in a 0-60 race, twice in the 1/4 mile - that doesn't add up to much, not enough to make up the loss in WHP.

And if you're power shifting, which testers don't do, while you're shifting the engine its building up and storing energy that is released when you reengage the trans. This offsets the loss when it's not engaged mostly so acceleration improves over standard shifting

[Antetokounmpo]

Originally Posted by Fundguy1

5% difference between manual and auto is typical. A manual usually leaches 15% how from crank to rear wheels. Autos typically 20%. This has been the standard since the 60s. A different torque converter will change this and get back some of this as well as shift faster, but it's not as smooth and is more jolting and jerking a shift which is why manufacturers don't use these. I didn't know if the percentage declined slightly as power increased or not. It was a question not a statement.

I'm talking about where you said "all autos, dct or not" have about 5% more losses... the DCT part is way off. The looses on a DCT are almost identical, if not identical to a manual because well... it's literally a type of a manual with automatic clutches.

[Fundguy1]

So yes, manuals have about 5% more rwhp than autos with the same engine. The dct compensates with a virtually instant shift. Power shifting can almost nullify that advantage, and the last part of the equation would be what are the different gearing ratios between the manual and the auto.

[hassmaschine]

I think he's saying the DCT uses a clutch like a manual (actually two), so it's losses are basically the same. The DCT is more than likely faster, but it's still boring.

[Antetokounmpo]

Originally Posted by Fundguy1

So yes, manuals have about 5% more rwhp than autos with the same engine. The dct compensates with a virtually instant shift. Power shifting can almost nullify that advantage, and the last part of the equation would be what are the different gearing ratios between the manual and the auto.

No, the losses of a DCT are the same as a manual. Toruqe converter autos are a different story.

Originally Posted by hassmaschine

I think he's saying the DCT uses a clutch like a manual (actually two), so it's losses are basically the same. The DCT is more than likely faster, but it's still boring.

Yes, DCT uses a dry clutch like a manual so there are no additional looses like a torque converter or a wet clutch automated manual. And yes, they are way more boring.

[Fundguy1]

Dct with the clutches is different. You're probably right with the way it shifts vs the others so I'm guessing from the extra weight and parts it's more like a 1% difference vwith Power shifting it would be close but probably a slight edge to dct. Launches with manual would probably be better as you can drop the clutch with motor reving. But you can get a line lock etc to allow this with a dct and auto. Poor man's launch control essentially.When I got mine there weren't dct available yet so I didn't dig into them as much.