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This is what I had posted in the thread op. Could we please stick to the title please rather than digressing?

AT is equally suited for both diesel as well as petrol cars. It has nothing to do with where the engines develop power etc. An AT GB will take care of the shifting!

I dont see the connection if torque is great at the bottom end of the engine or at the higher rpm!

In many ways Diesels may be bigger candidates - smaller power bands, turbo lag, etc. However, the natural markets for small AT's are not normal Diesel AT territory so they just do not bother.

A Diesel can be easily mated to an AT by redefining the shift points in the ECU.

For me after having driven both (not aware of the technology much) i find that AT does a much better job within city and on the highway the manual would do a better job as far as catching up with speed is concerned. But if one sees the comfort of long runs and day long drives then AT can be a real blessing albeit at the loss of speed your endurance can be much longer.

But also remember almost all newer CRDI Diesel engines suffer from turbo-lag, it ruins the low end, once the turbo spools up there is a sudden spike but before this Diesels are weak.


Because of the narrow Power band, at lower speeds Diesels might require more or as much shifts, it's at the time of highway cruising when they require lesser shifts due to the torque advantage. Also because Diesels make more torque and to accommodate narrow power band they get considerably taller set of ratios.

I Believe Diesels are better suited to AT because of the narrow power band, accelerating from standstill they require more gearshifts. The other advantage is better efficiency which offsets the FE loss in case of older less efficient AT boxes. My 2000cc 1500+ kg Laura DSG delivers almost 2X FE in city driving compared to the much smaller 1200cc Hyundai i10 mated to the antique AT gearbox, all mainly because of Diesel efficiency advantage and a modern transmission which does not sap much power.

Mating DSG to a turbo charged engine has its own set of problems, more so in case of engines which suffer from prominent turbo lag, because DSG does not have torque converter to multiply torque to compensate for turbo lag. One example is the earlier Laura PD DSG, the PD engine had a prominent lag, the DSG felt quite unresponsive at lower rpm while the newer CR engine with the same peak torque and BHP is much better suited to DSG, it has much lesser turbo lag and starts making good amount of torque as low as 1500 rpm.

Actually, you're absolutely right Anshuman. Never thought of it like that :p.

In typical city-driving conditions, we generally keep the car under 2k RPM (thereabout) most of the time and that's when the boost kicks in for many diesel cars. So we'd find ourselves changing gear as often if not more. Such driving warrants an the need for an AT.

What we need to clarify here is what kind of automatic transmission we are talking about. As the two basic types are clutchless automatic transmissions that use a torque converter (by far more common), and the other type is dual clutch transmissions like those used in the latest generation Porsches and Ferraris. I think even the VW DSG gearbox uses a dual clutch mechanism but im not 100% sure.

The fact is that conventional automatics (using torque converter) are SLOWER in a drag race than a manual transmission. The main reason for this is NOT shift times but power loss in the transmission.

Conventional Autos use a hydraulic fluid coupling called a toque converter rather than a clutch, in order to serve as a buffer between the engine crankshaft and the gearbox to prevent damage to both from speed/torque disparities. Anytime you have a fluid coupling in an open looped system there is a certain amount of energy lost in the form of heat within the coupling, as it slips to accomodate speed differences.

Think of how inefficient an airplane propeller is as opposed to a driven wheel at propelling a vehicle along the ground, and you'll get the idea. Driveline loss in autos is also significant enough to cause a decrease in fuel economy across the board by a significant amount.

On the other hand you have dual clutch transmissions like Porsche's PDK system that literally shifts gears faster than you can blink your eye and has the exact same driveline power loss as a conventional manual. This is because Dual clutch tracnsmissions have 2 gearboxes and transfer power from the engine via a solid clutch, similar to manual transmissions. One gearbox has gears 1, 3, and 5 while the other gearbox has gears 2, 4, and 6. The transmission shift gears by pre-selecting the next gear on the other transmission and simply shift gears by disengaging one clutch on the first transmission and engaging the clutch on the other transmission. The time between when one clutch disengages to when the other clutch engages can be very near zero. The shift time for the Porsche PDK system is arouund 50 Milliseconds = 50/1000th of a second.

With the same power loss as a manual transmission but MUCH faster gear shifts, a dual clutch transmission is FASTER than a manual transmission on a dragstrip and around a race track.

Now addressing the question of whether Automatic transmissions are better suited to Petrol Engines of Diesel Engines:

The only diesel AT that I have driven for a significant period of time is the Ford Endeavour 4x4 AT (Diesel Automatic) that I own and is my daily drive. I have driven several petrol automatics in India and the U.S though. One thing I find is that diesel automatics are much much more fun to drive because the engine is always in its torque or power band. When I press the accelerator, if the engine is anywhere above 1,500 RPM it has tons of GO. The Endeavour AT behaves in such a way that the rpm never drops below 1,800 RPM when I depress the gas pedal even slightly so it always has torque to pull. Much fewer noisy and jerky downshifts are required when I depress the accelerator.

Petrol ATs however are much more frustrating to me. To get any decent acceleration from a petrol engine, you have to be at about 4,000 RPM minimum. Most of the time in the city, the AT transmission of a petrol automatic never gets above 3,000 rpm. When I want to overtake, I depress the accelerator and the car does one of two things:
1) The most frequent thing it does is it downshifts, jerkily accelerates forward with accompanied with loud and harsh engine sounds as it revs away at 6,000 RPM. This is very annoying to me.
2) If I press the accelerator forward slowly, the revs will build up to a point to where it will not downshift any longer and accelerate away slowly. The problem with this is that I have to sit and wait for the revs to build up from 2000-3000 RPM to a point where it actually starts making power.

Instead of this behavior, I much prefer a manual transmission petrol car so that I can still depress the accelerator fully at about 3,000 rpm and have the car use available torque to accelerate instead of having the AT downshift and have the engine loudly rev to 6000 RPM with the accompanied JERKY behavior.

So my conclusion is that ATs are better suited to Diesel vehicles because they are much easier to drive, are less frustrating, require less downshifts when accelerating and at the end of the day, is a lot more fun to drive because you always have torque to play with regardless of the RPMs.

@Endeavour333; Let me say that a few things are common. At low speed esp from static some mechanism for slipping the clutch is necessary. Similarly (excluding a pucca CVT) for smooth shifts again some mechanism has to engage. This is these areas where the inefficiency of the AT shows up. Higher end ATs have some mechanism to bypass this mechanism above a certain speed.

So while agreeing they are different there are bits common to all.

I think you are referring to lock up torque converter.

Lock-Up Torque Converter - YouTube
Quote from Wikipedia:
Source: Torque converter - Wikipedia, the free encyclopedia


__________________________________________________ ______

ZF 8-Speed Automatic Transmission - YouTube
ZF8 is the best Torque convertor AT i have driven. This also is one of the most widely used Torque converter type automatic transmission in premium cars today. Used in all these cars-

• Audi A4 North American version
• Audi A5 North American version
• Audi A6
• Audi A7
• Audi A8
• Audi Q5 North American version
• Bentley Mulsanne (2010)
• Bentley Continental GT V8
• BMW 1 Series
• BMW 3 Series
• BMW 5 Series
• BMW 5 GT
• BMW 6 Series
• BMW 7 series
• BMW X1(India yet to get this gearbox)
• BMW X3
• BMW X5
• BMW X6
• BMW Z4
• Chrysler 300 V6
• Dodge Charger V6[13]
• Jaguar XF
• Lancia Thema V6
• Range Rover[14]
• Range Rover Sport
• Land Rover Discovery
• Rolls-Royce Ghost
• Rolls-Royce Phantom (2013)
• Volkswagen Amarok

(List taken from Wiki article)

The Ford Endeavour has a lock up torque converter. With Overdrive on (O/D Switch) the Torque converter locks up at 90 km/h (5th gear), with O/D Off, it locks up at 70 km/h (4th gear).

@.anshuman: Thanks for the videos and info you linked above. They were really helpful.

Let's just take the case of the BMW 530d. I have driven the automatic (only option available in India) and the manual 530d in Europe. There is no comparison with the seat of the pants feel and the linearity that you get in a manual. Even in tiptronic mode in the automatic, the car does not give you a feel anywhere close to what the manual gives. At the end of the day, it's upto individual preferences, but purely in terms of driving feel, everything else being equal, nothing comes close to a manual.

Now this Skoda Laura 6 speed DSG AT. Are there any warning signals before the DSG fails? When it goes for service, of course I suppose they look for errors and trouble signs but service interval of Laura being every 15K Kms which is a long interval, can we predict DSG failure by asking the service to check it bit more frequently, say every 5K?

^^ The 6-speed DSG 'box is a tried and tested unit. So it shouldn't fail.

What you need to be worried about is the 7-speed 'dry-clutch' DSG you get in the petrol Superb. That's the troublesome 'box.

@ Endeavour: Minor correction. The PDK 'box in modern Porsches isn't two gearboxes. It's one gearbox with two clutches. One clutch takes care of the even gears (2, 4, 6) and the other, the odd gears (1, 3, 5, 7) :)

Porsche's PDK is the same concept as VW's DSG. The Audi people call it S-Tronic. Ford says DCT. It all means two clutches operating on the same gearbox.

Also, while the 6-speed wet clutches are indeed less notorious than the troublesome 7-DSG as you say, friend manson suffered a mechatronics failure in his Laura couple of months ago and the car had to be driven home in limp mode. So I would stop short of predicting "It won't fail" :D

I have read the thread already and though the numbers are less, sporadically it continues. Also may be they are improving with each failures. Also in China they have further extended warranty on both 7 and 6 speed DSGs after govt forced them to take action. So still they are not as reliable as MT was my impression. May be after 5yrs they will be foolproof.