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-   -   6th gear for a car - Need, criteria, pros and cons (https://www.team-bhp.com/forum/technical-stuff/88515-6th-gear-car-need-criteria-pros-cons-4.html)

Let me rephrase that. A turbo diesel can make 'better' use of a 6th gear than a similar size engined Non turbo charged petrol can.

For example.

Swift Petrol vs Swift Diesel.

Swift Petrol is dead as a duck in 5th gear after 145 plus, hence a 6th gear would be of value only if rest of the gears are closely spaced.

Swift Diesel pulls like it was on rails from 85 in 5th gear to around 160 on the speedo after which the progress slows down till it touches 175 arounds.

Turbo kicks in at 2000 rpm in 5th gear at 85 kmph and at 120 kmph the car is ticking at 2900 rpm and after 3400 rpm arounds the torque starts tailing off.

A 6th gear which makes the turbo spool at 2000 rpm in 6th gear would make for an excellent cruiser on the highway, lowering the engine rpm's at 140 kmph or even 120 kmph for that matter with the engine being able to sustain that those speeds because of maximum torque of the DDIS kicking in at 2000 rpm.

Hence a 6th gear in a Swift diesel would not only improve kmpl but also impart it to be a better highway star.

I fully agree with you that it will be a boon once you cross 100 kph. :-)

Absolutely love the 6th gear on my Lodgy. Renault has perfectly mated the gearbox to the K9K engine and this enables effortless cruising in 6th. By effortless, I mean the ability to speed up without downshifting when the need arises. From 90 kmph, I can engage 6th and any time I need to go higher, it's just a dab on the accelerator. Due to the low RPM, high speed cruising in 6th, it's easy to attain high speeds without realising it.

It's not just about high mileage but also the ability to keep the engine relaxed and right in the turbo zone for quick overtakes. Given the choice, I would pick a 6 speeder over a 5 speeder any day.

As the automobile sector is evolving, many new technologies and inventions have become a part of the new passenger cars which are on sale today. One such change is the availability of a Six speed gearbox on almost every new car which is being launched.
Here in this post, I want to highlight the advantages and disadvantages of both the gearboxes. All these observations are by my personal experiences. I have driven cars with both the gearboxes extensively under mixed traffic conditions.



SIX SPEED GEARBOX

Advantages

• New and latest development
• Small gearing results in brisk acceleration
• Can result in better fuel efficiency on highways as the engines is more relaxed at high speeds due to the inclusion of an extra gear
• Useful for engines with high torque output
• Result in lesser NVH levels

Disadvantages

• Small gearing results in much more no. of gear changes, specially on the highways. I have found myself downshifting to the 5th gear while overtaking. It is not the case with most of the cars with 5 speed units.
• Not everyone is comfortable with the six speed units
• Comparatively new in the market, may have some reliability issues.

FIVE SPEED GEARBOX

Advantages

• Tall gearing results in lesser gear shifts. In most of the cars, 2nd and 3rd gear is enough for the city runs. Out on the highway, put the car into 5th gear and forget about the gearbox.stupid:
• Conventional, can be more reliable and easy to repair
• Can be used in powerful as well as comparatively weaker engines
• Gives peace of mind

Disadvantages

Thanks. An interesting read out and it shows you have given it some thought and brought your own experience into this. Most of your points would be valid if you could buy a car with a 5 or a 6 speed gear box. In most cases you can’t, so comparing apple to apple becomes difficult.

And only then could you compare it nothing else changes. So really an identical car/engine/engine management/differential/wheels/tyres and only 5- or 6 speed box.

In all honesty, I don’t think you can compare, generically speaking a 5 to a 6 speed gear box. There is really only one difference, uh one gear!

You don’t buy a gear box stand alone. You buy it as a total package, car, engine, differential etc.

Obviously, a 6 gear box has one more gear than a five box. How that feels from a driving experience, how much less or more shifting is required has not so much to do with the gear box as well as the other factors. (E.g. engine power and torque, differential ratio, wheel size etc).

The more torque an engine has over a wide RPM range the less gears you will need. At least for performance as in terms of acceleration. For best fuel economy you will need an infinite number of gears preferably of course. 6 won’t do at all!

You don’t necessarily get better fuel economy out of a 6-box compared to a 5 box. That really depends on many other factors. (E.g. engine power, tongue, gear ratio’s, differential ratio, wheel/tire size).

Neither can you claim a 6 box is good for engine with high torgue. you can make a very valid point that the higher torque the engine produces the fewer gears it requires to get the same performance, at least from an acceleration point of view.

Whereas on many 5 speed gear boxes, 5th gear used to be effectively an overdrive, that is not necessarily the case anymore on 6 speed gear boxes.

We are already seeing 8 speed gear boxes on some cars. Albeit mostly if not always, with automatic boxes rather than manual.

The number of gear changes required is not so much a function of how many gears you have, (although it is a factor) it is heavily influenced by the engine torque.

My Jaguar has a five speed auto box. It doesn’t even use first gear unless in sports mode. Normally it pulls away in second gear as it’s 4.0L supercharged V8 has so much torque! It will only use first gear if you require Warp speed acceleration.

Again, I don’t think comparing 5 to 6 speed gear box is possible or relevant without taking the complete power train into consideration. It’’s the total package that determines the performance, acceleration, how often you need to shift, fuel economy etc.

Jeroen

4 speeds were standard years ago and we saw 5 speeds evolving and eventually taking over. Same is the case with 6 speeders. They are evolving; 5 speeds will becomes a rare thing in future. Though there are certain points on which comparisons can be drown, it is a fact that 6 speeds will be the thing in future.

Sir I understand that the observations which I have may not be 100% correct and I am a no expert in the technical aspects. I drove different cars with similar specifications like a 1.4L CRDi vs 1.5L TDCi. I know it is very difficult to compare like this and few may term it as baseless.
The fact that few automatic cars have much more gears than a Five speed manual is also a different case here. I only compared manuals.

I am not saying that I am 100% right and I am not talking about the high end cars here. They cars which I considered were all below 20 lacs, most of them being affordable to a common man.

As an ardent fan of 6 speed MT, I'll agree with many points you've mentioned above but not sure about the bit I've quoted from your post. Here's why:

Downshifting to 5th often because of a 6 speeder? I think not.
I drive an i20 diesel and most times I can continue in 6th gear without downshifting. I've accelerated from as low as 70kmph in 6th without any knocking. Of course, if you're driving pedal to the metal, it's a different story. I'm a sedate driver. Alternatively, my previous ride, Lodgy, will mostly require a downshift in 6th when slowed down to say 90 or below. The point I'm making is, it's to do with the torque curve rather than the gearbox itself that causes this. I'm sure my Lodgy would behave in a similar fashion even if it had a 5 speed box, the reason being it's a peak torque engine. The i20, with its flat torque, picks up easily and it's more forgiving when trying to accelerate from low speeds.

Not everyone is comfortable?
How can that be a disadvantage? :D That's a subjective opinion rather than an objective one. As I mentioned earlier, I shift a lot lesser in my diesel i20 than my previous Lodgy or Ertiga (both diesels) because of a good spread of torque and mind you the Ertiga is a 5 speeder.

Comparatively new in the market, may have some reliability issues.
There's nothing technically complex about a 6 speed gearbox for it to pose some reliability issues. There's just an extra cog and I don't see any difference in reliability with regard to a 5 speed gearbox.

Finally, I reckon there's thread already with a similar topic. http://www.team-bhp.com/forum/techni...dvantages.html Perhaps this thread could be merged with it.

I seriously don't understand the point behind this thread. Without proper comparison of gear ratios, this discussion makes no sense. There are 6-MT cars with short gearing(Altis diesel but the engine is equally at blame here) and then there are 5-MT with super tall gearing(like the Esteem or even the Abarth has sorted ratios).

All the points that you have mentioned like frequent gear changes.etc. Are also dependent on the engine. And how is a 5-MT more reliable than a 6-MT?

Sir, I too have an i20 elite in our family and I have driven it for a long time. There have been plenty of instances where I was cruising around 100 kmph and due to some reasons the speed went around 55-60 kmph. I had to downshift to 5th gear. However, in my Ford Aspire, the 5th gear itself handled it. This is the case on most of the roads especially on the 2 lane roads without dividers where we need very quick overtakes.

My 2011 Verna had a six speed MT. Pretty sure cars older than that had it too.

Exactly to the point some of us have been making. Nothing to do with the gearbox and the ratio itself. You are comparing one car's behaviour to the other and make conclusion about 5versus 6 speed.

As pointed out, engine, power, torque and other factors come into play whether you need to down shift.

Jeroen

These diagrams pretty much sum up the role of a transmission. Here you see 4-speed tractive force of an engine is used with a 4-speed transmission, and how tractive force is wasted without a transmission.

I'm not able to source the book from where I referred this diagram (Courtesy - Automotive Transmissions by G. Lechner and H. Naunheimer). So posting a screenshot of the diagram I made for my Thesis.

Spike:)

I agree to you. I am not protecting myself here but I think the Ford Aspire and i20 are evenly matched as the Aspire has 215NM torque and 99 BHP engine whereas the i20 has 220 NM and
approx 90 BHP. Also the range for the working of the turbo is also similar. Thats why I made these observations.
I still believe that I may be wrong and thus want everyone to share their knowledge as most of the TEAM BHP members are more experienced and informed than me.

Hi Pratheesh,
You can fill us in on one of the most important aspects of a gearbox - cost. Esp interested in cost comparisons between CVTs and slushboxes. (Low power applications.)

@OP - I learned driving on a 3 + R setup (and unsynchronised first).
The first autos were 2 speeds.
On the other hand, my bike had six speeds.
Nowadays one sees autos with more ratios than any manual. (There is a interesting reason why.)
My point - meaningless to try to isolate this one aspect.

Regards
Sutripta