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Old 19th September 2010, 05:39   #16
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Originally Posted by valhallen.282 View Post
From a secondary perspective I'm thinking that for a manual transmission cars more gears = more shifting = more clutch usage. While its not bad it can't be good for the mileage or the drivers left foot either. even in quite a few autos its been found that you end up consistently just hunting for which gear to jump to. As it is 5 gears tends to bother plenty of people. Like my dad just CANNOT get the car into 5th. (We had a van before) he says it confuses him how to get to it and out of it!
My experience has been - with a reasonably high flat torque characteristic engine and a reasonably quick and short shift, 6th is quite usable. I tend to avoid the 5th on the 1.6 ltr Optra but find it natural and easy to use the 6th on the Laura TSI on expressways in Mumbai at 80kph. Unfortunately the moment you brake even slightly , you end up having to downshift !
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Old 19th September 2010, 11:32   #17
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Originally Posted by GTO View Post
BTW, there's quite the ego war between luxury car marques on this topic. Mercedes came out with the 7 speed AT, Lexus outdid them with an 8 speed in the LS and now there's news of the next-gen S Class being equipped with a 9 ratio box!!
Well, why don't they shift to a CVT and close the issue
Theoretically speaking, CVT seems to be an ideal transmission since it keeps the engine in it's optimal power band and instead changes the gear ratios to achieve the desired speed. But the current implementation seem to leave something to be desired as people complain about the rubber-band effect when floored (where the revs go up without much happening before CVT starts winding).

BTW I have driven i10 on highways and the engine becomes so buzzy above 90 kmph, you wish it had a sixth gear. Even hatches can benefit from the sixth gear.
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Old 19th September 2010, 18:53   #18
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Originally Posted by Guna View Post
BTW I have driven i10 on highways and the engine becomes so buzzy above 90 kmph, you wish it had a sixth gear. Even hatches can benefit from the sixth gear.
Absolutely, small hatches too can benefit from a 6th gear, at least on highways.
The primary purposes for a 6th gear in these cars (and especially a hatch like a diesel i20) is to provide maximum fuel efficiency, (not top speed as someone mentioned), and that is in my opinion to real purpose and advantage of a 6th gear.
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Old 19th September 2010, 19:08   #19
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Originally Posted by kadanaJ View Post
The primary purposes for a 6th gear in these cars (and especially a hatch like a diesel i20) is to provide maximum fuel efficiency, (not top speed as someone mentioned).
KadanaJ, You are wrong in the case of diesel cars. The top speed of diesel hatchbacks will get affected after addition of a sixth gear. Here is How: Most modern common rail hatchbacks(except i20) hit redline in fifth gear(4k rpm, the max power point, not the peak revving point in case of Multijets). Even my Elantra with tall gearing hits redline in fifth cog.

With the addition of a sixth gear the Diesel cars will obviously experience a better top speed. I feel the ability to cruise in a more relaxed manner is worth the effort. Personally i miss the sixth gear in my Punto on open highways.
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Old 19th September 2010, 20:22   #20
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Having the sixth gear would let you keep the car in it's power band longer. This helps in it's overall performance. Also, it would let all the gears to be closer to each other and this would help in the city as it would mean less shifting. Granted, if you are racing thru traffic, you prolly would shift more than on a 5-speed, but if that's not what you are trying to do, then, I'd say oyu'd shift less within the city as it's easier to be in a gear which works at any speed and it's easier to skip gears. Again, this would give you better fuel economy.

And, to determine the ideal shift points for maximum performance, you would need a dyno graph of the car's engine and know the gear ratios. Then it's possible to make a graph showing you the points where to shift. Here, I made one using the 1.4 TDi on my vista as an example.

https://spreadsheets.google.com/ccc?...FOFlUNlE&hl=en

In either the power or torque graph, the ideal shift points would be where the lines cross. The Idea would be to maximize area under the graph. This would also make it easy to understand that with another gear, this area can be increased within the same speed width.

I originally posted these graphs here.
http://www.team-bhp.com/forum/techni...que-curve.html (Tata Indica Vista 1.4 TDi Torque Curve)

BTW, the graphs are not accurate as I made them as an experiment and not from real dyno data.
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Old 19th September 2010, 20:44   #21
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Having 6 gears does not automatically make a car faster or more economical. The ratios have to be well calculated, to match the engine's characteristics and the driver's cruising speed.

If a driver is used to cruising at 60kmph, then the rpm in 6th gear might be too low for that speed. Shifting into 5th might fetch him better FE.

We'l have to wait for the actual ratios that Hyundai has selected for this transmission, in order to see how efficiently it works under real driving conditions.

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Old 19th September 2010, 20:55   #22
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Originally Posted by Guna View Post
Well, why don't they shift to a CVT and close the issue
Current CVT technology has problems handling higher power outputs:- the type you see in luxury cars.

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Old 19th September 2010, 21:19   #23
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I don't know if this is revelant - I would have loved a 6th gear on my Pajero. It seems strained over a certain speed in 5th.

On the other hand my Audi has a 7 speed AT. When I switch on cruise control on good roads the motors runs in a very relaxed manner & the mileage per litre display shows almost 18-20 kmpl

Having said that = the taller gears are only of use on highway drives. Hence the relevence in a city driven car is 0

Hope that helps ?






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Old 20th September 2010, 11:56   #24
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Thanks ALL the fellow bhpians who have contributed to this thread!

Appreciate your time and efforts to clear my doubts.

Quote:
Originally Posted by SS-Traveller View Post
@snf: What you are proposing is to ask for a 6th gear to maintain higher speed at lower engine rpms, which you think would improve mileage. Assuming your current 5th gear has a drive ratio of 0.8:1, you want one more gear to perhaps give you a drive ratio of 0.7:1. But that is not what a 6-speed gearbox is about.
Thanks @SST -- your explanation was the biggest of all -- but there is a small problem ! I am a software engineer with heavy passion for cars and all this data has gone above my head. I dont understand the technical details to this extent, can live with the basics though.

So could you take some more time to make me understand what is a drive ratio and how is it calculated - [like 0.7:1 and all] ?

Quote:
Originally Posted by SS-Traveller View Post
3. Advantages of a 6-speed GB include more closely spaced ratios, allowing selection of the right gear more easily to let the driver accelerate and drive more comfortably, without the car running out of steam.
My problem is in 5th gear also I dont run out of steam in my Swift D [ a good problem to have actually ] ; even after hitting 140kmph or so. So basically what I feel is , I need one more gear for a more relaxed drive with no need of so much power at disposal. I also hope a 6th gear could bring down the humming noise from Swift D at those speeds. I could hear this with windows rolled up and slightly bothers me -- I keep the music volume high to avoid hearing this sound.

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Originally Posted by SS-Traveller View Post
4. Now for the disadvantages. I said theoretically, because a higher gear puts more load on the engine - with this extra load, the engine may not even be able to rev beyond a certain rpm. Invariably happens - the revs you can do on 4th gear (1:1 is the usual ratio), are not achievable in 5th gear.
Slight confusion here -- If the gear puts load on engine, how can it feel relaxed? With all the higher gears, usually the power [or torque?] reduces and drive becomes relaxed right?

Quote:
Originally Posted by SS-Traveller View Post
Plus, even if the manufacturer did provide a 6th gear ratio which is lower than the 5th of a 5-speed GB, your managing to achieve those higher speeds may not be supported by the car's dynamics. The chassis, suspension, tyres, brakes etc. can take just so much speed, and not more - speeding beyond this increases the risk factors.
I understand this point too -- But will there be such a drastic difference in the top speed?


Quote:
Originally Posted by SS-Traveller View Post
And of course, the biggest disadvantage is a higher cost.
Could you mention some approximate cost addition that will incur for a 6 Speed compared to a 5 speed with rest all factors remaining the same?

Quote:
Originally Posted by Ricky_63 View Post
Having said that = the taller gears are only of use on highway drives. Hence the relevence in a city driven car is 0

Cheers
+1 to this -- I want this purely for the highway drive - I usually drive in the nights and choose to drive between 100-120 mostly and usually can maintain it for long hours during my drives.

Also when I understand that this could lead to improved fuel efficiency, will anyone be able to tell me how much difference can be achieved? +2 kmpl under the same driving conditions of a 5th gear [that is if I get 20kmpl on a car with 5 speed gear box, will I get 22 kmpl on the same car with 6 speed gear box under the same driving conditions where I got 20kmpl] ?
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Old 20th September 2010, 19:18   #25
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Default Numbers of gears, gear ratio and their significance

My friend Ashwin (I don't know his team-bhp id) and I had brainstormed and googled up a lot on a discussion that started with how a 6th gear would help. Some of the conclusions we agreed on (please feel free to differ with explanation):
Before going ahead, this link will be very useful: Puma Race Engines Technical Guide - How engine power and top speed are related

Minimum number of gears:
This is an irrelevant question as of today as most cars have 5 forward gears as standard. But theoretically, it would be number required where each upshift would be done at the redline in lower gear to 1000 rpm in next gear till we get such a gear where we can no longer reach the peak power rpm. This number could be mostly 2 or 3 gears in most cars. For instance, most cars reach 40 Kmph in 1st gear. 2nd gear will be 40 Kmph at 1000 rpm. This 2nd gear will unlikely reach the redline before it hits the top speed.

Top Speed:
Top speed is roughly a function of max power, vehicle weight and other variable resistances (such as wind resistance, rolling resistance, etc. which can increase exponentially as speed gains). The link Puma Race Engines Technical Guide - How engine power and top speed are related does have a rough table indicating maximum speed for a given power. Max power vs top speed graph is not a linear one as the resistance is not constant. Yet, in simplistic terms, for a given car, a more powerful (more peak power) will always result in higher top speed provided an appropriate gear ratio is available. More on this in the next paragraph.

Does my car need a 6th gear or more?
I'm quickly coming to the top of the thread. As mentioned above, for a given car, more power is supposed to pull the car to higher speed. It indirectly means that a car is supposed to attain its top speed in its peak power rpm. If a car reaches its top speed in the last/tallest gear in an rpm, which is higher than the peak power rpm, but at which the power is much lesser than the peak power, then another gear will help pushing the top speed further. For example, Linea with 1.3 MJD 86 ps (before BSIV) reaches 168 Kmph in 5th (last gear) at 4500 rpm. But if you look at the power curve of Linea 1.3 VGT MJD, power at 4500 rpm is just about 75 compared its near 90 ps at 4000 rpm (refer power curve: http://www.petes.in/images/Fiat_Linea90BHP.pdf). If you look at the curve, the same engine produces 75 ps as early as 3000 rpm itself. So, another gear will run at an rpm anywhere above 3000 rpm at 168 Kmph will pull the Linea even to further speed. However, I'm not exactly saying that the 6th gear should be at 3000 rpm at 168 Kmph. If we do that, it may not reach 4000 rpm at all as the resultant torque on the front wheels are much lower than the resistance it faces on its way to 4000 rpm. Even then we may reach the best possible top speed, but will not be quicker ratio. From the table, 90 ps give a top speed of 110 mph, i.e. 177 Kmph. Lets assume this as the theoretical top speed of Linea. Lets assume that Linea's 6th gear reaches is at 3500 rpm at 168 Kmph (48 Kmph per 1000 rpm) and manages to go till 177 Kmph, it will be just about 3700 rpm. Compare this to a 6th gear ratio of about 44 Kmph per 1000 rpm, i.e. 3800 rpm at 168 Kmph. This later ratio will be much quicker to 177 than the earlier one.

Now you will know why we don't need another taller gear for Swift or Verna, because they already cannot reach the peak power rpm (4000 rpm) in their 5th gear itself. But i20 does 174 Kmph in its 5th gear at 4000 rpm. Since it has already hit (or very close to) the peak power rpm in 5th gear, it actually does not need a 6th gear. But the fact that it can touch the peak power rpm in 5th gear means that it can benefit from another taller gear in fuel efficiency (later about fuel efficiency).

Best gear ratio for best fuel efficiency:
As we know, an engine burns some minimum fuel per a given amount of time irrespective of the speed (either idle or slow or fast) unless it is a hybrid. So, it is by simple rule better to be fast. But as speed gains resistance increases and more energy is required to pull the car. So, the engine ends up burning more fuel. Different manufacturers and research indicated that 60 - 80 Kmph is an optimum speed for the internal combustion engines. I have no links for this number (please provide if you have one). Car manufacturers also keep minimum fuel burning at around 1100 rpm (i.e. when a gear is engaged, ECU trying to maintain 1100 rpm). So, ideally the most fuel efficient gear should be a gear that runs the engine as 1100 when the car speed is 60 Kmph, i.e. about 55 Kmph per 1000 rpm. But then to run at 60 Kmph (about 40 mph), an minimum power of 8 ps is required. 1.3 MJD engine just manages to have about 8 ps at 1100 rpm (refer the graph again http://www.petes.in/images/Fiat_Linea90BHP.pdf). So, theoretically we could have a gear which run at 60 Kmph at 1100 rpm. This should be the most fuel efficient speed and gear combination. But if the engine has less than 8 ps at 1100 rpm, it has to have a gear that produces just 8 ps at 60 Kmph (calculate whatever rpm it is). That should be the most fuel efficient gear.

But I cannot drive at 60 Kmph all the time just for efficiency
So, what speed you want to drive at? 100 Kmph? Ok, lets choose a gear ration that is just suitable for this. Minimum required power to drive at 100 Kmph (60 mph) is about 20 ps. In the graph again, 1.3 MJD produces 20 ps as early at 1350 rpm. So, we need a gear that runs at 1350 rpm at 100 Kmph (i.e. about 74 Kmph per 1000 rpm, am I crazy??). As far as I can say, that is the gear you need if you want to drive at 100 Kmph. But as you know, it will be really difficult to accelerate from such gears. Well, I could always downshift if I'm in a hurry... just give me such a gear to cruise at 100!!

By now, we can see that we can benefit a lot on fuel efficiency from taller gears and even on top speed in case of some of the existing cars. I think Toyota Altis did the right thing for a heavy car with 90 ps. More number of gears also benefit in city driving conditions where we want to find most fuel efficient gear with right amount of pull at various lower speeds. More than 5 gears manual shift may bog down drivers and thus we may end up not getting the benefit, but in an automatic, higher gears are a must according to me, unless it is a continuously variable (infinite) gear system.
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Old 21st September 2010, 00:31   #26
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Heavy trucks use as many as 13 gears. And some early automatics as few as 2. And we have not even gone into one of the most important criteria:- cost.
All engineering is a matter of compromise. One needs to know the constraints put on the designer.
IMHO, choice of ratios (in a 5 speed for our econoboxes) is more important than an extra cog.
Re: drivability, try driving your car only in 1st and 5th. (or maybe 1st and 4th). And it wont be economical to boot.

Regards
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Old 21st September 2010, 10:15   #27
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Nice summarizing on our discussion, Opendro! However, whether the Linea MJD will even manage to reach 3500RPM on the 6th (with whatever ratio that is) is a big question
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Old 21st September 2010, 11:04   #28
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Contd from http://www.team-bhp.com/forum/techni...ml#post2076669 (6th Gear for a car ! Need, Criteria, Advantages & Disadvantages)

Would a tuning box help achieve more top speed?
I forgot to add this point in the last update. "Theoretically", more peak power means higher top speed. However, if the car already crossed the peak power rpm in its tallest gear at its top speed before tuning box, it is unlikely that a tuning box will improve its top speed. The reason is that the tuning box usually does not change the nature of the power/torque curve. It just proportionally increase the torque at the rpm ranges and most of the torque benefits are in the power band (roughly peak torque rpm to peak power rpm). But in case of cars like i20, where 6th gear manages to reach 172 Kmph at 3500 rpm, it is sure to push a little further beyond this 3500 rpm with tuning box. In a similar manner, I would expect Swift to go far beyond its top speed of 158 with a tuning box in its 5th gear. My rough estimate for a Swift DDiS with the 90 ps tuning state will reach 175 Kmph in 5th gear. Swift and i20 owners with tuning boxes can correct me here.
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Old 21st September 2010, 13:55   #29
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Quote:
Best gear ratio for best fuel efficiency:
As we know, an engine burns some minimum fuel per a given amount of time irrespective of the speed (either idle or slow or fast) unless it is a hybrid. So, it is by simple rule better to be fast. But as speed gains resistance increases and more energy is required to pull the car. So, the engine ends up burning more fuel. Different manufacturers and research indicated that 60 - 80 Kmph is an optimum speed for the internal combustion engines. I have no links for this number (please provide if you have one). Car manufacturers also keep minimum fuel burning at around 1100 rpm (i.e. when a gear is engaged, ECU trying to maintain 1100 rpm). So, ideally the most fuel efficient gear should be a gear that runs the engine as 1100 when the car speed is 60 Kmph, i.e. about 55 Kmph per 1000 rpm. But then to run at 60 Kmph (about 40 mph), an minimum power of 8 ps is required. 1.3 MJD engine just manages to have about 8 ps at 1100 rpm (refer the graph again http://www.petes.in/images/Fiat_Linea90BHP.pdf). So, theoretically we could have a gear which run at 60 Kmph at 1100 rpm. This should be the most fuel efficient speed and gear combination. But if the engine has less than 8 ps at 1100 rpm, it has to have a gear that produces just 8 ps at 60 Kmph (calculate whatever rpm it is). That should be the most fuel efficient gear.
I'm not too sure about the 8ps@60kmph theory. The power required to maintain a constant speed of 60kmph comfortably, would alter with weight, road conditions/elevation, coefficient of drag, rolling resistance, amount of torque being transferred to the wheels etc....

A turbo diesel doing 8ps will have way more torque at that rpm than a high revving NA engine doing 8ps.

The way i see it, there is no fixed speed or engine rpm for maximum fuel economy. The driver needs to improvise, depending on the type of car he is driving and the driving conditions.

Shan2nu

Last edited by Shan2nu : 21st September 2010 at 14:11.
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Old 21st September 2010, 15:47   #30
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Yes, I agree Shan2nu. As I mentioned, fuel burns more on the one hand as speed increases, but on the other hand it is covering more distance. There has to be some trade off somewhere. But theoretically, you have to agree that higher the speed, higher the amount of energy to travel the same distance (because resistance is more) for a given weight of vehicle. But the combustion engine does not allow that. It is a completely different game for hybrids. Their best fuel efficient speed could be as low as 20 Kmph. For instance, a Toyota hybrid will give more mileage in city (less resistance) than a highway (more resistance due to higher speed).

6 ps required to reach 60 Kmph was an indicative figure on flat road (again not mine). If we have to factor in all conditions, there will never be a thing like top speed. Isn't it?

BTW, 60 Kmph theory was not mine. It is recommended by almost all manufacturers (again please don't ask for links )

Last edited by opendro : 21st September 2010 at 15:50.
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