Why 6 speed gearboxes?

[Garfield]

We have just bought a Ford s-max 2.0 tdci with a 6 speed box. Why are manufacturers introducing this gimmick?, why can't they simply increase the difference in ratios of a 5 speed?

I recently set the cruise control at 55mph and after about 15 miles the fuel computer was reading 61 mpg, I then realised I was only in 5th so 15 miles after changing to 6th the fuel computer was reading 57 mpg. Some people beleive that driving without cruise control is more economical, I think that this is unlikely given that the car's normal average is 40 mpg. I have yet to tow with it but I expect 6th to be redundant with the caravan on the back.

Our old VW Sharan was an excellent stable tow car but the S-max is like driving a go-kart compared to the shaz so I expect that throttle response will be the only clue that our 20' ta caravan is on the back

 

[Lutz]

The number of gears is increasing in order to make it easier for the engine to always be working in its most economical rpm range, whatever the speed of the car. This is necessary for the manufacturers to be able to fulfil their commitment to reduce fuel consumption. The first seven speed cars are already on the market (these are automatics, though, not manual).

 

[cliff_5040076]

This may be a silly question but why buy a car with a 6 speed box when you would prefer one with a 5 speed box?

My CRV diesel has a 6 speed box and once you get used to it,it is no problem. I only engage 6th when travelling over 40 mph so around town its just like a 5 speed. On motorways the extra gear allows for lower rpm for a given speed.

My van tows superb at 60mph in 6th at 2000rpm which is when the engine delivers maximum torque so changing down a gear is a very rare occurrence. I chose the car to do this but if you have a really heavy van and the engine is not powerful enough you are going to have to change down on hills. The only answer is a more powerful car or one with different engine characteristics.

I set the cruise a 60mph and thats the speed I maintain (traffic conditions permitting).

Cruise control is no less economical than your foot but most people tend to slow on hills when cruise will automatically depress the throttle to keep you at the preset speed. So you are not comparing like for like.

 

[Lutz]

Easy; car manufacturers determine the fuel consumption of their vehicles by performing tests which are standard for the industry and which define prescribed cycles, supposedly simulating average city and open road driving conditions. If they find that they need a six speed box in order to fulfil their fuel consumption targets, they will. They don't do it for the fun of it (after all, a six speed box is, of course, more expensive than a five speed) nor is there any real demand from the customers (other than for an economical product).

 

[Guest]

All true but it does mean that either the box changes gear for you, or you do it yourself, more often. In an auto box it is often the changing of the gears that creates more heat than allowing the box to remain in one gear for longer, so I suspect it is a case of heads you win, tails you lose. It is obviously going to be exacerbated if a van is on the back. Many car manuals state 'lock' down a gear with auto drive, to stop changes when towing.

It is something that does concern me as I tow a lot with a 5 speed auto, and do so in the hot climes of the Med. So as my car has a computer that tells the garage (not me) if there is any overheating I asked for this to be cecked. Nothing was recorded yet I was towing in 40 degrees of heat in the summer, so although happy, am not really any the wiser.

 

[Garfield]

Easy; car manufacturers determine the fuel consumption of their vehicles by performing tests which are standard for the industry and which define prescribed cycles, supposedly simulating average city and open road driving conditions. If they find that they need a six speed box in order to fulfil their fuel consumption targets, they will. They don't do it for the fun of it (after all, a six speed box is, of course, more expensive than a five speed) nor is there any real demand from the customers (other than for an economical product).

In reply to comments,

We bought a six speed because a five speed was not available on the model we chose,

The s-max is doing nearly 70mph at 2000rpm in 6th, 57mph at 2000rpm in 5th

Certainly at 55mph 6th gear is no more economical than 5th, Going back to my old Golf GTi which did less than 40mph at 2000rpm in 5th but was still capable of 50mpg on a run, high gearing does not always equal economy,

 

[Lutz]

When the car manufacturers do their tests to determine the max towload, they would normally check whether the transmission is compatable with the load that they specify, even under tropical conditions. After all, they have to cater for the conditions in their export markets, too.

 

[emmerson2]

But why bother with gears anyway, no matter how many? Let the car do it for you! Drive auto.

 

[Garfield]

Just got back from our annual drive to Andorra in the new S-max, driving overnight on deserted french autoroutes allowed me to do some playing to releive the boredom.

Cruise control set at 80mph in 6th gear = 37mpg after 30minutes 5th gear = 39.5 mpg after 30 min, I didn't beleive these figures so I repeated it three times and got the same answer.

Dropping to 55mph in fog I tried 4th (54mpg), 5th (60mpg), and 6th ( 54mpg )

Compared to Cliff's crv, the S-max has a taller 6th (68mph @ 2000rpm)

Above 60 mph, 6th is quieter but that's its only use, with the van on the back it'll be a no go area. Otherwise the car is very good and handles like it's on rails even filled to the gunnels with ski gear and lots of duty free.

 

[giovanni_5060838]

All three of my cars have 6 speed gearboxes.

All 3 give better mpg at motorway speed than in 5th gear

the two that could tow,would very rarely get to use 6th

and unless the road was flat would properly give worse

mpg.

I can understand 6th gear being worse than 5th at 55 mph.

but at 80? very unlikely unless ford has made a mess with

where the real torque is...

 

[cris_408518]

My mate just said re six speed gear box's -

"It's so my Mum and Anna (lady friend) have more chance of finding a gear"

He says he's not sexist, you wouldn't want them driving your manual car either !

 

[STEPHEN_374226156]

I have a 6 speed Auto Vectra Tdi 150hp and tow a Coachman Amara 520/4 (1450 kilo) On a level road at 60mph it slips into 6th and drops to 1470rpm and maintains 36 mpg, Solo in 6th 52mpg is normal this is the first 6 speed auto I have ever owned in fact the first Vauxhall in a line of 27 tow cars inc 4x4s it is probally the best tow car we have ever had, in its first year we have covered 16600 miles 10320 of them towing both home and abroad I find the 6th gear to be an advantage with overall fuel economy, on board computors may be useful but the overall mpg is what counts For us that has been 31.7 mpg towing 41.8 solo and we live on the outskirts of London so endure poor traffic conditions

 

[michael_374229077]

1,extra gears are the manufacturer's answere to improved fuel consumtion under test (not always reflected under road conditions) &/or to disguise a narrow torque band (anyone remember the maxi,-both points applied!)

2,the best fuel consumption (other things being equal) will be obtained under conditions of least mechanical loss. because of cost or size constraints this can often mean that 6th gear is less efficient than 5th at certain revs (& 5th be less efficiant than 4th!). eg: it may involve more rotational components & therefore more friction & oil churning.

3, because it is easier to make an engine more efficient if designed to run within a narrow rev. band (eg: compare diesel with petrol: 2000 to 3000revs (6 gears) against 3000 to 5000revs (5 gears) for max torque bands) the future will dictate more ratios to compensate & thereby maintain an acceptable max. road speed. we are already at the realistic limit (6) for a manual box. hence the future will be semi-auto & then totaly auto as the no. becomes 7 then 8 ratios etc. & beyond what the average driver can be expected to control.

after a life-time in engineering the above is as simple as i can make it without geting over technical. i hope it is of some help to someone, somewhere.

 

[Themass31]

Did you take your van skiing gary.

craig

 

[Garfield]

No, we didnt take our van sking, I've been tempted but having done some accident repair work on our Lunar gemini and seen just what holds it together ( or doesn't hold it together ) and the lack of insulation, it is definitely not intended for winter use,

In reply to Michael, the powerful rev range of a diesel turbo is about 1600 - 4000 rpm (4000 - 1600)/4000 x 100 = 60% of max revs

for a petrol 3000 - 6000 (6000 - 3000)/6000 x 100 = 50% of max revs

In real terms the flexibility of a turbo diesel is every bit as good as that of a petrol, after 15 years in research labs, I am used to conducting experiments scientificaly, taking into account all factors

Past experience with petrols suggested that keeping in as high a gear as possible at all times without labouring the engine was the most economical way to drive but this experience with a turbo diesel suggests that there is a point of optimum efficiency regardless of road speed where the balance of engine speed vs engine torque is at its peak, beyond this, in a higher gear, using less revs and more torque to produce the same power is actually less efficient.

Perhaps as this car travels more miles and the engine and drivetrain loosen up, the mpg diiference between 5th & 6th will narrow

For those of us that have cruise control and average mpg display, why not try the experiment for yourselves, solo or towing?

My original point was that I have been unable to find the technical benefit of 6 gears, it's just another notch to find, one manufacturer introduces it on a sports model where, on a race track it has some benefit, before we know it 6th gear is a gimmick race throughout the fleet.

 

[seth1]

hello gary ,that must be one hell of a diesel that you drive,to make torque from 1600rpm to 4000rpm basically the redline ,i think you,ll find the torque will drop off at about 2500rpm although it might rev to 4000 there is no point in going up there.the big HP diesels all have narrower powerbands.its like a 130tdi golf is more useful on the road than a 150tdi.the powerband is broader and its more driveable.

 

[Garfield]

Although turbo diesels have a huge lump of torque at 2000rpm their max power is at 4000 and holding each gear up to this point will give maximum acceleration if you can stand the noise.

The last diesel that I drove that was all out by 2500 rpm was a Massey Ferguson

As far as the vw diesels go the 1.9 115hp 130 hp and the 150 all have exactly the same max torque of 229lbft @2000 rpm, the difference in power is due to the 130 & 150 having a flatter torque curve beyond 2000rpm producing more torque at higher rpm hence more power

The redline for the 1.8 - 2.0 diesels that I have driven is typicaly 4500 rpm

 

[rob_jax]

Hi guys,

Regarding economy and modern turbo-diesel engines and gearboxes - you should achieve the best fuel consumption when the diesel engine is operating in the maximum torque region.

In normal driving, you should see the "best" fuel consumption from the engine running at the very starting edge of the "flat torque region" - usually around 1800 rpm (varies from car to car).

I can quite believe that driving at 5th at one set speed may produce slightly better fuel consumption than a higher gear - where the engine is running below maximum torque. But increase the speed further, and suddenly 6th gear will become more efficient.

Gearing is quite carefully chosen to balance the capabilities of the engine with fuel consumption demands.

As Lutz says, my car has a 7 speed automatic gearbox and I can just about notice (usually from changes to the rev. counter) the gearbox adjusting itself to be in the "right" gear for the current speed.

And Lutz - if you read this - I was in Berlin at a CEN meeting in the DIN offices and spoke with some colleagues - would you PM me?

Robert

 

[Angus1]

Re. the VW diesels, dunno about the 115, but there was a difference with the 130`s. All but the sport had a 5 speed box and a torque figure of 210 lbs/ft, the sport having the 6 speed box and 228 lbs/ft. According to VW engineer this was due to the 5 speed box getting close to its safe torque limits for reliability and longevity, and the 6 speed box can handle much more. Having said that, the 5 speed box was never pleasant to use, with a wide gate and imprecise shift, the 6 speed being much nicer.

At the end of the day, you would expect it to be better and capable of handling more power as it is a newer design.

Why 6 speeds? Why not?

Why overhead camshafts when pushrods work?

Why multi-valve engines when two valves work?

Why turbo/superchargers etc........

I remember people asking why we needed 5 speed boxes when 4 were the norm.If anyone tried selling a new car with a 4 speed box with no synchomesh would there be any takers?

Its called progress.

You might not need it,like it, want it or understand it, but you can`t stop it ( unless the tree-huggers take over - then God help us all).

 

[seth1]

hello,i think people are getting mixed up with power and torque.although the average car diesel will make power to somewhere near 4000rpm(hp).the torque does die off around 2500rpm.now as a diesel is quite low reving its torque that really matters not hp.when figures are shown max torque at say 2000rpm thats it, no more to be had.it might run on for 500rpm or so either side of the peak.the idea of flat torque is torque without the revs ,=better fuel economy,but more gears are needed.

 

[Garfield]

Sorry Seth but I do know exactly the difference between torque and power and it's power that matters, you can exert torque but produce no power and no movement and its the torque available at the wheels that matters, thats why a 140 hp petrol with half the torque of a 140hp diesel can keep up by holding a lower gear for longer.

I think my experiment has proven that operating the engine at peak torque revs is not always the most economical, just as most drivers know that when going up a hill it is better to drop a few gears and let the engine rev, there comes a point for every vehicle where top gear is too high and you start to come back down the other side of the efficiency curve

 

[Icemaker]

from my experiance of many diesels it is of no use going beyond the engines max torque point regardles of how much bhp the engine sports, my alhambra 1.9 tdi 130 starts to pull strongly at 1600 rpm and falls off the power quickly at 2600 rpm if i change gear the car pulls strongly again and gains speed far quicker than reving the engine to its 4500 rpm limit. at 60 mph in sixth gear the engine is at 1700 rpm and right on the power band so unless the hill is steep i have no need to change down gears even with my coachman vip 530 at over 1600 kg on the back.

It is the same with diesel trains i drive, all of them are fitted with cummins diesel engines and voith gearboxes, when i take power the engine on full power is reving at 1730 rpm this is apparently its point of maximum torque, the power is fed to the gearbox and works through a torque converter up to 45 mph where the gearbox reverts to direct drive, at the trains max speed the engine will be doing around 1730 rpm, according to our engineers the engine rarely goes any faster than this as its pointless and uses far more fuel for no gains.

Six gears make my car better so why not have them, progress is great.

 

[seth1]

to gary,ive no idea what your talking about,ive never heard of it before,petrols and diesels behave totally different.saying torque doesnt matter,is totally wrong in my opionion.lets relate to a large diesel ,euro 3 cummins m11. 1300lbft @1100 -1500rpm but only 420 hp.relitively little power but lots of torque.typical of a diesel.surely ive no need to explain that due to this high gearing can be applied.and having a high reving engine doesnt equal high torque at the wheels.i own a 750cc sports bike ,130hp,14,000rpm redline,but absolutly no torque. seth

 

[Garfield]

How do you get 200lbft torque from a 100lbft engine?

Put a 2:1 reduction gearbox on it

Yes petrols behave differently to car diesels as they behave differently to commercial diesels

 

[seth1]

yes gary i can see what your saying on your last post.i had a friend who had that school of thought,liked to gear everything down.but whats the point in making something run at twice the speed.at the end of the day thats trying to get the torque increased is it not?now as were on a caravan forum i,d expect torque to be the required ingredent.on a drag strip forum i,d expect hp to be the required ingredent.i also would expect a small capacity diesel to act like a large capacity diesel,as long as their of similar design. seth