Why 6 speed gearboxes?

[spj]

Hi all,

Normally the higher the torque the better the tow vehicle, a diesel as any modern engine can have a small torque band and to get the best out of it more ratios can be used, a Cummins M11 engine as fitted to a number of HGVs will pull 44 tons at 56mph at only 1500rpm in 16th gear, pulling this kind of wieght requires lots of torque and little bhp hence the diesel engine and 16 speed box, there are no recent petrol HGVs because they lack pulling power compared to diesels, HGV engines have the same technology as cars except they had it first, Cummins used common rail on the L10 engine over 20 years ago and still do but they still have a narrow torque band needing more gears, a lighter vehicle such as a car fully loaded towing a caravan will benefit from more gears like a HGV so the engine can run at it's maximum torque, this can improve perfomance and economy. A petrol engine can produce reasonable torque but gives more bhp, they can hold gears for longer due to them revving higher and size for size are usually quicker but will not pull the same when left in gear towing.

spj.

 

[Garfield]

OK here's a basic physics lesson

Torque is measurement of rotational force, you can exert torque on a seized nut using a spanner but it won't move anywhere and no power is used / created, so torque alone will not move any vehicle.

Power = force X velocity (torque X rpm) ie when an engine exerts torque on a moving shaft to overcome resistance to movement, then we start to move.

When Ford state that the 2.0 tdci produces 340nm at 2000rpm you can calculate the power from this (71.2kw) similarly, from a stated 105kw at 4000 rpm you can calculate the torque (252nm)

The point of this post is that many people that buy new cars trade them in for another new car before things start to go wrong, this new car of mine is an absolute luxury that will have to last in excess of 100000 miles. 30 years ago cars simply fell apart and required replacing then audi started galvanising them and others followed, engines started to last longer, reducing the need to buy a new car. our last car, a 1999 Sharan with 115k on the clock still had an excellent engine and bodywork but it was in system failure, air con wipers, window motors etc packed up one by one till we'd had enough and replaced it.

I always say the best car I ever had was a 1989 Golf GTI, it had electronic fuel injection and ignition which made it reliable, powerful and economical but it had no power steering no electric windows or any other complex electronics to go wrong

 

[MONKEY1]

Little bit of anecdotal evidence but worthy of consideration. Following on from the posts I conducted the following experiment.

On Monday I drove from Kent to Cornwall , 300 odd miles conducted at 65 mph in 6 gear result 51.6 mpg returned in similar weather conditions for the return @ 65 mph but in 5th result 56.5 mpg.

I know it's only one run and I only take the green zone for 5th is more optimised at 65 mpg than in 6th. simple

 

[Garfield]

Interesting! so it perhaps not just our car, anyone else done the same test, why not post the results but include mph @2000 rpm in 5th & 6th

For us 5th = 56mph @2000, 60mpg @56, 39.5 mpg @80

6th = 68mph @2000 54mpg @56, 37mpg @80