Any - Why so many gears? | Motoring discussion | Back Room Forum

Fri 25 Mar 2011 13:11

Any - Why so many gears? - RaineMan

In the past week I have been driven in two (turbo) diesels; a Passat and a Renault (?). Both of there were around 2l and had six speed gearboxes. With modern technology they seemed quite happy at 30 mph in 6 th . This set me wondering if they are so torquey why do they need six gears? I have quite a bit of experience of “classics” and whilst I accept that three speeds are awkward at times (e.g 100E, Consul) they are quite adequate (apart from motorways) on cars like Zephyrs/Zodiacs and Crestas. I tend to feel that 4 + O/D is about right. So why are these cars supplied with six speed boxes – or is it just to make you want something you do not need?

Reply | Report as offensive | Link

Fri 25 Mar 2011 14:35

Any - Why so many gears? - craig-pd130

Although modern diesels develop fantastic amounts of torque and power, they do it over a relatively narrow range of rpm, compared with a petrol engine.

A typical petrol engine of up to 2 litre capacity (whether turbocharged or not) will have a useable rev range from around 2,000 to 6,000rpm.

A typical turbo diesel up to 2 litre capacity has a useable rev range from around 1,500rpm to 4,000 ... most are red-lined at around 4,500rpm, and if you look at the engine's power curves, they often peak around 4,000 and so there's little point revving beyond that mark.

So the narrower band of useful rpm means having more gears to fully exploit the car's performance. You mention that both cars seemed happy at 30mph in 6th -- that sort of use isn't very good for clutches / flywheels / gearboxes. The cars will do it, but it isn't good for them.

Sure, they don't really NEED 6 gears; 5 is probably enough for most cars and most drivers. But having 6 gears means the car can have an ultra-long top gear (good for economy on motorways) while have 4 useable ratios for other road conditions (first gear is not really useable on most diesels apart from town crawling).

Reply | Report as offensive | Link

Fri 25 Mar 2011 16:40

Any - Why so many gears? - Sofa Spud

Our VW Touran 1.9 TDI has a 6-speed box and I have to admit that it would be just as good with 5 gears like my old Passat TDI in normal motoring. If the car was heavily loaded and / or towing a caravan, then 6 gears might have the edge over 5.

Reply | Report as offensive | Link

Fri 25 Mar 2011 18:40

Any - Why so many gears? - RaineMan

The point I was making was that with modern ECUs (assume diesels have these or equivalent) these cars were pulling smoothly at very low RPM. The last 2l petrol manual I owned struggled in fourth at these speed and would not have pulled 5th.

Reply | Report as offensive | Link

Fri 25 Mar 2011 18:48

Any - Why so many gears? - Boulder2k9

A better question might be Why do BMW's and Lexus need 8 speeds in their respective gearboxes??

Reply | Report as offensive | Link

Fri 25 Mar 2011 19:05

Any - Why so many gears? - Andy P

To quote JC - "more is better"

Enuf said :-)

Reply | Report as offensive | Link

Sat 26 Mar 2011 13:10

Any - Why so many gears? - SlidingPillar

My Morgan three wheeler only has two (and no reverse...).

But first is a monumentally high ratio and redlined it would be doing 45 mph. It's a pain in traffic queues.

Reply | Report as offensive | Link

Sat 26 Mar 2011 13:50

Any - Why so many gears? - craig-pd130

@SlidingPillar, which model JAP engine do you have?

Reply | Report as offensive | Link

Sat 26 Mar 2011 14:17

Any - Why so many gears? - SlidingPillar

LTOW ie the overhead valve dog ear engine.

With its environmentally friendly total loss oiling system... These days, the waste oil is caught in a tin, but as designed, it lubricated the roads.

Reply | Report as offensive | Link

Sat 26 Mar 2011 22:16

Any - Why so many gears? - Sofa Spud

QUOTE:...""My Morgan three wheeler only has two (and no reverse...).""

Wasn't the TVR Speed 12 originally designed with a 2-speed manual gearbox, dropped for the production version?

Reply | Report as offensive | Link

Mon 28 Mar 2011 14:23

Any - Why so many gears? - Roly93

In the past week I have been driven in two (turbo) diesels; a Passat and a Renault (?). Both of there were around 2l and had six speed gearboxes. With modern technology they seemed quite happy at 30 mph in 6 th .

Sorry I find this hard to believe frankly. I have had two six speed diesels both of them from VaG, and I dont think a diesel Passat would be anywhere near happy and in its powerband at 30mph in 6th. That is unless your definition of happy is zero acceleration and the engine shuddering around on its mounts like a dimented jelly !

Diesels need the extra gears to cope with the narrow powerband/turbo boost region.

Reply | Report as offensive | Link

Mon 28 Mar 2011 20:39

Any - Why so many gears? - Sofa Spud

My Touran 1.9 TDI, mentioned earlier, is happiest in 4th at 30 mph, althought it would probably pull in 5th and shudder along in 6th at that speed. My old Passat TDI, also mentioned earlier, was definitley not happy even in 4th at 30 and so needed to be kept in 3rd of its 5 speed box.

Reply | Report as offensive | Link

Wed 30 Mar 2011 19:33

Any - Why so many gears? - RaineMan

In the past week I have been driven in two (turbo) diesels; a Passat and a Renault (?). Both of there were around 2l and had six speed gearboxes. With modern technology they seemed quite happy at 30 mph in 6 th .

Sorry I find this hard to believe frankly. I have had two six speed diesels both of them from VaG, and I dont think a diesel Passat would be anywhere near happy and in its powerband at 30mph in 6th. That is unless your definition of happy is zero acceleration and the engine shuddering around on its mounts like a dimented jelly !

Diesels need the extra gears to cope with the narrow powerband/turbo boost region.

Whilst I would agree they had next to no acceleration neither was shuddering in the least. This was on the level. When the Passat driver came to a slight uphill stretch he did change down to 4th and then 3rd. But on the level in 6th quite smooth.

Reply | Report as offensive | Link

Wed 30 Mar 2011 22:07

Any - Why so many gears? - Lygonos

At 30mph in 6th the cars would have been running at about 850rpm.

Not good.

It's possible it felt like 30mph but was more like 40-45 where the turbo would be just about to chime in.

Reply | Report as offensive | Link

Thu 31 Mar 2011 17:12

Any - Why so many gears? - Glaikit Wee Scunner {P}

40mph is the slowest I dare go in my 6 speed Octavia diesel. At 30mph , as said above, there would be no aceleration and the engine might be below its idle speed and attermpting to detach from its mountings. I usually use 4th at 30mph.

Ask Honest John

Sell Your Car

Real MPG

Fuel Calculator