Hi,

It seems the world is now more and more obsess with numbers. 3 turbos, 4 turbos, 4 angle steering wheels etc.

The new Porsche 911 would have a 7 seed MANUAL gear box.

Whilst the DSG in Skodas, VWs etc also have 7 gears and the Mercs and Lexus have 8 gears, they are all in effect automatics, meaning no hassle shifting up and down.

But why would you want a 7 speed manual gear box?

Obviously the trucking world have had 12 gears for a long time to cope with extra hauling weight etc, and overdrives have featured in manual classic cars (eg TR6 with 7 forward gears, 2nd, 3rd and 4th with overdrive).

I understand the Porsche is making the first 6 gears sporty and short, whilst the 7th is a crusing gear, isn't it a little OTT?

THe amount of gear changes you need to make from stand still from a service station to crusing speed is strange.

I have always had a theory that a 4 speed auto is as good as a 7 speed auto if the car has enough torque and power.

And what happened to overdrives, why have they disappeared?

I can see the benefit on a 7 speed manual with a car designed to go 170+ mph.

You probably don't use 7th until at 70mph and wanting to plod along at low rpm getting good economy.

Hi,

It seems the world is now more and more obsess with numbers. 3 turbos, 4 turbos, 4 angle steering wheels etc.

The new Porsche 911 would have a 7 seed MANUAL gear box.

Whilst the DSG in Skodas, VWs etc also have 7 gears and the Mercs and Lexus have 8 gears, they are all in effect automatics, meaning no hassle shifting up and down.

But why would you want a 7 speed manual gear box?

Obviously the trucking world have had 12 gears for a long time to cope with extra hauling weight etc, and overdrives have featured in manual classic cars (eg TR6 with 7 forward gears, 2nd, 3rd and 4th with overdrive).

I understand the Porsche is making the first 6 gears sporty and short, whilst the 7th is a crusing gear, isn't it a little OTT?

THe amount of gear changes you need to make from stand still from a service station to crusing speed is strange.

I have always had a theory that a 4 speed auto is as good as a 7 speed auto if the car has enough torque and power.

And what happened to overdrives, why have they disappeared?

Trucks need 12 speed gearboxes for similar reasons to cars needing 8 speed gearboxes - in theory, an engine with enough torque and power just needs a single gear. In practice, you can make your vehicle more fuel-efficient by using an engine with less peak torque and power, and a multi-ratio transmission letting you run the engine at the same revs for different road speeds. Your 4 speed auto is indeed as good in driveability terms as a 7 speed, with the right engine, but you lose out on fuel efficiency compared to the 7 speed gearbox.

Assuming perfect transmission efficiency (i.e. power in is the same as power out), and that all transmissions are the same weight, but with real-world engines, you want infinite gears, so that for any given power output required, the engine is running at the revs that gives you maximum fuel efficiency. In practice, gearboxes introduce their own losses, and so you compromise based on the weight of the gearbox, the loss it introduces, the complexity of working the gearbox, the expected workload of the vehicle (no point having 18 gears if you'll only ever use 12 of them), the torque and power curves of the engine and all the other engineering tradeoffs you have to make in designing a car.

As to why overdrives fell into disuse, your guesses are as good as mine - possibly car makers just feel that a 6 speed manual is easier to handle than a 4 speed + overdrive unit (as while the 4 speed + overdrive has 8 gears, 1st overdrive is typically useless, and to use all the gears appropriately requires constantly bring the overdrive in and out).

6 gears is too many in some cars.

Our BMW diesel had a 6 speed auto box at 70mph it would turn over at 1500rpm

the VRS manual petrol at 70mp in 6th is near 2500!!

7 and even 8 gears is excessive.

Typically, with a car, more gears are used for one of 2 things:-

1) To close up the ratio changes between gears, so you can keep closer to peak power, torque or best specific fuel consumption revs whilst accelerating, and tune the engine closer to PPTBSFC for hill climbing and cruising.

2) To produce a drivable set of N-1 ratios, and an over-driven top for cruising.

The theoretical ideal here would be a continuously variable transmission. Unfortunately, the Honda Jazz CVT is the only one I've ever encountered that can deliver PPTBSFC according to driver demands for acceleration or cruising.

As for why overdrives fell out of favour, it's a mixture of production costs (An N+1 speed box tends to be cheaper to make than an N speed plus an overdrive unit), reliability (ODs on 4+OD broke more often than top on a 5-speed box) and physical space (OD was only ever fitted to RWD cars; a quick look under a transverse engine FWD car should tell you why).

If anything, they should be putting more gears on cars with less power.

As an Example my Leon has ~260 bhp, 6 gears, and i very rarely need 4th and 5th. Yet my A6 has 130bhp, 5 gears, and desperately needs an extra gear for driving ~35mph, where 3rd is too low, and 4th is too high.

Nine is probably too many in a automatic family car. Six seems plenty in a manual box if you sequential shift although I often shift up to 2 or 3 gears in the manual TSI VRS.

Lots of cars have 8 now and I have 7 in my 1.8 TSI but I think it could usefully use 8.

7th is 29 mph/1000 and the engine revs to 7000 rpm but max torque occurs at 1500 rpm ie only about 44 mph and I feel it could easily pull a gear with about 31 or 32 mph/1000.

I see the 8 speed DSGs as inevitable and, paired with TSI cylinder cut out, fuel economy should continue to improve when these innovations are deployed by several percentage points.

Edited 7 September, 201114 yr by lol

When I told my dad I'd ordered a vRS and told him it had 7 gears he asked was I buying a car or a mountain bike . I've driven plenty of cars with 5spd boxes that oculd really do with a 6th or even 7th. The Mk1 Octy diesels definitely could.

Taken to an extreme you have the CVT which either has an infinite number of gears or 1 depending on how you look at it.

I recall redaing recently that a luxuary car brand (Merc I think it was) was going to launch a 9 speed auto with two reverse gears!

Mad!

When I told my dad I'd ordered a vRS and told him it had 7 gears he asked was I buying a car or a mountain bike . I've driven plenty of cars with 5spd boxes that oculd really do with a 6th or even 7th. The Mk1 Octy diesels definitely could.

Interesting; I've got a Mk1 TDi, and often skip-shift 3rd to 5th if I've been overtaking something doing under 40mph.

Postmanpat's new bike has 27 gears

Interesting; I've got a Mk1 TDi, and often skip-shift 3rd to 5th if I've been overtaking something doing under 40mph.

My dad's Octy was the 1.9 SLX TDi 110 and I drove it to Middlesborugh and back from Swansea. I just found that while 5th had plenty of poke, at 70mph the revs were a bit high and another gear would have been nice. We still got 75mpg out of it though over 600 miles which is excellent.

I recall redaing recently that a luxuary car brand (Merc I think it was) was going to launch a 9 speed auto with two reverse gears!

Mad!

If you're doing a dual-clutch transmission (DSG-type), two reverse gears isn't quite as insane as it sounds - you put one on each clutch, so you have reverse-odd and reverse-even. Not quite sure why you'd want it, though - I don't spend enough time going backwards to care about the reverse ratio, as long as it's similar to 1st.

And 9 forward speeds on a DCT makes sense for a performance car - sit at a nice efficient engine revs which you tune to be as quiet as possible pretty much regardless of road speed, then be able to rapidly downshift for power and a suitable noise when you decide that you're going to overtake. After all, if the car's doing all the gearshifting, you don't actually care if it does 8 gearshifts stopped to 130 km/h, or 5 gearshifts stopped to 130 km/h - you just want it to be quiet and comfortable for cruising, and to deliver lots of power when you decide to rag it.

My dad's Octy was the 1.9 SLX TDi 110 and I drove it to Middlesborugh and back from Swansea. I just found that while 5th had plenty of poke, at 70mph the revs were a bit high and another gear would have been nice. We still got 75mpg out of it though over 600 miles which is excellent.

Mine's the Elegance 110 (205/55R16s rather than 195/65R15s), and geared to just under 30mph/1000rpm, so maybe 2400rpm at 70 actual. I've doubts about a higher top being usable on anything other than a dead flat road.

See (and hear :p ) David Coulthard drive a Williams FW15 F1 car with CVT:

Does a CVT prolong an engine or reduce it's life under constant full throttle?

or to rephrase, does variable revs (changing gears and jerking) prolong the life of an engine that is always at full revs?

I personally think constant full revs would kill the engine (I mean constant), but surely the jerking and changing revs may also affect the engine in some way?

Does a CVT prolong an engine or reduce it's life under constant full throttle?

or to rephrase, does variable revs (changing gears and jerking) prolong the life of an engine that is always at full revs?

I personally think constant full revs would kill the engine (I mean constant), but surely the jerking and changing revs may also affect the engine in some way?

If max torque is what you're after you get it well below max revs (say at 4,000 rpm for a petrol engine with the red band at 6,500 rpm and over)

However, going at 4,000 rpm constantly demands good sound insulation :p

'Common sense' (not always so sensible) tells me that an engine constantly running at 4,000 rpm will break before one that most often purrs at 2,500 but that might be completely wrong.

Does a CVT prolong an engine or reduce it's life under constant full throttle?

or to rephrase, does variable revs (changing gears and jerking) prolong the life of an engine that is always at full revs?

I personally think constant full revs would kill the engine (I mean constant), but surely the jerking and changing revs may also affect the engine in some way?

Full throttle - how is a CVT under full throttle all the time? You control the throttle, not the gear box?

The 1970's big block American autos had only 2 gears as they didn't need any more with their 7.3 litre V8 engines and 55 mph limits !

Does a CVT prolong an engine or reduce it's life under constant full throttle?

or to rephrase, does variable revs (changing gears and jerking) prolong the life of an engine that is always at full revs?

I personally think constant full revs would kill the engine (I mean constant), but surely the jerking and changing revs may also affect the engine in some way?

As others, I don't understand your point (or possibly what you're asking).

A well setup CVT (and yes I've driven one) holds the minimum constant revs it needs to give you the acceleration or steady road speed you're looking for. Accordingly, the only time it will ever venture above peak power revs is if it's under-geared and you're doing a max speed run. During an acceleration test it will hold peak torque revs until it needs to rev higher to give you higher road speed.

That's not entirely obvious from the video, because that's a racing engine, and turns at 2 or 3 times what a road engine does, but only has about the same difference between peak torque and peak power revs.

To clarify, what I mean is when the throttle is pushed to the ground, the car would maintain maximum revs and would this be better for the engine or not?

Might be easier to clarify with a video I found. Under acceleration the rev is near the red-line.