I've always had a pretty good grasp of the mechanics of it all,
including how the TC works and the subtleties of what it does.

I was kind of thinking that torque multiplication rather than the
'disengaging' effect of low torque transmission at low RPM was the
underlying reason they were first used, and I suppose there was then
no real reason to change things later.

It has several functions. First it acts as a 'clutch' when starting from
rest. Next it acts rather like a continuously variable gearbox by allowing
the engine speed to exceed its output speed by up to around 2000 rpm and
converting most of that speed into extra torque at lower output RPM. Not
all though, as it gets hot in doing so. ;-) It also - and crucial for most
autos - cushions the actual gearchange.

When autos had two or three ratios, the torque multiplying feature was
crucial to reasonable performance - albeit at the expense of economy.
Boxes now have up to what? 8 ratios, so this part isn't much needed. So
many just use it for starting off and each gearchange, locking it out of
action via an extra clutch for most of the time.

Then why do you *have* to have a torque converter?  Oh dear, I'll
never understand this...

Probably because you've already got 'clutches' in a planetary box which
can do the same thing. Didn't the Model T Ford use this principle? Later
pre-select boxes of course used a fluid flywheel for starting from rest.

That's a good argument *for* putting a planetary gearbox, rather than
a synchro. one, behind a friction clutch.  I'd love to know what I've
missed that would tell me why it's not done.

Gear life depends on accurate meshing - so the loads on the teeth are
correct. Much easier to achieve in a planetary setup. On a synchromesh box
there is a fair distance between bearings.

But the numbers in real-life of TCs and friction clutches among
different power levels of engines doesn't seem to bear that out.

Why would it make any difference whether there was a planetary gear
set or a mainshaft/layshaft gear arrangement behind the clutch?

Because the TC will handle a heck of a lot more torque application.  It
"gives" where the friction clutch wears out.

Eisboch

Not that I'm a fan of TC transmissions, but there is something just
inherently cool about a planetary gear set.  Just a simple arrangement
of a few cogs that stays fully meshed all the time, yet it gives you
two forward speeds and reverse simply by holding a different part of
it still.  It's almost like god invented it because he knew it would
come in handy when we got around to making cars.

I've mentioned it before, but I've often wondered why no one ever put
a friction clutch in front of one instead of a torque converter.

Because the SMG isn't sufficiently developed to replace a conventional
auto. Nor does it give the same driving satisfaction as a true manual.
Things may change, though. If you require a 'clutchless' change with total
control over the gears, both automated synchromesh boxes and torque
convertor autos are improving all the time. I'd not take a bet which will
win - although my money says the TC auto since it's intrinsically more
reliable.

Dave, you can take that position if you like, but why does BMW (and others)
offer three different transmission types for the Z4?

Manual
Automatic
SMG

Eisboch

In auto mode, yes. Different when doing the selection yourself and in a
hurry. The two aren't mutually exclusive - Audi manage it. As will BMW
when they adopt a twin clutch SMG shortly.

If it can change gear without any action on the part of the driver, it's
an auto. There is no argument possible about that. An auto isn't defined
by the presence of a torque convertor.

You  want it to be as smooth as an automatic?  Then buy an automatic.
That's not what it is.

And yes, I've experienced a decent automatic.  The  one in my 540 isn't bad

Have you any experience of decent autos? Not being funny, but the SMG
isn't in the same class, smoothness wise. And I've driven a recent M5.

You must be driving a  different SMG than I am.  I can choose the
'roughness' of the shifts.  1-2 means smooth.

4-6 means fun.

Tom

I agree. Until BMW join the 21st century and use a twin clutch SMG the
rough changes when in auto mode just ain't acceptable. It's one thing
having 'punchy' changes when enjoying a country road, but not when
trickling through traffic.

The M3 would be too much money for a weekend car.  And I am kind of old
school.  Automatic or manual.  The SMG may be nice but I prefer tried and
true technology.

You might want to at least drive an M3.  Try the SMG too.  Some people don't
like it, as the previous poster said.  I have it in my M3 and love it.

Definately not an automatic and if that is the 'feel' you want, the SMG is
not for you.  If you like a 6-speed, but want an auto mode in traffic, the
SMG is great.

Love mine.

But I have a 540 when I need an auto

They are two different cars.  The Z4 is not practical but it will be a
"weekend" car and not an everyday drive.  The 3 series would be my second
choice although there is  not much room in the back seat.  My third choice
would be a X3 but I don't like the expense and maintenance associated with
AWD regardless of the improved handling.

I would get the auto. From test drives, BMW autos seem pretty good to
me. I would suggest staying away from the SMG. I had a 2004 Z4 3.0i with
the SMG and although the technology was cool - I am a bit of a geek and
like that kind of stuff - it was not very smooth shifting. I would say
it is one of the reasons I traded the car in so soon (that along with
troubles with the top and wanting room for more than 2 people). I now
have a 2006 330i with a 6 speed manual and I think it is a much better
automobile all around.

Anybody have a preference of automatic over stick?

I will be buying a Z4.  I want an automatic because 1) I will not be driving
it like a race car and 2) the advances in automatic transmissions make them
as reliable as manuals and 3) I have been driving autos for over 30 years
and am ready for a change.