BMW

On Tue, 25 Mar 2008 00:12:30 +0000 (GMT), "Dave Plowman (News)"
> wrote:

>Not an expert, but when you scale anything up directly using the same
>materials the strength goes down. Hence two small bolts are preferable to
>one large one, etc. So like for like two turbos with the capacity of one
>larger one will have less mass and will react quicker. If that makes sense.

Kind of like how long it takes 500 Dremel tools to spin up, compared
to one wood chipper.
--
Dan.

On Mon, 24 Mar 2008 20:33:53 -0400, Dean Dark >
wrote:

>On Tue, 25 Mar 2008 00:12:30 +0000 (GMT), "Dave Plowman (News)"
> wrote:
>
>>Not an expert, but when you scale anything up directly using the same
>>materials the strength goes down. Hence two small bolts are preferable to
>>one large one, etc. So like for like two turbos with the capacity of one
>>larger one will have less mass and will react quicker. If that makes sense.
>
>Kind of like how long it takes 500 Dremel tools to spin up, compared
>to one wood chipper.

Fabulous analogy! <golf claps>

/daytripper
'00 s4 6spd

In article >,
Dean Dark > wrote:
> On Tue, 25 Mar 2008 00:12:30 +0000 (GMT), "Dave Plowman (News)"
> > wrote:

> >Not an expert, but when you scale anything up directly using the same
> >materials the strength goes down. Hence two small bolts are preferable
> >to one large one, etc. So like for like two turbos with the capacity of
> >one larger one will have less mass and will react quicker. If that
> >makes sense.

> Kind of like how long it takes 500 Dremel tools to spin up, compared
> to one wood chipper.

Brilliant. Wish I'd thought of that. ;-)
If only I knew what a wood chipper was.

--
*A journey of a thousand sites begins with a single click *

Dave Plowman London SW
To e-mail, change noise into sound.

On Mon, 24 Mar 2008 22:06:10 GMT, dizzy > wrote:

>daytripper wrote:
>
>>On Mon, 24 Mar 2008 01:22:46 GMT, dizzy > wrote:
>>
>>>daytripper wrote:
>>>>
>>>>More to the point: two smaller turbos have much less inertia than one
>>>>equivalent turbo, which goes directly toward reducing boost lag...
>>>
>>>I've read that, yes, and perhaps it's true. I'd like to see the math.
>>>It's not obvious to me why two "half sized" is better than one "full
>>>sized" turbo.
>>
>>It is simply a function of physics: all else being equal (materials used,
>>turbo design, etc) you need a larger impeller to make the one turbo provide
>>the same volume of charge air as the two. That single impeller will have a
>>larger diameter as well as higher mass, thus it has a higher inertia to deal
>>with on spin-up, apparently a difference that cannot be fully countered by
>>having all of the exhaust volume working on it vs the halved volume the two
>>smaller impellers receive...
>
>Well, it must be the phyics, and I'm not saying what you wrote (which
>IS the common wisdom) is incorrect.
>
>Still, I'd like to see the math - something along the lines of "the
>turbo's air-moving capacity goes up with the square of the diameter
>while the rotational inertia goes up by the cube" (and why).
>Preferably from an expert, not one of us usenet pundits. 8)

Dizzy - As one that has built many turbo engines in the early days 1982 - 1990
for a number of UK dealers that were sanctioned by the importers or makers to
supply tuned cars I can speak with a fair amount of knowledge.

The smaller the turbine and impeller the faster it accelerates

The problem with a fast spinning impeller is what marine engineers relate to as
"cavitation". That is when the propeller spins in turbulent water with bubbles
etc and the boat goes nowhere - the force of the screw is negated and it's like
clutch slip on a manual car.

Air is a solid - it doesn't feel like that or look like that but if it wasn't
then airplanes wouldn't fly - get the idea?

On a diesel that doesn't really rev to high speed compared to a petrol engine 2
x small turbos are ideal because they stave off the dreaded LAG and give low
down torque. BTW Diesel engines are restricted to a max of about 5K rpm because
the fuel doesn't burn faster and the deliver systems are a little slower then
petrol injection.

BMW actually use two different size turbos on the 3.0Lt six pot diesel to do
just that - the smaller one for low torque and the larger for higher revs - the
smaller is slowed when the exhaust drives the larger unit - all very nicely
balanced.

OTOH - some tuners fit 2 x turbos to serve 1/2 the engine like a V8 having one
on each bank or a V6 or 6 in-line serving 3 cyl each. Smaller turbos faster
pressurization. RACE cars and other specialist situations require different
solutions but think on it a little.

If going for the world land speed records you don't need fast acceleration but
you do need top end power so large turbos required
--

Sir Hugh of Bognor

The difference between men and boys is the price of their toys.

Intelligence is not knowing the answer but knowing where and how to find it!

Hugh Gundersen

Bognor Regis, W.Sussex, England, UK

On Tue, 25 Mar 2008 13:02:41 +0000, wrote:
[...]

>Air is a solid - it doesn't feel like that or look like that but if it wasn't
>then airplanes wouldn't fly - get the idea?
[...]

fwiw, "air" is considered to be a fluid, not a solid...

You can look it up ;-)

/daytripper
'00 s4 6spd

On Tue, 25 Mar 2008 11:36:55 -0500, daytripper >
wrote:

>On Tue, 25 Mar 2008 13:02:41 +0000, wrote:
>[...]
>
>>Air is a solid - it doesn't feel like that or look like that but if it wasn't
>>then airplanes wouldn't fly - get the idea?
>[...]
>
>fwiw, "air" is considered to be a fluid, not a solid...
>
>You can look it up ;-)
>
>/daytripper
>'00 s4 6spd

Good point and you are correct my misquote of course its a fluid but the problem
is some people won't drown in it......
--

Sir Hugh of Bognor

The difference between men and boys is the price of their toys.

Intelligence is not knowing the answer but knowing where and how to find it!

Hugh Gundersen

Bognor Regis, W.Sussex, England, UK

On Sun, 23 Mar 2008 20:43:33 -0400, "Pete" > wrote:

>"DCA" wrote
>> Ah - but from what I am advised, they are not both fitted to provide
>> twice the boost (which of course a single big one could do), they are
>> to provide different boost for different engine running conditions? Am
>> I wrong?
>
>They are two identical turbos running in parallel, each one supplying
>compressed air to three cylinders. The advantage of having two small
>turbos vs. one large one is that the smaller turbos spool up faster and
>therefore minimize turbo lag. The disadvantage is that small turbos run
>out of steam at higher rpms.
>
>What you're probably thinking of is a bi-turbo setup where there is one
>small turbo that engages at low rpms and one larger turbo that engages
>at high rpms. That is not the case in this BMW engine.

I suggest you check again Pete. The 3.0L diesel 6 does have differing size
turbos. But then you don't get this model over there - it's Euroland's biggest
seller apart from the V8 diesel in Germany and the rest of Europe we British
don't get the V8 in RHD as it's too costly to re-engineer the firewall and
steering around the V* - so they say but we do get the V8 & V12 petrol variants.


>
>Cheers,
>Pete
--

Sir Hugh of Bognor

The difference between men and boys is the price of their toys.

Intelligence is not knowing the answer but knowing where and how to find it!

Hugh Gundersen

Bognor Regis, W.Sussex, England, UK

wrote:

>Dizzy - As one that has built many turbo engines in the early days 1982 - 1990
>for a number of UK dealers that were sanctioned by the importers or makers to
>supply tuned cars I can speak with a fair amount of knowledge.
>
>The smaller the turbine and impeller the faster it accelerates

Sorry, but that statement helps not at all, as it is devoid of *any*
conditions. One can only guess that the conditions are "everything
else equal", but that makes the statement absurdly obvious.

You see, with two smaller turbos, each gets only half the exhaust to
drive it, comparted to a single larger turbo.

wrote:

>On Sun, 23 Mar 2008 20:43:33 -0400, "Pete" > wrote:
>>
>>What you're probably thinking of is a bi-turbo setup where there is one
>>small turbo that engages at low rpms and one larger turbo that engages
>>at high rpms. That is not the case in this BMW engine.
>
>I suggest you check again Pete. The 3.0L diesel 6 does have differing size
>turbos.

Conversely, in the Toyota Supra Turbo, possibly the best-known
"sequential" twin-turbo, the turbos are actually the same size.

On Wed, 26 Mar 2008 00:06:45 GMT, dizzy > wrote:

wrote:
>
>>Dizzy - As one that has built many turbo engines in the early days 1982 - 1990
>>for a number of UK dealers that were sanctioned by the importers or makers to
>>supply tuned cars I can speak with a fair amount of knowledge.
>>
>>The smaller the turbine and impeller the faster it accelerates
>
>Sorry, but that statement helps not at all, as it is devoid of *any*
>conditions. One can only guess that the conditions are "everything
>else equal", but that makes the statement absurdly obvious.
>
>You see, with two smaller turbos, each gets only half the exhaust to
>drive it, comparted to a single larger turbo.

Dizzy - are you an arsehole by trade or were you born like it.

Try this experiment.

Small bucket - or paint can - 1 quart - fill with water and tie a piece of
string to it about 3 foot long - holding end of string spin the water filled can
in a circle above your hear in a vertical plane - may require a few swings to
get going. Try accelerating it spinning faster.

Now stop and empty the water out - repeat - spins easier and faster.

Now try same with a longer piece of string.

Finally tie string around neck and dangle from tree.

--

Sir Hugh of Bognor

The difference between men and boys is the price of their toys.

Intelligence is not knowing the answer but knowing where and how to find it!

Hugh Gundersen

Bognor Regis, W.Sussex, England, UK