740d - invest
the
future
To
make the diesel 740d matching 735i in terms of performance, the price
is
rather heavy. You can count the additional components involved : a
common
rail direct injection system, a pair of turbochargers with variable
geometry
turbines, a pair of intercoolers, a specially machined crank bearing
block
and a cast iron block made by a special process. When BMW announced a
price
of only 5% more than the 735i, no doubt the 740d is sold at
considerable
loss.
Although beating
Mercedes
and Audi to launch the first V8 turbo diesel, the Munich firm expect to
sell just "a few thousand units" annually. They know this is just an
experimental
step. Brighter future will appear for V8 turbo diesel cars since 2004,
when the toughest-ever emission regulation be effective. According to
their
understanding, large capacity petrol engines are unlikely to pass the
regulation,
leaving diesel engines the only possible solution.
We rarely talk
about diesel
engines. If you are not familiar with their characteristics, please
read
AutoZine Technical School - Modern
Diesel Engine. Now I assume you are OK.
The heart of 740d
is a 3901c.c.
V8 with aluminium head and cast iron block. 2 banks of dohc, 16 valves
head has the same design as 320d and 530d. The high pressure
common-rail
direct injection system is doubled in the V8. Like many diesel engines,
a strong cast iron block is adopted to cope with the 18.0:1 compression
ratio plus the turbocharged pressure. Noise reduction is very crucial
to
diesel engines, so BMW added vermicular graphite to the block during
casting,
this also lighten the block by 20%. Anyway, with so much components
added
and without aluminium block, 740d found itself 150kg heavier than 735i.
Added with twin
turbo and
intercoolers, the result is 245hp (versus 238hp of 735i). However, the
advantage of diesel is always torque - 413 lbft at 1750-2500rpm. In
other
words, this is even more than the 369 lbft of M5's
5-litre V8 and 361 lbft
of 750i's 5.4-litre V12 ! To make good use of the
low-rev character (peak
power occurs at 4,000rpm), a very small final drive ratio is used, just
2.65. Also because of the abundance of torque, only 5-speed Steptronic
automatic is provided.
The advantage of
735i could
be noticed in 2 places : first, the throttle pedal. Once the
turbochargers
engaged at around 1600rpm, the engine is far more responsive than the
petrol.
Although the 0-62mph time of 8.4sec is 0.2sec slower than 735i, this is
mostly due to the turbo lag at very low rev. In real world, its passing
ability is superior. Second, the fuel gauge. Despite of the 150kg
weight
penalty, 740d returns a combined fuel consumption of 28.8mpg, compare
with
735i's 23.3mpg.
However, as an
additional
150kg is put on the front axle, handling must be deteriorated. Steering
is reluntant, with too much understeer. The second problem is the
noise.
Although sound insulation is quite effective, it
can't change the uninspiring
style of the noise.
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