Ron,
Here's an explanation provided from the Pantera forum (not sure if the poster is "in-the-know or not):
<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR> The Ford modular V8 packaging was compromised for transverse mount
installations. When the engine was designed, Ford believed many of
them would be installed in transverse front wheel drive applications.
This dictated the engines be as short as possible longitudinally so
they would fit in the planned engine compartments when installed
transversely. The bore spacing is very narrow resulting in very
small bores and long strokes for their displacements. The 4.6L and
5.4L V8's have bores about the size of my little Buick 215 cube V8.
To get the displacement desired, the blocks had to be relatively tall.
The small bores limit the valve curtain area and, ultimately, the
maximum flow the heads can achieve and power the engine can develop.
This forced the designers to 4 valve per cylinder heads for the high
performance versions. Unfortunately, the heads and cam drives were
made very bulky. Combine the enormous cylinder head castings with the
tall decks needed to contain the long stroke cranks and you get an
engine that is very tall and wide and doesn't lend itself to swapping
in many engine bays. In a Pantera that can easily swallow a 460 big
block, either the heads must altered (shaved for clearance) or the
inner fenders clearanced. Even in aluminum, the engines are heavy
for their displacement. A look at this comparison shows why:
http://www.vorshlag.com/pictures/motor-4.6-4V-004.jpg
With the limited bore spacing, stroke and extra cylinders are your
only ways to get extra displacement. The 5.4L V8 (with a taller deck
to accomodate the longer stroke crank) and 6.8L V10 are the direct
result. The 5.4L V8's have an even worse bore/stroke ratio than the
4.6L, resulting in very high piston speed. A bunch of the Lightning
truck 5.4L engines have snapped connecting rods and ventilated blocks
due to this.
Given these (and other) deficiencies, it's quite clear why Ford has
had to resort to supercharging to stay competitive. John Colleti,
the head of Ford's Special Vehicle Team recently gave an interview
in which he explained why the Ford's 2003 Cobra is supercharged.
He said they were able to reach their power goals with the normally
aspirated 4.6L DOHC V8 but they couldn't keep the motor together
for the durability cycle, even with premium aftermarket race rods
(2003 Cobras use Manley H-beam forged rods and forged steel cranks).
They also reverted to an iron block from the previous Cobra's aluminum
block (Mustang Cobra weight is up to nearly 3700 pounds now). There's
probably a reason for that. As it is, the 5.4L supercharged V8's in
the Lightnings are exploding at an alarming rate. A friend has one
and is on an owner's forum. At last count, over 40 owners have blown
engines with the usual failure mode being (predictably) the connecting
rods.
Dan Jones
<HR></BLOCKQUOTE>
Andy
