I bought privately and took the official word of Maserati as gospel, so did not ask the seller anything regards to the valves! They confirmed all 4.7 models and exhaust valves fitted from factory, which I now know is not the case.
It may be possible that the 4.7s models have valves in their exhaust systems, but Maserati's own QP literature from the period makes it clear that only the 4.7 Sport GT S has what they refer to as the Racing or Sports Exhaust. This would explain the discrepancy between the sound in S vs Sport GT S cars, and is the reason the Sport GT S kicks out slightly more hp. Hope this is helpful
From the QP brochure:
"QUATTROPORTE S ENGINE - 4.7 LITRES. Under the bonnet of the Quattroporte S is the last word
in the new family of wet sump engines: the new 4.7 litre engine unit delivers a top speed of 280 km/h and acceleration
from 0 to 100 km/h in 5.4 seconds, thanks to the new eight-cylinder 90 degree V8 engine, with displacement of 4,691 cc,
which can generate a power of 430 Hp at 7,000 rpm, and a maximum torque of 490 Nm at 4,750 rpm. Identified by its
red cylinder head covers, the new 4.7 litre engine has been specially developed to optimise the car’s power and make the
Maserati Quattroporte S even more exciting to drive, with the smoothest of rides and breathtaking performance. The technical
features adopted ensure that this engine is exceptionally quiet and reliable, with very low friction levels for reduced
fuel consumption, providing performance in which the qualities shared by all the latest Maserati engines, massive specific
power and rapid rise and fall in rpm, are raised to new heights. Generating 82% of its torque at just 2,500 rpm, the 4.7 litre
V8 engine accelerates impressively even in high gears: this means relaxed driving on long trips, and in general full use of the
engine throughout its operating range.
"QUATTROPORTE SPORT GT S ENGINE - 4.7 LITRES. This version is fitted with the most powerful
engine in the Maserati Quattroporte range. Capable of 440 Hp at 7,000 rpm and a maximum torque of 490 Nm at
4,750 rpm, it raises the car’s performance to unprecedented levels. These figures have been achieved not only by modifying
the engine’s electronic control parameters, but also through in-depth development of the racing exhaust system installed
on this version. The system is controlled by air valves that open in SPORT mode to reduce back-pressure in the exhaust;
this increases the maximum power available, and creates a throaty roar at low engine speeds and full symphonic sound of a
genuine sports car at higher rpm."
SPORT EXHAUST SYSTEM
“Normal” Mode - exhaust valves closed.
In “Normal” mode, the exhaust valves are closed, forcing the exhaust gases to take a longer route; this reduces the energy
level and thus tones down the sound of the engine. “Normal” exhaust system control mode is designed to optimise comfort
inside the car by lowering the level of audible engine noise heard, while also limiting the maximum power available to 430
horsepower.
“Sport” Mode - exhaust valves open.
In “Sport” mode the exhaust valves open to offer the exhaust gases a broad, straight path: in contrast with Normal mode, this
allows all the energy to discharge through the tailpipes without any restriction, enabling delivery of the maximum power of
440 horsepower combined with a truly thrilling deep sports- car roar. “Sport” mode, which the driver activates by pressing
the button provided on the central console, is designed to make full use of the car’s performance potential."