The ram-air hood and custom...
The ram-air hood and custom 17-inch wheels gave a clue to the one-off GP's performance potential. Paint was the same Picante Red metallic as the concept car.
Often when a concept car is being developed, several different systems are addressed by different teams in different locations. One team may do the exterior design, another the interior, and yet another the powertrain. Such is the case with the '95 Pontiac 300 GPX concept car. As well as featuring a fresh new exterior design courtesy John Manoogian II and his team at GM Design Center, the powertrain development was handled by Pontiac Special Vehicle Engineering under the direction of SVE's manager at the time, Tom Goad.
Known as "The Mule," this particular GTP was built in November of 1994, just 2 months before the 300 GPX's debut in January of 1995 at the North American International Auto Show in Detroit. In addition to showcasing a variety of new features seen on the concept car, there was also a lot in common with the upcoming Grand Prix that would debut for the '97 model year.
Compared to one of its production counterparts, the Mule was entirely different under the hood. Where a standard-issue '95 Grand Prix would be powered by either a 3100 V-6 or 3.4L Twin Dual Cam V-6, the engine bay of this GP featured a supercharged and intercooled version of the 3800 Series II V-6 found in the Bonneville.
The interior was all factory-issue,...
The interior was all factory-issue, save the custom gauge panel to the right of the steering wheel that housed VDO gauges for monitoring oil pressure, vacuum/boost, and voltage.
Though no version of the 3800 was ever offered in the First-Gen front-drive Grand Prix, they were available in the Buick Regal, which shared the same platform as the GP. The engine was transplanted using off-the shelf mounts and dogbones (which connect the engine to the front of the unibody structure) from the Regal.
In actuality, the '95 model year was sort of a transitional one for the 3800 engine series. While the 225-horse supercharged version was still based on the earlier 3800 Series I V-6, the normally-aspirated version was actually based on the new Series II architecture and the differences were quite significant. Many of the changes made to the engine were in the interest of packaging and efficiency, but power was significantly increased as well.
For starters, the Series II block featured a 1-inch shorter deck height than its predecessor, which reduced the block weight by 11 pounds and allowed for easier packaging in smaller engine compartments. The reduction in deck height required a shorter piston and rod combination, which reduced reciprocating mass, increasing the engine's rev range. The block itself was also upgraded with 4-bolt mains and 2 crossbolts per journal for added durability.
Even today, the crisp lines...
Even today, the crisp lines of the '88-96 Grand Prixs still look great. Building a clone of this car would not be an impossible task and would be a great car to show or run at a GP gathering.
Series II cylinder heads were extensively revised as well, using equally-spaced intake and exhaust ports and new valve locations, which necessitated an updated camshaft design with new lobe locations. This updating is similar to what Pontiac engineers did when designing the Ram Air V. The ports themselves were designed with information gathered from the Buick Stage II V-6 racing program and closely resembled the race units.
The valvetrain was also redesigned to withstand higher engine speeds. Lightweight valves, springs and springs and retainers reduce valvetrain weight, which contributed to overall efficiency and brought the redline of the engine from 5,400 rpm to a full 6 grand. Horsepower for the naturally aspirated 3800 jumped from 170 to 205 hp at 5,200 rpm and torque jumped 5 ft-lb to 230 at 4,000.
With such a solid foundation for a performance powerplant, it seemed natural to put a supercharger on this engine and take the performance to another level. The supercharger in question was not the Eaton Model 62 used on the 3800 Series I, but the larger Model 90 that displaced 90 ci per revolution.
This prototype version of...
This prototype version of the 3800 Series II V-6 put out a full 300 hp, thanks to an Eaton Model 90 supercharger, air-to-liquid intercooler, custom headers and ram-air. Boost was kept to a conservative 8 pounds, though the potential for much more was there.
The larger blower allowed the maximum 8 pounds of boost to be generated at a lower overdrive ratio, which reduced blower speed and charge air heating. It was mounted on an experimental intake manifold that featured an integral liquid-to-air intercooler. The intercooler had its own separate cooling core and pump and used a mild glycol solution for heat transfer. Its throttle body was fed through a special ram-air system that inhaled cool outside air, further reducing charge-air temperatures.
To complement the forced-induction system, the exhaust system was also upgraded from the factory cast-iron manifolds. A pair of one-off headers were fabricated by SVE with 1.75-inch primaries, a 3-inch collector/crossover and a 3-inch catalytic converter. From there a dual-outlet exhaust system featuring 2-inch pipes routed spent gases out.