The custom airbox was very...
The custom airbox was very similar to the one used on 300 GPX concept car. Triangular opening connected with ducting in the hood to provide ram-air effect.
All of this upgraded equipment made a huge difference in power. The Bonneville's stock normally-aspirated 3800 Series II V-6 put out 205 hp at 5,200 rpm, with 230 ft-lb of torque at 4,000. With the addition of the supercharger, intercooler, and exhaust upgrades, the engine power was boosted to a full 300 hp at 5,500 rpm with 335 ft-lb of torque at 3,500. Boost was held to a conservative 8 pounds due to the use of production pistons.
This abundance of power was transferred to a modified 4T60-E transaxle that was upgraded with a prototype Torsen limited-slip differential. Despite the fact that the 4T60-E was only rated for 275 ft-lb of torque, the transaxle held up admirably. There was some speculation that the transaxle may have been a prototype for the beefier 4T65-E, though this was never verified.
Even though the 300 GPX Mule car was a mechanical prototype, some exterior upgrades were also added. The most obvious change was the prototype ram-air hood. It was constructed of a composite material similar to fiberglass known as GRP (glass reinforced plastic). Like its more famous concept car counterpart, the Mule was painted in the same Picante Red metallic as the 300 GPX. A set of custom Borbet 17-inch wheels were fitted with Goodyear Eagle GS-C 275/45ZR-17 tires. The overall effect was attractive and aggressive without being obnoxious.
Its custom hood was made of...
Its custom hood was made of composite material. The design did away with accent lines and added the scoop. Ducts inside routed cool air to the engine.
Though the exterior was upgraded to match the increased power under the hood, the changes to the interior were only in the area of gauges. The stock saddle-colored leather interior was retained. A factory in-dash tach featured a revised redline and an auxiliary gauge panel housed a trio of VDO gauges for monitoring oil pressure, vacuum/boost, and voltage.
Driving the Mule
Your author had the privilege to put some seat time in this GP prototype way back in May 1995 and the experience is still fresh in my mind. Though saddled with a software glitch that caused it to occasionally stall at idle, the supercharged 3800 proved to be very flexible, with an abundance of low- and mid-range torque and a delightfully throaty exhaust note.
With a curb weight under 3,500 and 300-plus hp under the hood, things were likely to happen quickly and this GP did not disappoint on any level. Pontiac quoted 0-60 times in the high 5-second range and our results were right in there as well. Torque steer was minimal and the Torsen limited-slip differential could be felt when making slow-speed turns. The sensation was actually quite similar to what one would experience in a rear-drive Pontiac equipped with Safe-T-Track.
On the street, the Mule was a blast to drive, a combination of low-speed torque and revability. The 3800 would enthusiastically whiz up to 6 grand almost instantaneously, which would break the big Goodyears loose at anything below 40 mph. Likewise, the tranny shifted at exactly the right time and did so with firm, authoritative control.
The wheels were a custom Borbet...
The wheels were a custom Borbet design and measured 17x8 inches. They mounted a set of Goodyear Eagle GS-C 275/45ZR-17 tires.
Though the Mule made quite an aggressive performance statement, it was also remarkably civilized. Like many late-model performance cars, the supercharged 3800 V-6 didn't really give anything up in the pursuit of more horsepower. It was still very smooth and responsive, with no flat spots or other undesirable manners.
After the 1995 show season and the debut of the new generation Grand Prix in 1997, the Mule was put out to pasture for a few years, languishing in outdoor storage. Unfortunately, the crush order came from on high in the summer of 2000 and it was destroyed, but not before a couple of parts came off of it. The custom 300 GPX fiberglass engine cover, which was identical to the showcar's (and not on it when these shots were taken) was removed and raffled off at the 2000 Grand Prix Gathering. A set of keys and the remote control also survive.
In addition to giving a glimpse of the upcoming powertrain for the '97 Grand Prix (which was introduced with a non-intercooled 240-horse version of the 3800 V-6), the 300 GPX Mule Car also gave a preview of what sort of performance upgrades that the performance industry would offer for 3800s. From ram-air systems to headers to intercoolers to custom PCM programming to limited-slip differentials, the aftermarket has done a great job of stepping up and supplying parts that GP fans want and more is undoubtedly on the way.