I was first exposed to SLP's big-inch GTO during a 2004 track day at Raceway Park. It was still early in the aftermarket's crusade to defile the latest LS1-powered hot rod from GM; that's what made the booms from the Yellow Jacket Goat's Ram Air H.O. hood and crackles from the Loud Mouth exhaust so sexy. That soundtrack was thanks to SLP's ZL-402, an in-house stroker package created with iron LQ9 and aluminum LS1 and LS2 blocks. This version used a 6.0L LQ9 block and a 4-inch forged-steel crank. It had an 11.3:1 compression ratio and a race cam--.621/.603 lift, 256-/256-degree duration at .050, 108 centerline, and 112 lobe separation. Its lope was so exaggerated it seemed like each individual combustion event could be heard--violent explosions that threatened to blow a piston out of the block and through the fiberglass bonnet.
"It'll hold together," SLP Director of Engineering Brian Reese laughed, while detailing the meticulous assembly process that all ZL-402s go through before being sold. He was right--nary a piston went lunar, but a solid one-two powershift on drag radials shredded a stock CV shaft. The best time that day was an 11.83 at 122 mph.
Number X03 is nearly a complete package, even though it was created to prototype new SLP GTO parts. Its ZL-402 puts down 496 rear-wheel horses and 461 lb-ft through an Exedy twin-plate clutch, six-piston, 13-inch Wilwood brakes stop the Goat on a dime, and SLP chassis and suspension upgrades give the Pontiac a sharper feel over stock. The only aspect of the car left almost completely stock is the drivetrain. In '04 Goats this powerful, the driveshaft, stub axles, and CV shafts (surprise!) should be upgraded.
When SLP loaned the car to GMHTP in January 2005, we had already experienced its strengths and weaknesses at the track. This much cam isn't meant to be street friendly--that being said, the company's name is Street Legal Performance. We didn't expect a perfect-idling Gen III and the street manners of a Civic when we took delivery, and if ex-GM engineer Reese was confident that his race Goat's manners could be passable on the street, GMHTP would be happy to run it through the ringer. Simply put, we took it to see if it could take it.
Interestingly enough, Mother Nature and the gridlock of a New York-to-Jersey commute teamed up to raise absolute hell with SLP's Pontiac. Freezing temperatures pushed the factory ECM to its limits during idle and part-throttle transition--it was just too much cam to control during winter. The near-OEM-quality tune fought valiantly though, with a searching idle and an occasional stall as the only problems.
A snowstorm came along that week and dumped over a foot of the white stuff in Queens, where I had unceremoniously parked the Goat. Ever tried to get out of an iced-up parallel parking space with a six-speed and 450-plus lb-ft? It was hairy, but I managed without denting up the hardware.
My time with the cam/clutch combo during a 5-mph commute through the South Bronx and into Jersey persuaded me that this cam was too much for constant street use. The Exedy twin-plate clutch's power capacity is huge (as is its pedal effort), and combined with the enormous camshaft, it resulted in low-speed chatter and bucking if you didn't balance the gas and clutch pedal perfectly from a stop. However, those of you who live outside of the urban areas and don't deal with a ton of traffic might not mind it as much in return for a 7200-rpm redline.
The most disappointing part of that week was that this 402, which was the actual dyno mule during SLP's testing, had a penchant for throwing its crankshaft pulley bolt. This happened a few times, tossing the serpentine belt and leaving us stranded. SLP hasn't determined whether the aftermarket bolt or the balancer is to blame.
So we reached out to ZL-402 buyers to get a different perspective. One gentleman from Harrisburg, Pennsylvania, bought one of the very first SLP short-blocks built. He installed AFR heads, a 90mm F.A.S.T. intake, and TSP's Magic Stick 3 (.603/.609 lift, 237-/242-degree, 113 LSA) cam. After a November 2004 install into his manual-trans '99 SS he ran across two problems: the crank pilot bearing was the wrong size and it spun freely on the crank, and an oil galley plug was damaged. "I called SLP about it and they put forth a good effort to get it resolved. SLP mailed me replacement parts which cleared [the problem] up," he said. "I was a little disappointed about that, but the power (515 at the wheels) and the way it drives takes the sting out of it." When SLP was asked for comment, we were told that its very first batch of short-blocks' potential for an oversized (incorrect) clutch pilot bearing bore had long since been corrected. Also, the buyer hasn't experienced any problems with the crank pulley coming loose, so it appears that ours was an isolated incident.
All that aside, engineer Reese's comment about the 402's power coming on like a two-cycle motor was dead-on. There are two ranges with this motor: tiny GTO tires planted; tiny GTO tires spinning. The latter happens around the 4500-rpm mark and continues through four gears, easy. After a few days of sliding around I actually turned the traction control on--even then, the GM TC can't rein in the power fast enough when this 402 explodes to 7200. Big inches plus the race cam meant power everywhere, and we would have loved to drop this mill into a live-axle F-body, bolt up some slicks, and done a few clutch drops. The ZL-402's acceleration in a word--violent.
The '04 GTO got a fairly tame suspension from the factory, and this is another area where SLP decided to do some tweaking. The company designed a 19mm rear swaybar (stock is 16mm) and poly links to improve handling and traction and at least put a dent in the Goat's inherent wheelhop. SLP's progressive springs work with a rear camber kit to drop the ride height by 1-inch, although the foul weather forced us to ride on the stock 17x8 rims, SLP has a set of ROH 18 by 8s that work great with that spring package. With the above changes and riding on 17s, the ride is still very supple for sports-car standards, but the handling and looks are both improved.
The GMHTP editors loved the on/off switch cam and race-car feel of this GTO, but wanted to try a more street-friendly camshaft before writing this review. SLP was happy to swap out the .621/.603, 256-duration whopper for a mild-by-comparison .596/.596, 250/246-duration, 114-centerline, 114 LSA cam. This bumpstick, which carries a 51015 SLP part number, shifted the 4000-7000 power range to 2500-6000--and really clicked with the big 402. Granted, the weather was 30 degrees warmer, but the GTO's manners were night and day with this new camshaft. It was much easier to drive at low rpm, the idle was much smoother, the motor didn't stall, and the same twin-plate clutch wasn't as finicky. Dyno pulls put 484 horses and 452 lb-ft to the wheels with this cam, meaning that it made almost all of the race cam's power without all of the headaches.
The ZL-402 simply transformed this GTO--50-plus more cubic inches, a good cam, and great tuning is known to do that. For more details on SLP's ZL-402 packages see our stroker story in this issue; that is, if you like affordably violent horsepower.