Before installing the 375HP...
Before installing the 375HP Pac, we dyno-tested the 350HP Pac for our baseline, which spun the rear tires to 289 rwhp equating to 347 hp at the crank.
We tried to strip-test SLP's...
We tried to strip-test SLP's 375HP Pac after dyno-testing. With 4-year-old worn and dry-rotted stock tires, we just couldn't hook up with the additional 21 hp and 13 lb-ft of torque. It looked like the T/A was off to a clean launch, but the tires would still spin even if we stepped on the pedal slowly at 20 feet out of the hole. It was my worse time at the track in many moons. We're anticipating a return with sticky meats.
SLP's 375HP Pac includes an...
SLP's 375HP Pac includes an 85mm high-flow MAF that the company says will add 5 to 10 hp (depending on the combination). We used the #23060 MAF sensor for the '98-'00 LS1. The SLP 85mm MAF (left) will flow 895 cfm; a stock 75mm unit only flows 805 cfm with the screen. If the screen is removed, the SLP MAF flows over 1,000 cfm.
Another component of SLP's...
Another component of SLP's 375HP Pac is the No. 50185 1.85-ratio rocker arm package (adds 10-percent lift) that retails for $599.95. The '98-'00 LS1 will need the No. 51185 rocker arms w/springs, retainers, and keepers due to its higher-lift cam (0.500-inch lift).
Here's a look at the stock...
Here's a look at the stock rockers, springs, and retainers after removing the valve covers. We were glad to see a clean and well-maintained motor. If this 83,000-mile world-class engine was neglected, abused, and loaded with sludge, we wouldn't have attempted this rocker and spring change.
It's important to have air...
It's important to have air pressure in the cylinder (80-120 psi; see air hose leading to the spark plug hole) to prevent dropping a valve while replacing the springs. We started the job with SLP's inexpensive valvespring removal/installation tool (PN 53306, $61.75). The kit also includes the spark-plug fitting adaptor for connecting compressed air to the cylinders. The basic tool gets the job done and is OK for a one-time swap at home, but professionals will opt for a more expensive set of custom tools to speed things up.
After a satisfying strip-test of SLP's 350hp PerformancePac on a bone-stock '99 T/A (owned by Firebird-freak Kelly Holloway) in the Dec. '06 issue of HPP, we decided to step up to the company's 375HP Pac. To keep everyone updated, the 350HP Pac delivered us improvements of over 0.3 second and 3 mph. We were impressed: It's like picking up 30 hp!
SLP's 350HP Pac consists of the Flow Pac cold-air induction, Loudmouth II after-cat exhaust, and the cool-looking fender badges. All the parts from the 350HP Pac are included in the 375HP Pac, plus an 85mm high-flow MAF, 1.85:1-ratio rocker-arm package, DiabloSport II handheld programmer, and the 375HP fender badges. For this story, we decided to dyno-test and strip-test the SLP 375HP Pac.
Parts installation and dyno testing was performed at SLP's Performance Parts Engineering, Research, and Development Testing Center in Tom's River, New Jersey. Luckily, the SLP facility is only 45 minutes from the famous Englishtown Raceway Park dragstrip.
Upon arrival, the SLP guys, Brian Reese, director of engineering, and Ken Estelle, engineering fabricator, put the Poncho on the lift and plugged in the manual controller for the T/A's automatic transmission. The trans controller prevents downshifting during WOT dyno pulls, so our testing has true 1:1 Third-gear dyno results. (Many chassis-dyno operators test in Second gear, not knowing how to stop the downshifting. Lower-gear dyno-testing shows higher power readings due to more torque multiplication in the lower gear.)
Brian, Ken, and myself strapped down the Bird to SLP's in-house SuperFlow chassis dyno for some baseline pulls with the 350HP Pac. After a few pulls, the LS1 spun the rollers to a best of 289 rwhp at 5,100 rpm and 303 lb-ft of torque at 4,100 rpm in steambath-like 94-degree air. Flywheel horsepower equates to 347 using 20-percent driveline loss as a guideline. At 347, we were down 3 hp from the guaranteed 350 hp. Brian explained that the SLP PerformancePacs are based around the '01 and '02 LS1s, which come from the factory with the LS6 intake as standard equipment, unlike our '99. An LS6 intake swap (usually good for 10-15 hp) could easily put us past the Pac's power ratings.
The Mods
Off the rollers and into SLP's installation facility (50 feet from the dyno room), the Pontiac received its next round of improvements. Ken, Brian, and I worked together changing the rockers and valvesprings.