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.
Note: the '98-'00 LS1s need to have the valvesprings changed due to the 0.500-inch-lift cam ('01-'02 LS1s have a 0.464/0.478-inch-lift cam). The 0.500-inch max valve lift becomes 0.544 lift with the 1.85:1 rockers and could cause coil bind with the stock springs. The SLP spring package is made by Manley and includes titanium retainers to reduce weight at the valve tip and lessen the odds of valve float at high rpm (5,000 and up). Rocker and spring installation took us nearly four hours to perform (the motor was hot when we started). Some of the time was spent fixing and replacing a few cracked emissions hoses. SLP recommends only competent and experienced people attempt the rocker and spring change.
We changed from the stock 75mm MAF to SLP's 85mm unit, which flows 895 cfm compared to the stocker that flows only 805 cfm.
On The Dyno
Back to the dyno with the aforementioned parts installed, we noticed that the temperature rose to a sweltering 98 degrees. Still, Brian put the hammer down, and we were up to 301 rwhp and 308 lb-ft of torque at 5,200 and 4,200 rpm, respectively. But wait, we weren't done yet. We needed to plug in the DiabloSport II Programmer to complete the 375HP Pac installation.
The hand-held power programmer installs SLP's custom high-performance tune. Optionally, it allows you to trim the air/fuel ratio and spark advance for mods beyond SLP's package. It also functions as a scan tool to read engine diagnostic codes. This electronic tool can improve performance, drivability, and efficiency.
Once the power tuning was set, we spun the rollers and observed 310 rwhp and 318 lb-ft of torque! Overall, the 375HP Pac proved to be worth 21 hp at the rear tires. Flywheel horsepower would equate to 372 using 20-percent driveline loss as a guideline. That's a significant gain when you consider we didn't go digging into the motor as you would for a cam or head swap.
On The Strip
When we strip-tested the 350HP Pac, it was with cool springtime air. Knowing we would be following up with a strip-test in the summer heat, we decided it would be wise to install SLP's 160-degree thermostat and the three-position fan control switch. Once done, Brian custom-tuned the PCM (powertrain control module) to enable the fans to activate at a lower temp (SLP proprietary) to work with the new 160-degree thermostat. He also increased the transmission shift firmness.
On the ride home, the trans shifted like we just changed the valve body (much quicker and firmer), and the temp gauge never went over 180 degrees, unlike before when it would hang near 210 degrees.
We drove the T/A 90 minutes to meet our co-workers from GM High-Tech, Super Chevy, and Muscle Mustangs & Fast Fords magazines. They arranged for a test day at our favorite track, Raceway Park in Englishtown, New Jersey. Our test day was in typical hot August weather, so we popped the hood and flipped the fan switch to let the Bird cool down for an hour.
Before we began testing, the temperature was a warm 86 degrees. Our front tires were inflated to 40 psi, and the rear were deflated to 28 psi. When we factored the 350HP Pac's 13.10 baseline from that 65-degree test day, our new baseline was 13.22 for this 87-degree day.
We pulled up to the line and slowly stepped into it coming out of the hole, but now the tires were spinning too easily from the newly added power. I looked up and noticed a lousy 2.20 60-foot time but continued down track to a disappointing 13.30 at 106.07. I came back for a hotlap and stepped into it even more slowly, and the tires still spun worse to a 2.23 60-foot with a 13.37 at 105.91. For the rest of the day, no matter how gently we tried to come out of the hole, we were 0.3 off our 60-foot times from when we tested the 350HP Pac.
Besides the added power causing traction woes, the owner replaced one of the rear tires (four years ago; stock size 245/50R-16) with a different brand (and therefore, a different tread and compound), so the tires weren't working together. The fronts were also worn too unevenly to rotate, and nobody had a pair of sticky tires to lend us. This was a real-world occurrence that even happens to us magazine guys.
Conclusion
Once again, we were impressed with SLP's PerformancePac. This time, the Stage 2 package was "put to the test" and delivered the goods on the dyno. The added power to the pedal with the old hard-compound radials made our strip test impossible but was much more fun on the street. With slicks or drag radials, we're sure 12s would have been possible. Tune in next time for the 400HP Pac installment and test-with good tires.
