We were back with our subject '99 T/A (LS1, auto, 3.23) to test SLP's 400HP PerformancePac. This time, a pair of Nitto Extreme Drag Radials (245/50R-16) were mounted on the stock wheels for good hook and strip-test results.
Our plan was to test the merits of SLP's PerformancePac line on an LS1 T/A. To recap what was accomplished thus far, we successfully strip-tested SLP's 350HP PerformancePac on a bone-stock '99 T/A owned by Kelly Holloway (Dec. '06 HPP), which lowered e.t.'s over 0.3 and added 3 mph to trap speed. Next (Jan. '07 HPP), we moved up to the 375HP Pac. Though we attempted a strip test, the additional 21 rwhp and 13 lb-ft of torque revealed in dyno testing and the four-year-old, dry-rotted tires conspired to limit traction. Because of that bad track experience, we ordered a set of Nitto Extreme Drag Radials to properly test the SLP's next stage-the 400HP Pac.
Dyno-testing the 350HP Pac provided 347 flywheel horsepower as converted accounting for 20 percent driveline loss using the 289-rwhp figure of SLP's chassis dyno. After installing the 375HP Pac, the LS1 developed 310 rwhp on the chassis dyno, equal to 372 hp at the crank. SLP's PerformancePac power ratings are based on the flywheel horsepower of the '01-'02 LS1s, which come from the factory with the LS6 intake as standard equipment. To ensure we could easily surpass those ratings with a '99 LS1, we opted for the popular LS6-intake swap for this story. SLP's LS6 intake includes the hardware and will enable us to retain the legal EGR emissions equipment.
The LS6-manifold swap was performed at SLP's installation center (which has been reopened to the public). During the swap, we cut off the part of the EGR tube that disrupts airflow into the intake. SLP's catalog states: "Add up to 15 hp to your LS1 with the LS6 intake manifold." After changing intakes, the blue Bird was strapped down to SLP's in-house dyno (which also has been reopened to the public). When we spun the rollers, we realized a 17-rwhp gain! That's impressive considering we didn't touch the computer or anything else. Now we were up to 327 rwhp and happy to be beyond the Pac's 375 hp rating with 392 at the crank.
400HP PerformancePac
Pleased with the results, we were anxious to install, dyno, and strip-test SLP's 400HP Pac, which includes everything we already installed and tested from the 350HP and 375HP Pacs plus SLP's tuned-length, long-tube stainless steel headers. The headers include a high-flow, 3-inch Y-pipe but require the install kit and high-flow cats to keep the Bird flying legal.
LS1 F-body header installation is not a walk in the park and needs to be done on a lift by a competent technician. It can take anywhere from 8 to 12 hours. SLP's engineering fabricator, Ken Estelle, handled the task in roughly 8 hours.
Upon post-install startup, we enjoyed the deeper and louder tone when revving it up, but the engine actually seemed quieter at idle. Remarkably, at highway-cruising speed, interior-exhaust drone was reduced. This is due to the long tuned-length tubes (SLP has a patent on its headers) that work together with each exhaust pulse to produce a smoother and more solid-sounding exhaust tone. Still, full-throttle blasts will get people's attention.
When we tested SLP's 375HP Pac (Jan. '07, HPP) we gained 21 rwhp but were only up to 372 flywheel horsepower. SLP's Pacs are based on the '01 and '02 LS1s, which sport the LS6 intake as standard issue, so we wanted one too. Here's the original LS1 intake (before the swap), equipped with the ugly EGR tube located right behind the throttle body. | 
The unsightly '98-'00 LS1 EGR tube protrudes into the intake and disturbs airflow due to its location right behind the exit of the throttle body. For '01-'02 engines, the LS6 intake was used on LS1 F-bodies and the EGR system was eliminated. | 
On the topside, it's hard to see a difference between the LS6 and LS1 intakes. The only notable one is the SLP LS6 intake has a brass EGR tube fitting to keep the '98-'00 LS1s 50-state emissions-legal. |
The LS6 intake provides an increase in plenum volume (5.06 liters to 5.19) that can only be seen by looking at the underside of the castings. On the left, the LS6 is flat versus the curved underside on the LS1. | 
SLP's LS6 intake includes the hardware, gaskets (already installed), and instructions to get the job done. Most important are the supplied LS6 coolant line (shorter) and block-off blocks (PN 30041K). The LS6 intake will not fit if you try to install it with the longer LS1 coolant lines in place. | 
We will remove these longer LS1 coolant lines, which run under the LS1 intake from the front to the rear of the heads, so the LS6 intake will fit with it's larger flat bottom. |