New GM castings form the basis of the PRC cylinder head lineup. In addition to the subject 5.3-liter aluminum castings (PN 12561706) used as the basis for the PRC 220cc 5.3-liter Stage 2.5 heads, PRC also offers ported factory heads, including the pictured LS6/LS2 (PN 12564243) heads in various levels of dress from bare to outfitted with the customer's choice of valvesprings. Stage 1 heads (shown) feature stock LS1 valve sizes of 2.00-inch intake and 1.550-inch exhaust valves while Stages 2, 2.5, and 3 utilize oversize aftermarket stainless valves supplied by either Rev or Manley. PRC's base head package includes its own dual spring kit (PN PRC-199-SPkit, retail $289.99) that features titanium retainers and is rated to 0.660 lift.
In the quest for performance for your LS1-equipped Firebird or GTO, the road to big-power increases usually progresses from bolt-ons into "Cam-Only" configurations, such as those presented in "LS1 Power Plan" (Aug.-Sept. '08). A well-thought-out combination of parts can result in a very street-friendly idle and wide power band that doesn't sacrifice driveability for an adrenaline rush on the top end.
After boosting power on an automatic LS1-equipped Bird to the tune of more than 380 rwhp, the car laid down a 1/4-mile e.t. of 12.79 seconds on a gorgeous day at the track. Although additional power could have been generated by a more aggressive cam, rather than compromising the excellent dual-purpose nature of the T/A, the quest became improving cylinder head airflow. With a dizzying array of both factory and aftermarket heads available, we turned to Lubbock, Texas-based Texas Speed and Performance for recommendations.
According to Jason Mangum, co-owner of Texas Speed, "In 2004, Trevor Doelling and I formed Precision Race Components (PRC) to offer enthusiasts low-cost, high-quality factory-based ported cylinder heads and related valvetrain components. To that end, we expanded our full-service speed shop to over 8,000 square feet to build a dedicated area for cylinder heads and invested in state-of-the-art CNC machinery for head-porting and valve jobs, along with the diagnostic and test equipment necessary to produce new CNC-ported factory heads and related cylinder head work for the Gen III engines.
"Although we offer aftermarket cylinder heads to satisfy hard-core racers, our primary market continues to be enthusiasts who are focused on maximizing engine performance on a budget for all Gen III/IV engine platforms. For the best bang for the buck on an LS1, our PRC Stage Series of heads based on either aluminum 5.3-liter heads or LS6/LS2 cores are very popular.
After extensive research to design a street port that would flow more than 300 cfm and retain the excellent swirl and velocity characteristics of a stock head, a CNC program was developed and executed, using a Haas five-axis CNC machine.
"For this application, we suggested using our PRC 220cc 5.3-liter Stage 2.5 heads that flow 312 cfm on the intake and routinely generate 50 or more horsepower at the crankshaft. Although the intake port volume is a bit larger than the stock LS1/5.3L (200 cc) or LS6/LS2 (210 cc), the evolution of our CNC and valve program has reached a point where the port velocity has been maximized, allowing owners to reap the benefits of the heads without fear of compromising the bottom-end torque curve at the expense of high-rpm power gains. For owners of small-bore Gen III engines, high-dollar aftermarket heads with big-runners and bigger flow numbers don't always translate well into real-world streetable power."
Priced at an affordable $1,199.99 (PN 199-PRC2.55.3), the heads include new GM 5.3-liter cores and Rev 2.02/1.575-inch stainless valves, along with PRC's own double-spring kit and titanium retainers. Since PRC offers the head without a spring kit as well as with either the Comp Cams 918 or 921 spring kit, we ordered up a set of heads without springs for $1,000, then swapped over the Comp 921 double springs and all the excellent Comp valvetrain components.
To obtain the clearance necessary to slide the Y-pipe out from the header junction, the aftermarket Spohn adjustable torque arm needs to be loosened. The four 12mm bolts that retain the torque arm to the body are removed.
In addition to the heads, Jason strongly recommended that the small 26.4-pound factory injectors be upgraded to a larger set of FAST 42-pound injectors (PN 304208, retail $372.33) to ensure the engine would be adequately fueled with the upgraded heads.
Before heading off to install the PRC cylinder heads, let's summarize the combination currently installed in Harold Baker's automatic '00 WS6 T/A. Sporting 66,500 miles, the Bird has an SLP LS6 intake manifold, Jet-Hot headers, an off-road Y-pipe and a Comp Cams hydraulic roller cam with a Comp dual valvespring kit and Hi-Tech pushrods. Real Performance Motorsports (RPM) in Lewisville, Texas, performed the installation and expertly tuned the T/A. In addition to the components from the "LS1 Power Plan" (Aug.-Sept. '08), the car has a long list of free mods and cost-effective mods, including a Corsa after-cat exhaust, BBK 80mm throttle body and an aftermarket suspension.
The torque arm is pulled down on the passenger side and the Y-pipe is simultaneously tapped backward until it separates from the headers. In this case, the header to Y-pipe junction was so tight that the Y-pipe was separated after the test tube. The Y-pipe is then twisted and wiggled loose from the junction to the after-cat and removed.
Follow along as Matt Garzonie at RPM installs the PRC cylinder heads in Part I, then in Part II completes the final assembly, including swapping out the factory injectors for the FAST injectors. Once that's done, we'll show you how the T/A performed both on RPM's Dynojet chassis dyno and in the 1/4-mile at San Antonio Raceway in Marion, Texas.
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