New GM castings form the basis...
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...
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.
Our fresh 5.3-liter castings...
Our fresh 5.3-liter castings come from GM with 61.15cc combustion chambers, which will yield a 10.9:1 compression ratio on a stock bore LS1. To ensure that the heads seal properly to the block, each casting receives a minimum cut of 0.005-inch on a Winona Van Norman mill, which marginally increases the compression to 11.1:1. The 5.3-liter heads are very popular with LS1 owners as their chamber size bumps compression by a full point.
The CNC machine performs the...
The CNC machine performs the modifications to the intake, exhaust, bowl area, and-in the case of the 5.3 heads-the combustion chamber, which is opened up to 62.5 cc's. According to Mangum, "The ability to test and develop a head program, then ensure repeatable, high-quality performance is paramount to our business. Rather than developing ports for race applications, we focus on the street application and the performance flow capabilities of our heads at low- and mid-range lifts."
The completed PRC 220cc 5.3-liter...
The completed PRC 220cc 5.3-liter Stage 2.5 heads in bare form are delivered with new GM valve seals and Rev 2.02-inch intake and 1.575-inch stainless valves. Compared to a stock GM LS1 casting, such as the owner's "853" heads (PN 12559853) that flow 227 and 180 cfm on the intake and exhaust respectively (as measured at 28 inches of pressure) at 0.600 lift, the PRC head flows 312 cfm on the intake and 234 on the exhaust.
The Newen CNC seat and guide...
The Newen CNC seat and guide machine at PRC is one of the most advanced machines of its kind in the industry. Proprietary valve angles and extremely accurate preparation of the seats and guides ensure that each valve will seal properly and optimize the flow characteristics of the heads.
Before beginning installation...
Before beginning installation of the heads, the negative battery terminal is disconnected, the car is raised on a lift, and the radiator is drained. In order to remove the heads from the engine, the Jet-Hot headers and Y-pipe have to come off. The front and rear O2 sensors are disconnected, then the factory issue band clamp that joins the Jet-Hot 3-inch Y-pipe to the Corsa after-cat exhaust is loosened, followed by the exhaust and band clamps that retain the header and test pipes (or high-flow cats, if installed).