By Stephen Kim
Improving upon a set of mediocre OE cylinder heads is easy. Simply open up the bowls and back-cut the valves on a set of Gen I small-block Chevy heads, and — bam — an extra 20-30 horsepower is yours. Up the ante to a set of GM LS7 castings, however, and wringing out any additional airflow proves much more difficult. Arguably the best factory small-block heads ever built, the LS7 castings move a phenomenal 360 cfm through raised runners and 12-degree valves. While factory CNC porting and monstrous 2.200-inch titanium intake valves further add to the LS7’s mystique, RHS welcomed the challenge of making them even better.
Granted the factory LS7 cylinder heads perform exceptionally well on a 427 c.i. motor, but today’s crop of mega-displacement race engines can easily push them past their comfort zone. In the decade since the LS7’s release in 2005, the explosive growth of aftermarket block and stroker crankshaft options have spawned an equally explosive growth in engine displacement. Today’s LS race engines can easily exceed 500 c.i., and when matched with modern valve train hardware that can turn 8,000-plus RPM with minimal effort, keeping the cylinders filled at elevated engine speeds requires some serious airflow.
By starting with a clean-sheet design, RHS has significantly improved upon GM’s LS7 cylinder heads with its Pro Elite Big Port LS7 castings. Key enhancements include CNC-ported 307cc intake runners, raised intake port entrances, massive 2.250/1.615-inch valves, beefy .750-inch-thick deck surfaces, and reinforced rocker rails. In addition to enlarging the intake ports from 275- to 307cc, RHS raised the port entries .220 inches higher than stock.
The straighter flow path from the port entry to the valve seats significantly enhances performance potential, and the RHS castings boast plenty of cross-section to support large-displacement combinations.
“The stock LS7 heads are very impressive, and since they are already CNC ported from the factory, there isn’t a lot left on the table,” Kevin Feeney of RHS explains. “We had to look outside the box, which is why we decided to raise the intake runners. This provides a much straighter line of sight for the intake charge as it travels into the combustion chamber.”
Bolt-In Simplicity, Enhanced Durability
While raising the height of the intake ports is one of the most effective methods of improving cylinder head performance, they often require intake manifolds designed specifically for the repositioned ports. To eliminate this added expense, RHS designed the Pro Elite heads to maintain compatibility with standard LS7 intake manifolds.
“From the very beginning, the RHS LS7 heads were built to accommodate a stock or aftermarket intake manifold designed for standard-location ports,” says Feeney. “To accomplish this, we added material to the intake face so the cylinder heads can maintain proper port alignment without having to use spacer plates.”
On the durability front, perhaps one of the biggest downsides of porting a factory cylinder head is the simple act of removing metal compromises the integrity of the casting. This is of particular concern at the roof of the runner, where porters risk grinding into the valve spring pockets in an effort to improve line of sight at the port entrance. Likewise, the elevated cylinder pressure and spring pressure race engines are subjected to place additional stress on the head casting. Consequently, RHS fortifies its Pro Elite LS7 cylinder heads in several key areas to maximize casting durability.
“In addition to raising the intake ports, we also raised the roof of the head to maintain spring seat thickness, and to provide adequate installed height for the tall valve springs required in high-lift applications,” Feeney explains. “We also reinforced the entire rocker rail area for additional valve train stability and to support aftermarket shaft-mount rocker arms. Head gasket retention is critical in high compression and boosted applications, so the RHS LS7 heads feature much thicker decks and a six-bolt head design.”
Air Quality, Not Quantity
Without question, the flow bench is an incredible tool that has dramatically accelerated cylinder head development in the last decade. However, factors such as port velocity, cross-section and chamber efficiency are often just as important as peak airflow numbers.
“The quality of airflow is more important than the quantity,” said Feeney. “The key to making lots of power is optimizing the combination of runner volume, cross-sectional size and valve diameter to design a balanced port that will maximize power and torque for the intended application without sacrificing throttle response. All these factors went into designing the Pro Elite 307cc castings specifically for larger cubic inch applications. Increasing the intake valve size to 2.250 inches complements the larger bore diameter of big displacement race engines very well. Compared to stock, the overall increase in cfm of the RHS heads might not be dramatic on a flow bench, but they still net a big increase in horsepower.”
While analyzing specs give some indication of potential performance gains, nothing settles the score like a real-world dyno test. To find out exactly how much of a horsepower boost the RHS Pro Elite cylinder heads are worth over a set of stock LS7 castings, we tagged along as the School of Automotive Machinists bolted them to a 451c.i. LSX-powered 1995 Camaro.
Built to compete in the NMCA LSX Real Street class, the Camaro serves as the ideal test bed for the RHS LS7 heads. Not only has the Camaro’s 451 reached the limits of the factory LS7 heads, it needs a big increase in horsepower to keep pace with the competition in Real Street.
Back in 2010 when the Camaro’s 451 was built by Late Model Engines, the first aftermarket rectangle-port LS cylinder head castings started trickling into the marketplace. However, the 360 cfm dished out by GM’s factory LS7 heads were deemed more than sufficient to run competitively in Real Street. The talented crew at LME still improved upon them, hand-porting them to 399 cfm on the intake side and 256 cfm on the exhaust side, at .700-inch lift. Matched with a custom 254/260 hydraulic roller camshaft featuring .646/.630-inch lift, the combo cranked out 720 HP. At the track, the Camaro ripped 10.20 seconds at 137 mph.
Those numbers were more than respectable back then, but the competition in NMCA Real Street has gotten much faster through the years. To help the dated 451 keep up with the pack, SAM Tech was tasked with updating the combination with a set of 307cc RHS Pro Elite LS7 castings and a custom COMP Cams 260/274 solid roller. Considering the factory LS7 cylinder heads on the 451 have been ported to flow an additional 39 cfm over stock, the RHS castings clearly had their work cut out for them.
Nevertheless, at the end of the dyno session, bolting up the RHS Pro Elite LS7 cylinder heads and a custom COMP solid-roller camshaft netted an additional 59 RWHP over a set of ported LS7 heads. Factoring in a standard 20-percent driveline loss equates to a 74 HP increase at the crank. Not bad for a day’s work.