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LS1 Cylinder Head - Head Games - Tech

Dyno-Flogging Rhs' Latest Ls1 Cylinder Head Offering

Richard Holdener Oct 1, 2007

Chances are you've heard of the Racing Head Service. Back in 1967, RHS started out life as a manufacturer of racing cylinder heads but soon grew into a full-service operation, offering turnkey engines and performance parts for race cars. According to company literature, it was among the veryfirst to offer crate engines and mail-order performance parts. During the '70s and '80s, RHS made quite a name for itself in circle-track and drag racing. In 1976, RHS started a little company called Competition Cams. Given the relationship between cylinder heads and cam profiles, the RHS/Comp Cams combination made for one heck of a one-two punch. RHS and Comp Cams enjoyed years of performance productivity in NHRA and short-track competition with their performance heads and valvetrain components. Sometime in the late '90s, ownership change and company realignment allowed RHS to slip into inactivity, all but retiring an automotive icon. Resurrected as part of the Comp Cams group, RHS acquired the resources of Pro Topline, including a high-tech cylinder-head foundry in Auckland, New Zealand. This was the start of a return by RHS to its position in the market as an innovator and producer of high-performance heads.

RHS offers a wide variety of cylinder heads for small- and big-block Chevy applications; our test involved heads from the new Pro Elite series. Like many aftermarket sources, RHS offers its LS1/ LS2 heads in two port volumes, since one cylinder head is definitely not ideal for all applications.In addition to the 210- and 225cc intake-port volumes, it is also possible to alter the combustion chamber size from 62cc all the way down to just 36cc. The massive change in chamber volume is possible thanks to a generous deck thickness of 0.800 inch (roughly twice the factory LS1 head). This not only allows dramatic changes in the combustion chamber size (and attending static compression ratio), but also makes the RHS heads ideal for forced induction or nitrous applications, where elevated cylinder pressures can distort the thinner factory heads. The use of the two intake-port volumes allows the RHS Gen III heads to be run on a wide variety of power levels and displacements, as a head designed to feed a stock 5.7L LS1 would not be the ideal choice for a high-rpm 427 stroker.

While the RHS Gen III heads obviously feature full CNC porting to improve the flow rates considerably over a production head, the RHS heads are much more than a set of ported stockers. Starting with a fresh slate, the dedicated RHS castings offer a number of desirable features in addition to big flow numbers. The Power by Design Gen III heads feature altered valve angles. Rolling the valve angle back from the 15 degrees used on the production heads down to 11 degrees greatly improves airflow. The altered valve angle also increases piston-to-valve clearance, allowing RHS head owners to run wilder cam profiles without resorting to notching (or replacing) their existing pistons. Remember that a typical small-block Chevy runs a 23-degree valve angle and the hot small-block race heads dropped this down to 18 degrees. In addition to the change in valve angle, the RHS heads also feature improved valve-guide material and hardened valve seats to extend durability. Another trick feature is the raised (0.500) valve-cover rail. The extra material provided the necessary clearance for aftermarket rockers without resorting to aftermarket valve covers or valve-cover spacers-or hacking the insides of your stock valve covers.

The CNC porting of the RHS Pro Elite Gen III heads maximizes flow rates. This program includes precision machining of the intake and exhaust ports along with optimizing the combustion chamber design. When combined with 2.04-inch stainless intake and 1.57-inch exhaust valves, the result is impressive numbers. The smaller 210cc RHS heads offered well over 300 cfm on the intake.In fact, according to data supplied by RHS, the 210cc heads flow 323 cfm at 0.650 lift and 320 cfm at a more realistic 0.600 lift. But RHS has been in the business long enough to know there is much more to a good cylinder head than just big flow numbers achieved at max lift values. The RHS 210cc heads exceeded 300 cfm at 0.500 lift while still offering impressive low- and mid-lift flow (see airflow data). The exhaust flow numbers are equally impressive, though the numbers generated would have been much higher had RHS tested the heads with a flow pipe on the exhaust. Flowing 214 cfm without the aid of a flow pipe is pretty remarkable, since all that intake flow must also find its way out. Try coaxing 320 cfm out of your old Fuelie, or even an L98 head, and you'll come to appreciate just how impressive these new Gen III RHS heads really are.

While the RHS Pro Elite Gen III heads sounded like a serious performance piece on paper, the proof, as they say, is in the pudding. In this case, the pudding was a direct back-to-back test on the engine dyno. What better way to illustrate the power gains offered by a set of heads than a direct comparison? To put the new RHS heads through their paces, we first selected a suitable test motor. Our LS1 started out life as a GM Performance Parts 5.7L crate motor. Originally destined for the now-defunct (hopefully soon-to-be resurrected) F-body, the 5.7L came as a complete ready-to-run engine assembly. Not content to test the heads on a stock motor, we made a few modifications to the LS1 first. Replacing the stock parts were a set of Hooker long-tube headers feeding a pair of 3-inch Magnaflow mufflers; a Keith Wilson-designed FAST LSX intake with matching 78mm FAS tthrottle body; and an XR265HR grind from Comp Cams, which offered a 0.522/0.529-lift split, a 212/218-duration split at 0.050, and a 114-degree lobe separation angle. Mild as performance cams go, the XR265HR cam is both streetable and powerful, a perfect combination for our test motor.

We ran the LS1 on the Super Flow engine dyno at Westech Performance. Rather than rely on the factory electronics, the modified LS1 was run using a FAST XFI engine-management system, allowing us to dial in the air/fuel and timing curves while simultaneously removing the (restrictive) factory mass-air meter and attending air-inlet system. The idea was to run the motor first with the stock heads and then with the RHS heads.It's important to note that you have two options when it comes to rocker arms. You can reuse the factory rockers, but in order to do so you'll need to order Comp Cams' stud rocker stand, PN 54201-2. The alternative is to go with its line of shaft-mount rockers, PN 1509, perfectly suited for more aggressive applications. Lastly, our mule was optimized with the XFI management system and produced 480 hp and 453 lb-ft with the stock LS1 heads. As we have come to expect of these powerful Gen III motors, the torque production was impressive, exceeding 400 lb-ft from 3,000 rpm (actually lower rpm had we tested below that engine speed) to 6,300 rpm. Try building a conventional 23-degree small-block that makes 480 hp with a 212/218-duration cam.

After running the test motor with the stock heads, off they came, replaced by the new RHS Pro Elite Gen III heads. We like to use ARP head studs when running head tests, but a snafu (by the author) in ordering the ARP hardware forced us to run a set of new stock bolts. Never try to reuse the torque-to-yield bolts, as they will not provide sufficient clamping force on the head gaskets. The RHS heads were installed with a fresh set of Fel Pro head gaskets. After adjusting the Comp rockers and installing the FAST LSX intake, we were ready for action. All we can say is Wow, as the new RHS heads improved the power output of the modified LS1 by as much as 50 hp over the stock castings. All the impressive features included with the RHS heads combined to improve not only the peak power numbers but also the all-important average power production. The new heads picked up power from 3,000 rpm all the way to 6,500 rpm. Torque production from our LS1 test mule now exceeded 450 lb-ft from 4,100 rpm to 6,100 rpm and never dipped below 420 lb-ft. If these Gen III heads are any indication, it looks like the boys from RHS are back in full effect.


Racing Head Service
Memphis, TN 38118



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