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RHS LS1 Head Upgrade - Swap ’til Ya Drop

An RHS LS1 head upgrade nets 43 horsepower over stock with no loss of bottom-end power

Richard Holdener Jul 6, 2011
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While it's true that cylinder heads abound for the LS motor, we are always excited about testing the very latest offerings. Case in point, the new Pro Action heads from Racing Head Service (RHS). Having been in the game long before the introduction of the Gen-III and Gen-IV small-blocks, RHS made a name for itself by performance-porting the original small-block Chevy heads. Factory fresh, the 23-degree iron heads were obviously not high on performance, but having a set of RHS-ported Chevy heads stepped up performance considerably.

Its success with the original small-block has carried over to the modern Mouse. Research and development from RHS is in full swing, as its engineers continue to increase the number of offerings from an ever-growing line of performance cylinder heads (and blocks). Though we anxiously await the new CNC LS1 heads, to say nothing of their LS7 offerings, we took a long look at their current crop of as-cast LS1 heads from their Pro Action line.

According to RHS, the Pro Action line of cylinder heads was designed to combine the performance of CNC porting with the affordability of as-cast heads. This was achieved through a combination of race-proven head design and what RHS has labeled ôClean Cast Technology.ö Power output is (partly) a function of head flow, which in turn is a function of port design. The Clean Cast Technology is said to provide just such an optimum as-cast port design that maximizes flow without the need for additional porting. The Pro Action RHS heads were available in a couple of different configurations, including as-cast P-ports with no valve job and a pair of assembled heads featuring either 205cc or 225cc intake ports. For our stroker application, we chose the larger 225cc intake ports, but in all reality, the smaller 205cc heads would easily support the modest power output of our mild street motor. We selected the larger 225 heads more for what the future may hold than the current combination.

The 225 designation for the RHS LS1 Pro Action heads was derived from the volume measurement of the intake port. While this may seem a tad on the large side for enthusiasts who grew up on the original SBC, rest assured that acceptable port size has increased through the years. Where we might be tempted to run a set of 195cc heads on a typical 23-degree street small-block, we wouldn't hesitate to install a set of 205cc heads on a diminutive 5.3L (roughly equivalent to a 327 Chevy). Testing has shown that these port volumes are not detrimental to low-speed power production.

One need only look at the (relatively) massive port volume of the current LS3 offerings (260cc) to illustrate this point. In addition to the more-than-adequate port volume, flow from the 225cc RHS heads was optimized through a 2.05/1.60 stainless steel valve combination and highly efficient combustion chamber design. All this was achieved using stock valve angles and centers, stock port locations with all the necessary accessory holes drilled and tapped. Basically, the RHS Pro Action LS1 offerings were ready-to-run, high-performance bolt-on cylinder heads.

Call us skeptics, but what sounds fantastic on that dimly lit press release is oftentimes somewhat less so in the light of day. What we wanted to know was just how good were the new RHS Pro Action heads, and (as always) we conjured up a way to find out exactly that. What better way to test the merits of the new heads than to compare them directly to their production counterparts?

Before we could compare the RHS 225 LS1 heads to a set of stockers, we needed a suitable test motor. While the RHS heads were certainly capable of supporting impressive power levels (likely above 600 hp on the right normally aspirated application), we decided to build an engine more representative of the average enthusiast. To that end, we assembled a 383 stroker by machining a production 5.3L block to accept both a 4.0-inch stroker crank and a set of 3.902-bore forged pistons. The 4340 forged-steel stroker crank and matching 6.20-inch rods came courtesy of ProComp Motorsports, while the forged pistons came from Probe Racing. Additional power was available with a flat-top design (thanks to higher compression), but future turbo plans limited our choice to a 21cc dish piston to keep the static compression near 9.5:1.

L&R Automotive was responsible for boring, honing, and assembly of the finished 383 short-block. Our cam choice was in keeping with our mild street-motor motif. The dual-pattern XE265HR cam (XFI RPM Series) from Comp Cams offered a .522/.529 lift split, a 212/218 duration split, and an idle-friendly 114-degree lobe separation angle. We have run this cam many times in the past with excellent results. In every instance, the XE265HR improved the power output over the stock cam through the entire rev range.

More importantly (for daily street use), the Comp cam did so without sacrificing idle quality or drivability. There are wilder (and more powerful) cam choices, but the XE265HR is one heck of a good street cam. The hydraulic roller cam was combined with a new set of lifters and hardened pushrods from Comp Cams. The buildup also featured ARP head studs, Fel Pro head gaskets, and a new Sealed Power oil pump and timing chain set. The stock oil pan and windage tray were replaced by a new setup from Moroso, which included a remote oil filter.

The 383 was configured on the engine dyno with a FAST management system, a Meziere electric water pump, and a set of 1-3/4-inch long-tube headers from American Racing. The induction system consisted of a Fast LSX intake and throttle body fed by a set of 36-pound injectors. Prior to testing, the motor was broken in with conventional motor oil, but we made the switch to 5W-30 synthetic (both from Lucas Oil).

Run with stock LS1 heads, the mild 383 produced 465 hp at 5,500 rpm and 485 lb-ft of torque at 4,400 rpm. Swapping out the stock heads for the RHP Pro Action 225 heads increased the peak numbers to 506 hp at 5,700 rpm and 503 lb-ft of torque at 4,700 rpm. You know you have done well when your head swap improves the power output through the entire rev range, and that is exactly what happened. Torque production from our mild stroker now exceeded 450 lb-ft from 3,600 rpm to 5,800 rpm, making one sweet (and effective) torque curve. The best thing is that our RHS heads have plenty of flow in reserve, giving them the ability to support future (wilder) combinations. Should you see the need to install wilder cam timing, a blower, or nitrous, you have all the cylinder head you need.



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