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.