After receiving the heads, we sent them to Porter Racing Heads for some minor porting and blueprinting. While the Edelbrock heads can be run out of the box, we prefer to pull them down to double check guide clearances, lap in the valves, and give everything a close inspection before spending a lot of time on the dyno at wide open throttle. These Performer RPM heads have the same small port size as stock heads, but they flow quite a bit more air. The Edelbrock heads flow 290 cfm on the intake at .600-inch lift right out of the box. The porting work by PRH picked up the flow to just a little over 300 cfm on the intake at .600 lift, which we felt should be good for about 600 hp if everything else is properly tuned.

Mopar big-block cylinder heads have two basic port sizes, standard and Max Wedge. These Edelbrock heads are the smaller, standard port size, which has a cross section of 2.88 square inches. The classic McFarland formula tells us that a 505ci engine with 2.88 square-inch cross-sectional heads will have a torque peak near 4,000 rpm. We always double-check our engine designs with the McFarland formula, and it is usually pretty good at predicting the actual results.

As with most aftermarket heads, the spark plugs in the Edelbrock RPM heads are angled, which can cause some fitment issues. Many header designs have been updated to fit the newer head designs, but older headers might require clearance work. As with most aluminum cylinder heads, a 3/4-inch reach spark plug is required.

Cam and Valvetrain
We knew that the size of the cylinder head ports would limit the power peak to less than 6,000 rpm, so a decision was made to go with a hydraulic-roller camshaft. Big-block Mopar engines never came from the factory with hydraulic-roller camshafts, but the aftermarket has stepped in to develop retro-fit parts. COMP Cams is one of those vendors with retro-fit parts for the Mopar big-block, including a line of Xtreme Energy hydraulic-roller camshafts that use their new XFI lobe family. For this engine, we went to the bottom of the page, and selected the XR292HR-10, which is the largest hydraulic-roller cam COMP had in the catalog. The XR292 has an advertised duration of 292/300, and a .050 duration of 242/248. The COMP catalog says this cam will have a radical idle but in our 505-inch engine the idle was actually quite mellow.

To complement the big roller cam, we selected a set of 1.60-ratio stainless steel rocker arms from Rocker Arm Specialist (RAS). These rocker arms aren't very well known in the Mopar world, but they are a great product that we have used over the years in different motors. The RAS rocker arms come complete with heavy-duty shafts and hold-downs as well as high-quality adjuster hardware. These stainless rocker arms have a roller tip and a bronze bushing for the shaft. The 1.60 rocker arm ratio gave us a total valve lift of .584/.579 on the intake and exhaust. A set of COMP 26120-16 beehive valve springs was selected based on excellent results in the past with similar combinations. These beehive springs have been shown to work well with the XFI cam lobes on other projects, so we felt comfortable with this choice.

Intake and Carb
We have wanted to get a healthy big-block Mopar with this CH-28 dual-carb intake on the dyno for some time. The CH-28 intake is an old favorite from Edelbrock that has been around for many years. It is a low-profile dual-plane intake, so it isn't going to make killer power, but it looks great, and we thought it might just run pretty hard on the right engine. Edelbrock has linkage kits, air cleaners, and fuel lines available, while Hughes Engines offers a porting package for this intake. Hughes Engines also has a linkage kit designed to work with the stock Mopar throttle cable, which makes this intake a bolt-in deal for many Mopar muscle cars.