KJ Jones
Brand Manager, 5.0 Mustang & Super Fords
January 1, 2009
Photos By: KJ Jones

Horse Sense: Our normal M.O. for this type of project is to evaluate cylinder heads as part of an overall top-half package that includes a camshaft and intake manifold. Since the word on the street is that Edelbrock's new CNC-ported RPM Xtreme heads will make a marked difference in a stroked 5.0's power output without changing a street/stripper's bumpstick and intake, we're bolting the heads on by themselves for this test to let the dyno tell us the real deal.

By now, most of our longtime readers (and even some of you newcomers) are hip to our tactics when it comes to finding the value-from a performance perspective-of various cylinder head/camshaft/intake manifold (H/C/I) packages for street-driven '86-'93 Mustangs.

First, we determine a Pony's as-is rear-wheel horsepower/torque on the chassis dyno, then we swap old parts for new. We typically close our power-and-torque evaluation by running the upgraded 'Stang on the dyno once again, noting the changes-good or bad-for the reports you read in this magazine.

Leroy's 'Stang is street driven, but some components, such as air conditioning, heat, and power steering, have been deleted for the sake of racing. With this being the case, disassembly is easy for technician Jerome Anderson, as it only requires disconnecting various lines, hoses, and wires for fuel, vacuum, water, and the alternator, as well as removing the upper intake, valvetrain, and long-tube headers for access to the heads that are coming off.

Thus far, our research has focused primarily on H/C/I combinations for mostly stock (with the exception of acceptable bolt-ons such as cold-air systems and free-flowing exhausts) 5.0 engines. All of the packages we've tested have proven themselves by showing impressive power gains on the dyno, so we think you can't go wrong using any one of them when you're ready to pump up the performance of your untouched Mustang.

That's the way it is for stockers. However, on the other side of the coin, aluminum heads, hotter hydraulic-roller cams and better-flowing intakes already have starring roles in stroked engines, such as the 331s and 347s that thump under the hoods of many street/strip Foxes. So, with this being the case, making meaningful changes in any of those areas on a stroker can be a bit more challenging.

Since we're always up for a good challenge, we decided to see how Edelbrock's latest small-block Ford aluminum heads perform on a non-stock, 302-based engine. Our first look at the company's new RPM Xtreme castings (PN 51259; $2,199.98/set) came at the '07 SEMA show when your tech editor spied the heads sitting inconspicuously among other Edelbrock products on display. Xtreme is shortened to simply XT on the face of the heads

While RPM Xtremes are derived from the company's ever-popular Performer RPM heads, the new pieces bring much-improved airflow into the mix thanks to a radical blend of as-cast and CNC-modified ports. That's right, engineers at Edelbrock designed a killer CNC program to open up and improve the intake- and exhaust-port openings and bowls where necessary, leaving some areas alone and putting a significant amount of focus on porting the combustion chambers on the new heads. The folks at Edelbrock say the design changes were made with stroker engines in mind, and they yield power gains that could help make RPM Xtremes perfect for aggressive street Mustangs that occasionally see action at the race track.

Big Daddy Performance's top tech Mike Rozman removes the aluminum cylinder heads from the 347. Before cries of disparity ring out, note that the stroker's original heads are similar in makeup to the Edelbrock RPM Xtreme pieces. The heads we removed are 18-degree (valve angle) pieces that feature 185cc intake ports, just like the XT heads. Combustion chamber sizes are not exactly the same (although close), as are valve sizes. Actually, with their 2.02 intake/1.60 exhaust valves, the original heads have slightly bigger exhaust valves than the Edelbrock's, which come loaded with 2.02 intake/1.57 exhaust stainless pieces.

Since our job is to try and validate statements like this, we acquired a set of RPM Xtreme cylinder heads and arranged to have them installed on a 347-powered '88 LX coupe at Big Daddy Performance in Lakewood, New Jersey, during the first leg of KJ's three-week, tech-project odyssey on the East Coast. Mike Rozman is the lead tech in Dwayne "Big Daddy" Gutridge's spacious 'Stang spa, and with the assistance of fellow technician Jerome Anderson, Mike completed the head exchange and dyno testing in a day's time.

We explore only the cylinder-head swap for this project effort, mainly because the engine's existing hydraulic-roller camshaft's profile (0.577-intake/0.580-exhaust lift; 0.280-intake/0.286-exhaust duration at 0.006) is close to the cam specs that Edelbrock suggests will work most efficiently when combined with XT heads.

On the intake side, we opted to continue using Edelbrock's Performer RPM 5.0 II upper/lower combo (PN 7123; $636.87). For optimum performance, the Victor 5.0 (PN 2945; $599.99) EFI manifold that we originally considered requires more cam lift than the engine's existing piece, and we really wanted to test the new heads' mettle on a street/strip stroker in out-of-the-box, bolt-them-on, no-extra-components (camshaft, intake, pistons) fashion.

We've seen notes posted in several Mustang-enthusiasts' Internet message forums where curious 'Stangbangers are in search of details and answers regarding the way RPM Xtremes perform. We're all about getting to the bottom of that type of mystery, so keep reading and see what we learned once the XT heads were sitting atop Leroy Howell's 347 and the rollers of Big Daddy's chassis dyno reached full speed.