5.0 Mustang & Super FordsHow To Engine
Fox Mustang Cylinder Heads - Xtremely Impressive
You Won't Be Disappointed By Hopping Up Your Street Stroker With Edelbrock's New CNC-Ported XT Heads
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.
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.
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.
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As direct, bolt-on projects go, this certainly was a good one. In straight, no-extras trim, Edelbrock's new RPM Xtreme small-block Ford cylinder heads delivered the goods, boosting the performance of Leroy Howell's notchback Fox to the tune of 413.16 peak horsepower and 420.25 lb-ft of torque at the rear tires.
Before you start thinking that's not much for a 347, we know there are others that do slightly better. (For example, the 350ci stroker in our T-top coupe makes an estimated 450 naturally aspirated horses and 416 lb-ft of torque, but it has bigger heads and a much-bigger camshaft than this engine.) It's important to recognize that the 347 in our test 'Stang mustered only 382.46 hp and 404.51 lb-ft of torque in our baseline test.
"I think these heads would be great to offer as part of a complete stroker kit, or assembled engine," Big Daddy says. "Using the Performer RPM 5.0 II intake manifold was a good call for Leroy's setup. With better headers and an x-pipe, the numbers probably would have been even better."
We went a step beyond with this tech effort and took Leroy's 'Stang to the dragstrip to see of the heads made any improvement in e.t. or miles per hour. After several passes to establish new launch rpm and shift points, Leroy's Mustang annihilated its previous best-ever time of 11.47 (at 116 mph) with a quickest-ever pass of 11.24 at 120 mph.
All we did was bolt-on new heads, guys. Even though streetability will likely become a bit more compromised if a higher-lift hydraulic-roller or solid-roller camshaft and a Victor 5.0 intake are added, doing so might prove well worth the effort.