By Richard Holdener
When it comes time to modify your motor, there are a number of different routes available to improve performance. The most popular options include increased displacement, basic bolt-ons, and power adders (which include both boost and nitrous oxide). Spoiler alert: The best method to maximum performance is to combine these.
Start with a stroker, add the right heads, cam, and intake, then add boost and/or nitrous. This route obviously assumes you are both looking for every last ounce of power and have the means to afford it. For most of us, the choice of one is more than enough, and just the fact multiple avenues exist is reason enough to run a quick dyno comparison. After all, the All-Motor (bolt-on) guys don’t think much of the boost camp, and vice versa. Let’s see how the two compare on a small-block Ford.
In reality, our blue-oval test motor started out life already blessed with additional displacement. Rather than run the test on a stock 5.0L, we decided to step up to a 347 stroker assembly. Since we had to assemble all the components anyway, the 347 crank was no more expensive than a 302. The 3.40-inch stroker crank and 5.40-inch forged connecting rods were supplied by Speedmaster. The forged crank and rods were combined with a set of .030-over, forged pistons from JE to produce the desired 347 inches. The pistons featured valve reliefs that allowed us to successfully run both the stock 5.0L cam and the more aggressive XFI stroker grind from COMP Cams. It should be noted the 347 will be much more receptive to the bolt-ons than a smaller 302, as the additional inches can better take advantage of head flow and wilder cam timing.
The game plan for this test was to run the 347 stroker first with the stock 5.0L Mustang components, including the stock iron cylinder heads, camshaft, and a GT-40 intake (we didn’t have a stock H.O. intake handy for testing). Tuning each combination was a FAST XFI management system, so there was no need to run the mass air meter or attending air intake assembly. Having the FAST management system allowed us to quickly dial in each of the three different 347 combinations.
Additional components in the build-up included a Moroso oiling system, MSD ignition, and Speedmaster 28-ounce balancer. The GT-40 intake was fed by an Accufab 70mm throttle body, while fuel was supplied by a set of FAST 36-pound injectors. Exhaust exited through a set of Hooker 1 ¾-inch, long-tube headers into 18-inch collector extensions. Keeping things cool on the dyno was a Meziere electric water pump.
Run with the stock heads, cam, and GT-40 intake, the 347 produced peak numbers of 307 hp at 4,700 rpm and 401 lb-ft of torque at 3,300 rpm. Obviously, the mild cam and stock heads were greatly limiting the package, but that was all about to change.
The first modification on the list was to add boost to the current combination. This could then be compared to the bolt-on upgrades that included a set of RHS aluminum heads, wilder COMP XFI cam, and Edelbrock Performer RPM II intake. Boost for the stroker came from a Vortech S-trim supercharger. The Vortech S-trim required a dedicated oil feed and return back to the pan. We chose the Vortech S-trim both for its ease of installation (and testing) and for its ideal sizing for our low-boost and power needs. The Vortech was capable of supporting more than 750 hp — more than enough for our needs — yet was plenty efficient at lower power and boost levels.
The S-trim supercharger was equipped with a 3.80-inch blower pulley and 6.75-inch crank pulley, which produced a peak impeller speed of 36,769 rpm at a peak engine speed of 6,000. On our 347 test motor, this equated to a peak boost reading of 8 psi at 5,700 rpm. The rising boost curve from 2.4 to 8 psi brought peak numbers of 421 hp and 462 lb-ft of torque. Even down at 3,300 rpm, the Vortech supercharger increased torque production of the 347 from 401 to 461 lb-ft, though the mild cam and stock heads were still limiting power production AND artificially increasing the boost pressure (really just back pressure) in the manifold. Now, it was time for the bolt-on brigade!
Off came the Vortech supercharger, as well as the stock E7TE, iron heads, GT-40 intake, and stock 5.0L cam. These mild components were swapped in favor of a set of RHS aluminum heads, an XFI hydraulic roller camshaft, and Edelbrock Performer RPM II EFI intake. We also stepped up to the larger 75mm throttle body from Accufab.
The dual-pattern XFI236HR-14 cam supplied by COMP Cams featured .579 lift (intake and exhaust), a 236/248-degree duration split (measured at .050), and a 114-degree lobe separation angle. The cam was combined with 200cc, as-cast aluminum heads designed as a direct bolt-on for hydraulic-roller cam applications. The RHS heads improved intake flow from 166 to 274 cfm, enough to support nearly 550 hp on the right application.
Topping the RHS-headed combination was the Edelbrock RPM II EFI upper and lower intake. Like the RHS heads and XFI cam, the RPM II represented a sizable jump in performance potential over the GT-40 intake. Equipped with the new, all-motor (bolt-on) combination, the power output of the 347 jumped from 307 hp and 401 lb-ft of torque to 448 hp and 420 lb-ft of torque.
Looking at the results, we see the 347 produced 448 hp with the bolt-ons and 421 hp with the Vortech supercharger, but (as always), the peak numbers do not tell the whole story. The supercharger improved the power output through the entire rev range, but the bolt-ons actually lost power below 4,000 rpm. In retrospect, a slightly milder cam profile might be a better choice on this 347 to help improve low-speed power production.
The boost level of the Vortech supercharger was as much a function of the lack of flow from the stock components as it was the ability of the supercharger. Boost is actually a measurement of back pressure in the intake system. If we ran the blower at the same speed on the bolt-on combo, the boost would decrease while the power output increased. Restrictions in the system increase boost pressure, and flow gains (from ported heads, cam profiles, and better intake manifolds) decrease it. This should indicate the best 347 would be a combination of bolt -ons and boost, but we also know that independently, they offered impressive power gains as well.
Sources: Accufab, Inc., accufabracing.com; COMP Cams, compcams.com; Edelbrock, edelbrock.com; Holley/Hooker, holley.com; JE Pistons, jepistons.com; Speedmaster, Speedmaster79.com; Racing Head Service, racingheadservice.com; Vortech Superchargers, vortechsuperchargers.com