If you are going to properly test a set of ported heads, you need to build a good test motor.

Words by Richard Holdener

Our dilemma was this: The good folks over at ProMaxx supplied a set of CNC-ported, 210cc aluminum Ford heads, and we to choose between two headless horsemen (Mustang motors) to test them on. Truth be told, the two test motors actually numbered three, as we also ran these heads on a larger (and wilder) 392 with excellent results (comparing them to stock heads no less). You will have to read about that test in another installment, but the results are definitely worth the wait.

For this go around, we had to choose between a run-of-the-mill 5.0L (302) short block and a larger 347. The 5.0L was obviously more common, but given the flow potential of the 210cc heads (well over 300 cfm), we elected to abide by the bigger-is-better philosophy and go with the extra inches offered by the stroker. At least one of our headless horsemen was about to get some serious new headgear.

On the surface, our choice might seem as simple as the fact that bigger motors make more power than smaller ones. This is certainly true, but so too is the fact the larger motor will be better able to take advantage of the flow potential of the impressive headgear. By no means will this 347 be able to take FULL advantage of an intake port that flows more than 320 cfm (translation-able to support 650-plus hp), but the stroker will certainly put up a better fight than the smaller 302. It would take one heck of a high-winding, 5.0L (or 347 for that matter) to exceed 650 hp, but we liked having enough head flow for future growth. If we ever decide to upgrade to a 351W, or 408 stroker version, it is nice to know the ProMaxx 210cc heads will be able to support just about any combination we decide to build.

For the uninitiated, the extra inches offered by the 347 come via an increase in both bore and stroke compared to the standard 302. The 347 is one of the most commonly available stroker assemblies offered for the 8.2-inch deck-height, 5.0L motor. The standard 5.0L (and earlier 302) achieved its displacement by combining a 4.00-inch bore and a 3.00-inch stroke. The 347 stepped up the bore size to 4.03-inches, about the limit for most production 5.0L blocks. The increased bore size is combined with a 3.40-inch stroke.

Those quick on the calculator will realize this alters the bore-to-stroke ratio from 1.333:1 for the 302 down to 1.185:1 for the 347. To put these numbers into perspective, a larger number here indicates a big-bore combination that might be more prone to high-rpm power production. Don’t let the slight change in this bore-to-stroke ratio convince you that the 302 is a better choice, as the 347 will always make more power and has plenty of rpm potential.

Truth be told, things like bore-to-stroke, and even rod, ratio have much less of an effect than the power production components. The three major contributors that determine not only the peak power and torque, but the overall shape of the curve, include the heads, cam, and intake. We obviously had the cylinder head portion of the equation covered with our 210s from ProMaxx. The heads featured 210cc intake ports that flowed more than 320 cfm. The heads featured full CNC porting, 60cc combustion chambers and a 2.08/1.60 valve package. In truth, these heads were better suited for a 392 or 408 stroker, but this test would show they were equally at home on a milder 347 street motor (with the ability to grow).

The heads were supplied with valves but no spring hardware, so we installed a suitable combination from COMP Cams. The springs were set up for use with our sub .600-lift hydraulic roller cam. COMP supplied 939 springs, 4771 spring seats, and 732 titanium retainers. This spring package allowed us plenty of coil bind clearance for our .579-lift XFI cam, and the ability to rev the motor cleanly out past 6,500 rpm, where the 347 wanted to make peak power.

The 347 short block featured a forged crank and rods supplied by Speedmaster, along with a set of forged flat-top pistons from JE. The pistons featured sufficient valve reliefs to allow for our XFI cam lift, and were sealed using a ring package from Total Seal. Completing the short block was an XFI236HR cam and adjustable, double roller timing chain from COMP Cams. Designed for Ford stroker applications, the XFI cam featured .579 lift, a 236/248-degree duration split, and 114-degree lsa. COMP also supplied a set of drop-in hydraulic roller lifters and hardened (6.45-inch) pushrods.

The short block was sealed up using a factory oil pan, Mellings HV oil pump, and ARP oil pump drive. The heads were secured to the 347 short block using Fel Pro 1011-2 head gaskets and ARP head studs, then completed with a set of 1.6-ratio Ultra-Gold roller rockers from COMP Cams. A set of Speedmaster, cast-aluminum valve covers completed the short block.

With our short block complete, it was time for the finishing touches, including the damper, induction system, and ignition. The rotating assembly was internally balanced, so we applied an internally balanced damper from Speedmaster to complete the ensemble. Not wanting to lose the damper at high rpm, it was secured using an ARP damper bolt.

Knowing the heads and cam would push peak power higher in the rev range, we selected a single-plane Funnel Web intake. The intake was topped by a Holley 650 XP carburetor. Initially, we were concerned about the 650 limiting the power output, but running a larger 750 resulted in no power gain. It just goes to show you how good the XP series really is.

ProMaxx Headed 347 Ford
We love it when an engine package comes together, and this 347 was a perfect example of when things go right. The combination of the Funnel Web intake, COMP cam, and Promaxx heads worked well on the 347. The single-plane intake, XFI cam timing, and free-flowing heads allowed the little stroker to rev cleanly up past 6,500 rpm, where the small-block Ford produced 459 hp and 409 lb-ft of torque. The torque curve was plenty broad, exceeding 375 lb-ft for more than 2,000 rpm. Given the flow rate of the heads, it will support a much wilder combination, but worked well nonetheless on this milder street stroker.

The final touch was an MSD distributor and ignition amplifier. Once dialed in, the ProMaxx-headed 347 pumped out 459.9 hp and 409 lb-ft of torque. No matter how hard we tried, it would not give us that last .1 hp to round it out to an even 460 hp, but our headless horseman was headless no more!

Sources: ARP, arp-bolts.com; COMP Cams, compcams.com; Holley/Hooker/NOS, holley.com; MSD, msdignition.com; ProMaxx Performance, promaxxperformance.com; Speedmaster, speedmaster79.com