Demon 347 Short-Block Cylinder Assembly - Fast Ford Formula Part 1

It's every bit as easy to add basic bolt-ons to your stock 5.0L as it is to a stroker version; ditto for forced induction or nitrous oxide. Basic bolt-ons are the most popular due to the low cost, but it takes a great many bolt-ons to equal the power offered by a stroker short-block or a power adder.

At MM&FF, we decided to combine a number of different performance routes- including displacement, bolt-ons, and even forced induction-all in one test. The idea is to illustrate the power gains offered by changing the heads, cam, and intake on a typical 5.0L assembly.

The pistons featured valve reliefs for inline-valve heads, which allowed us to successfully replace the factory 5.0L cam with a wilder XFI unit from Comp Cams. We chose the 347 stroker since most 302 builds include a displacement upgrade. The stroker assembly makes good financial sense since the cost of building a 347 is really no different than building a 302 or 306. Of course, that changes if you already have a 302-but fear not, as the modifications and boost work equally well on the smaller 302.

In Part 1, we look at what happens when you upgrade the normally aspirated 347 stroker short-block. The stock 5.0L heads, cam, and GT-40 intake were replaced by a set of as-cast RHS aluminum heads, an XFI hydraulic roller camshaft, and an Edelbrock Performer RPM II EFI intake. Both combinations were run with an Accufab throttle body and matching EGR spacer. After dialing in the normally aspirated combinations, we will follow up by adding a Paxton centrifugal supercharger running 8 psi. For now, it is time to go into details of the first test.

Next came the factory 5.0L E7TE heads. The heads featured stock (as-cast) ports but were previously modified for testing in our series "Ultimate Guide to Cylinder Heads." The improvements included machining the spring pads, and drilling and tapping for guideplates and screw-in rocker studs. This allowed us to run the heads with adjustable roller rockers on some of our larger test motors. For our needs, the as-cast ports meant they would represent the power potential of the stockers. The spring upgrade would allow us to test higher in the rev range, though the factory H.O. heads, cam timing, and GT-40 intake would surely limit anything resembling high-rpm power.

That these components were installed on the larger 347 (instead of the factory 302) would also reduce the engine speed where the combination produced peak power. Basically we had a stroker version of the stock 5.0L fuelie motor, though the static compression ratio did increase from the hike in displacement (from 9.35:1 to near 10.0:1).

Prior to making power pulls, the motor was treated to a computer-controlled break-in procedure using conventional Lucas Oil. While the break-in procedure wasn't necessary for the factory hydraulic roller cam, achieving proper ring seal is every bit as important. Rushing things such as the break-in procedure is a surefire route to reduced engine life. The FAST XFI engine management system was hooked up, and after dialing in the air/fuel and timing curves, the normally aspirated 347 produced 307 hp at 4,700 rpm and 401 lb-ft of torque at 3,300 rpm. Though the factory heads, cam, and GT-40 intake limited peak power production, torque production from the Demon stroker exceeded 350 lb-ft out to 4,600. Had we tested the motor lower in the rev range, torque production was over 400 lb-ft at 3,300 rpm and would likely have exceeded 350 as low as 2,500 rpm. Not exceptional power, but the stock components were limiting what was otherwise a stout short-block.

Naturally, the cam required a decent set of cylinder heads to take advantage of all that lift, so we installed a set of aluminum heads from RHS. A legendary name in the cylinder-head business, RHS offers a number of both cast-iron and aluminum heads for 5.0L Ford applications. It even has new CNC-ported versions we hope to test on a future build, but this 347 received a set of assembled, as-cast 200cc aluminum heads designed as a direct bolt-on for hydraulic roller cam applications.

Topping off the mix was an Edelbrock RPM II EFI upper/lower intake. Like the RHS heads and XFI cam, the RPM II represented a sizable jump in performance potential over the GT-40 intake.

The three components were swapped on the engine dyno with the motor still warm from the previous baseline testing with the stock components. Off came the stock 5.0L heads, cam, and GT-40 intake, and on went the RHS heads, XFI cam and RPM II intake. The Accufab throttle body was carried over, as were the 36-lb/hr injectors, FAST management system, and Hooker headers.

Once tuned to perfection, the 347 pumped out 448 hp and 420 lb-ft of torque. Note that the peak torque value only increased from 401 with the stock components to 420 with the new combo, but the big news was the shift in the torque curve. The 347 produced peak power at 4,700 rpm and peak torque at 3,300 rpm with the stock components, but these numbers jumped to 6,200 rpm and 5,200 rpm with the new heads, cam, and intake.

Naturally the sizable shift in the torque curve resulted in a loss of low-rpm power as the stock components offered more torque up to 4,000 rpm. Had we tested the stock components out past 6,000 rpm, the difference would have been as much as 200 hp in favor of the modified motor.

In truth, we'd probably recommend a slightly milder cam (the XE274HR is an excellent choice for a fuelie 5.0L) for this normally aspirated combination to improve low-speed power production. The XFI cam would work better with a shorter-runner intake like the TFS Box R system, but the combination responded well once we added boost to the equation.

But would these gains remain consistent once we added the Paxton supercharger to the mix? By this we mean would the supercharged modified motor produce another 141 hp more than the supercharged motor with the stock components? Since we were limiting the boost pressure to a street friendly 8 psi, would the stock motor with the blower produce as much power as the modified motor with the blower? And since we didn't plan on performing any pulley swaps, what would happen to the boost pressure between the two combinations? Would the added airflow offered by the modifications reduce the boost pressure compared to the stock components?

These are but a few of the questions we plan to answer in Part 2 of our Fast Ford Formula.