Demon 347-Stk vs. Modified...
Demon 347-Stk vs. Modified
It should be obvious from the gains in power that replacing the stock 5.0L heads, cam, and GT-40 intake is worth considerable power. Installation of the RHS 200 aluminum heads, XFI cam, and Edelbrock RPM II intake increased the power output of the 347 stroker from 307 hp and 401 lb-ft of torque to 448 hp and 420 ft-lb of torque. Note that the peak torque output was increased substantially, but shifting that peak torque number from 3,330 to 5,200 rpm resulted in a dramatic change in peak power. The stock E7TE heads, 5.0L cam, and GT-40 intake would limit power production on a stock 302, but they downright strangled this larger 347 stroker. Changing one component will naturally improve the power output, but the combination is only as powerful as its most restrictive component.
According to RHS, the intake ports on the 200cc aluminum heads flowed 274 cfm at 0.600 lift, while the exhaust flow checked in at 199 cfm. These flow figures compare to roughly 160 cfm and 113 cfm for the stock E7TE heads. As is evident, the RHS heads offered considerably more flow, and therefore more power potential. Judging by the figures, these RHS can support over 550 hp-much more than we were looking for with our 347.
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.