Muscle Mustangs & Fast FordsHow To Engine
617HP Paxton Street Stroker
The only thing better than a stroker motor or boost is a stroker motor and boost!
The Xtreme Energy cam featured a relatively wide 112-degree lobe separation angle (LSA), something that helped improve the idle and broaden the power band. Since our supercharged stroker would easily exceed the strength of the production block, we erred on the mild side of cam timing to improve idle quality and drivability. If you have more than enough power, why not have it idle like a stocker? As mentioned, Comp Cams also supplied the necessary hydraulic roller lifters, spider hold-down assembly, and hardened pushrods. Comp also supplied aluminum 1.6- ratio roller rockers. The 331 stroker short-block was finished off with Fel Pro oil pan and front cover gaskets, a 28-ounce balancer, and Melling's HV oil pump (making sure to include an ARP HD oil pump drive).
Next, it was time to address our cylinder head needs. While our combination was purposely kept on the mild side, we stepped up big time on the cylinder heads. Combining a mild cam and high-flow heads yields a much better power curve than the other way around. Knowing this, we installed a set of CNC-ported RHS Elite heads on the stroker. The Pro Elite heads offered exceptional flow thanks to CNC-ported 205cc intake ports, a multi-angle valve job, and CNC combustion chambers. Further improving flow was the fact that the 20-degree heads were assembled with a 2.055/1.60-inch stainless-steel valve package. The heads included rockers studs and guideplates for the 1.6-ratio roller rockers, and the heads were secured to the stroker using Fel Pro 101-2 head gaskets and 7/16-inch ARP head studs. The heads were topped off with an Edelbrock Performer RPM Air Gap intake and new Mighty Demon carburetor.
The carburetor worked well on both the naturally aspirated and supercharged combination. An MSD billet distributor, Meziere electric water pump and 1-3/4-inch Hooker headers completed the normally aspirated combination. Exhaust from the Hooker Super Comp (Fox chassis) headers was channeled through 18-inch collector extensions.
The 331 stroker would prove to be plenty powerful in normally aspirated trim, but the low-compression, mild-cammed Ford combination was built specifically for boost. Boost in this case came from a Paxton Novi 1200 supercharger supplied as part of its carbureted Mustang kits. The kit came complete with the supercharger (you can also choose a slightly more powerful Novi 1500), carb enclosure, blower mount, pulleys and fuel system. We opted for the driver-side mount, but the carb kit is also available with the blower mounted on the passenger side a la 5.0L EFI motors. We relied on a 750 Mighty Demon carburetor set up with a 89/91 jet and 0.028 air bleed combination.
The Paxton Novi 1200 was installed using a 7-inch crank pulley and 3.33-inch blower pulley, which produced a peak boost pressure of 9.4 psi at 6,000 rpm. Naturally the curve was still rising, but in deference to the stock block, we kept engine speed and boost to a minimum. It is possible to adjust the boost level provided by the blower with changes in blower and/or crank pulley size. With our static compression ratio below 9.0:1, the boost level was easily run on pump gas.
Before running the Novi 1200, the stroker was first treated to a break-in cycle then run in normally aspirated trim to establish a baseline. After jetting and timing sweeps, we ran the motor from 2,500 to 6,000 rpm in an effort to demonstrate the power gains offered in the streetable rev ranges. In normally aspirated trim, the low-compression 331 produced 392 hp at 6,000 rpm and 386 lb-ft of torque at 4,400 rpm. These were respectable numbers considering the mild tune.
After installation of the Novi 1200, the power numbers jumped considerably. Run at a maximum boost of 9.4 psi, the supercharged 331 produced 617 hp at 6,000 rpm, and 561 lb-ft of torque at 5,200 rpm. True to form, the carbureted Paxton kit added over 200 hp and 200 lb-ft of torque--all on pump gas.
The carb enclosure demonstrated its worth, offering not only plenty of power and a safe air/fuel mixture, but also impressive idle and part-throttle characteristics. A carburetor can never match a dedicated EFI system for optimum fuel control under every combination of load, throttle angle, and engine speed, but it performed every bit as well as your typical NA carbureted motor, and did so without an expensive electronic fuel injection kit.