Muscle Mustangs & Fast Fords
617HP Paxton Street Stroker
The only thing better than a stroker motor or boost is a stroker motor and boost!
Why are stroker Ford combinations so popular? The answers are many, but mainly, when it comes to building a performance powerplant, bigger is always better. That is to say it is always easier to produce a given amount of power with increased displacement. Where a 500hp normally aspirated 302 might be a tad on the grumpy side for daily street driving, a 500hp 408 would be much more manageable. Thus, the extra cubes make it not only easier to achieve the desired power output, but it can do so while simultaneously improving drivability.
After all, every build is not an all-out race motor. The vast majority of engine combinations go in daily drivers, or at least something that needs to offer sufficient street manners. A high-compression motor with wild cam timing that has a tendency to overheat in stop-and-go traffic is not going to bring many smiles, no matter how fast it is when you finally bring it up on the cam.
The only thing better than big power from a big motor is even bigger power from a boosted big motor. With that, let’s take a closer look at the benefits of a stroker motor before adding the Paxton supercharger to this 331 combo.
As indicated, the greatest benefit of a stroker is the sizable jump in power, but the gains come in at more than just peak rpm. An example works well here. Suppose we build a 5.0L 302 that puts out 302 hp. Given the available performance components, achieving 1 hp per cubic inch is pretty simple. If we produce the same efficiency (by applying the same performance components) on a larger 331 stroker, our motor should produce 331 hp. While an extra 29 hp is always welcome, the real benefit of the stroker can be found at engine speeds well below the power peak. The stroker adds a hefty chunk of torque, and not just at the torque peak, but everywhere along the curve. This extra torque greatly improves part-throttle acceleration, drivability, and it can eliminate the need to downshift when passing. In short, the extra power offered by the stroker adds fun any time you stomp on the loud pedal.
To illustrate the power offered by a streetable stroker, we decided to build one and run it on the dyno. Working with Probe Racing, L&R Automotive, and Procomp Electronics, we chose a 331 stroker in lieu of the larger 347. The stroker kit included a 3.25-inch forged-steel stroker crank. The 4340 forged steel crank was probably overkill for this street application, as the limiting factor in terms of power was certainly the strength of the production 5.0L block. In reality, a cast crank would work just as well on this application. We have run cast cranks in excess of 600 hp, so fear not if you choose a cast version to save some money.
Since we planned on keeping boost, engine speed, and eventual power output to reasonable limits, the components we picked were less about maximizing power as they were about building a combination for performance and drivability. The 3.25-inch crank was combined with forged I-beam connecting rods (from Procomp Electronics) and forged pistons from Probe Industries. The I-beam connecting rods were 5.4 inches in length, thus improving the rod-to-stroke ratio over a standard 302-length (5.09) connecting rod. The use of the 5.4-inch rod improved the rod-to-stroke ratio from 1.566:1 (with the 5.09-inch stock rod) to 1.66:1. The Probe dished pistons featured dual intake valve reliefs to allows use with either the standard inline valve locations of most 5.0L heads, or the offset locations featured on the Trick Flow Specialties Twisted Wedge heads. This came in handy as we planned on topping the stroker with a set of aluminum TFS Twisted Wedge heads.
Once assembled by Ford Performance Solutions, the 331 was treated to a mild Comp Cams Xtreme Energy (PN XE266HR) hydraulic roller cam, lifters and double-roller timing chain. The dual-pattern XE cam offered 0.544 intake lift and 0.555 exhaust lift. The duration was likewise skewed in favor of the exhaust, with 216 degrees of intake duration and 224 degrees of exhaust duration.
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.