Halo Stroker Kit

The URGE Designs Halo Stroker kits represent a culmination of over five years of meticulous redesign and part optimization, all aimed at preserving the high-revving spirit synonymous with Honda’s S500/600/800 and S2000 Halo cars.

URGE Designs is proud to offer two variants of the Halo Stroker kits:

1. The 99×9 Halo Stroker Kit, which features a 99mm K-Series based crankshaft, provides a significant increase in torque compared to the OEM S2000 while maintaining the iconic 9000 RPM redline of the AP1 S2000. Additionally, it achieves a reduction in OEM bearing pressures, which promotes extended bearing life.

2. The 10K Halo Stroker Kit, which employs a 90.7mm AP2 S2000 crankshaft, pays tribute to the heritage of the S500, S600, and S800. It supports an exhilarating 9500+ RPM redline while also reducing OEM bearing pressures to ensure longevity of bearing life.

Extreme Reciprocating Weight Reduction is key

Achieving high revolutions per minute (RPM), whether it’s propelling a 99mm stroke to 9000 RPM or a 90.7mm stroke to 10,000 RPM, presents challenges with inline-four vibrations, harmonics, and maintaining OEM-like bearing pressures. This challenge has driven a rigorous commitment to weight reduction. Precisely, every gram removed from the rotating assembly can decrease peak crankshaft tensile force by approximately 12 pounds per cylinder and reduce the pressure on each bearing by roughly 8 PSI.

Wrist Pins

The optimization of engine performance begins with the wrist pins, which lay the groundwork for reducing piston weight and subsequently allow for a decrease in connecting rod weight. Wrist pins are the most burdened components within a stroker kit, tasked with enduring thousands of pounds of force over a small area while ensuring the piston avoids collision with the cylinder head and, a fraction of a second later, transmitting the force of combustion through the connecting rod. The diameter of the wrist pin is a critical design parameter; our goal is to minimize it as much as possible while retaining the necessary strength to withstand the immense forces exerted over 150 times per second.

A smaller wrist pin diameter facilitates a reduction in piston compression height, thereby minimizing piston weight. Examination of modern high RPM racing engines or superbike pistons reveals they typically feature shorter compression heights, a design enabled by smaller diameter wrist pins. In the highly competitive arena of single-cylinder motocross, the focus on reducing pin diameter is paramount in lightening piston weight to maximize power output. Moreover, a reduction in compression height, resulting from smaller piston pins, affords the use of longer connecting rods, which in turn reduces the increased cylinder friction and peak reciprocating forces that are characteristic of stroker kits.

Initially, we utilized titanium for the wrist pin material, achieving a weight reduction of approximately 45% compared to OEM pins. While titanium boasts an impressive strength-to-weight ratio, its yield strength is about 20% lower than that of chrome molybdenum (chromoly) steel, necessitating a larger pin diameter to meet our 9,000 RPM and 99mm stroke objectives. This limitation impeded our ability to decrease piston compression height and overall weight reduction potential. To accommodate the desired smaller pin diameter, we turned to tool steel, which allowed for a diameter reduction of over 21% compared to OEM standards. However, tool steel’s lower strength-to-weight ratio, coupled with the need for thicker walls to support our performance goals, resulted in a wrist pin weight that was 10% heavier than titanium.

In pursuit of a material with superior strength-to-weight ratio compared to titanium, we found a solution in a material widely used in NASCAR. C350 marries double the yield strength of chromoly steel with a reduced wall thickness, enabling us to match the 45% weight reduction of titanium pins while bolstering pin strength by 40%. C350’s robustness ensures improved roundness, diminished deflection, and minimal wear, making it the most durable material for piston pins currently available.

C350 wrist pins also benefit from a Diamond Like Coating (DLC), renowned as the hardest and lowest friction coating available. DLC’s remarkable reduction in friction has been observed to decrease average piston temperatures significantly, as noted by engine builders who have reported minimal wear even with low oil pressures during dynamometer testing. URGE Designs’ C350 piston pins, with their small diameter, weigh a mere 59 grams—equivalent to titanium pins—but with a 40% increase in strength, providing an indispensable advantage for the most stressed component of the rotational assembly. This achievement represents a 45% weight reduction when compared to the standard 100+ gram OEM F20/22C pins.

Pistons

Pistons account for the majority of reciprocating mass within an engine, presenting the most significant opportunity for weight reduction. Manufacturers of high RPM automotive and superbike motors are intensely focused on reducing every possible gram from pistons. Such weight savings directly translate into enhanced horsepower and improved reliability by diminishing the stresses on reciprocating parts, curbing bearing wear, and reducing overall engine vibrations.

Our collaboration with a leading piston manufacturer has yielded a design that takes full advantage of the 21% smaller diameter and 45% lighter C350 wrist pins, resulting in a piston compression height reduction of 20% compared to OEM specifications. Our piston designer employed Finite Element Analysis (FEA) to precisely mill away any non-essential material that did not contribute to withstanding critical stresses. Significant material removal was achievable beneath the piston crown, around the supports for the small diameter wrist pin, and behind the piston skirts.

URGE Designs’ 89mm Halo pistons have achieved an impressive weight of approximately 240 grams, in stark contrast to the 340-360 grams of the OEM F20C and F22C 87mm pistons. While off-the-shelf aftermarket pistons typically range from 310 to 350 grams, and even competitive lightweight pistons weigh between 260 and 280 grams at 89mm, our design represents a substantial advancement in piston technology.

Connecting Rods

Just as with piston pins, selecting the appropriate material is crucial in the design of connecting rods. Titanium offers a superior strength-to-weight ratio compared to chromoly steels, but it also comes at a cost that is three to four times higher, a factor that could affect the affordability of the URGE Designs Halo stroker kits.

Typically, OEM or aftermarket piston weights designed for a 9000 RPM redline with a 99mm stroke or a 10,000 RPM redline at a 90mm stroke would necessitate the use of a 7/16” rod bolt to maintain the integrity of the bottom end. However, a bolt of this size would increase the necessity for additional material, thus adding to the weight of the bottom end of the rod. In alignment with our philosophy of optimized material utilization, we have selected MP35N for the rod bolts, which boasts a tensile strength exceeding 280 KSI and exceptional fatigue resistance. These characteristics are the reasons MP35N is the material of choice in high-stress applications such as Formula 1, NASCAR, and IndyCar. The extraordinary strength of MP35N rod bolts, in concert with our ultralight pistons and pins, allows us to employ bolts with a smaller diameter than usual, further reducing the weight of the rod.

Our design and material strategy have a cascading effect on the entire engine system. Starting with the smaller diameter piston pins permits a reduction in piston compression height, which in turn allows for weight savings on both the pistons and the top of the connecting rods. Coupled with the use of stronger bolts, this also enables weight savings at the bottom of the rods.

Our rod manufacturer employs a custom 4330M alloy, which exhibits approximately 40% greater strength compared to the conventional 4340 alloy rods, while still being cost-effective. Leveraging Finite Element Analysis (FEA), our rod manufacturer has engineered a 4330M rod for the 99×9 Halo kit that weighs roughly 440 grams—over 30% lighter than OEM rods and approximately 20% lighter than aftermarket 4340 rods.

99×9 Halo Stroker Kit 4330M rods weigh >30% less vs OEM

For the 10K Halo Kit connecting rods, our objective was to achieve an approximate 10% reduction in weight to accommodate the demands of higher RPMs. It is critical to recognize that reciprocating forces rise exponentially with increases in RPM, necessitating a significant decrease in mass. To achieve a titanium-equivalent weight without incurring the associated costs, we decided to employ the C107 super steel option provided by our rod partners. This choice allowed us to reduce the weight of the 10K Halo Kit rods to 400 grams, a notable achievement considering the longer rods required due to the shorter stroke.

10K Halo Stroker Kit C107 rods weigh >35% less vs OEM

Longer rods

The shorter compression height allows for a 6mm longer rod.  The longer rods improve rod to stroke ratio reducing friction, wear and reciprocating forces compared to similar stroke kits.

Comparative Weight

So, what are the cumulative benefits of all this meticulous design optimization? A comparative analysis reveals that the URGE Halo kit achieves a 29-31% reduction in reciprocating weight compared to OEM configurations, and a 20-25% reduction when contrasted with other stroker kits on the market. This weight savings is attained without compromising durability; in fact, strength is enhanced through the use of superior materials, such as C350 for wrist pins, 4330M alloy for connecting rods, and MP35N for connecting rod bolts.

This extreme weight reduction translates to faster acceleration, higher trap speeds and increased horsepower.  The stronger materials and lighter weight significantly reduce internal stresses and wear increasing the life of the motor.

99×9 Halo Crankshaft Options

We evaluated two primary options for the Halo stroker kit: an OEM Honda K24 crankshaft and a custom billet crankshaft.

The Honda K series crankshafts share bore spacing and main bearing journal dimensions with the F20C, allowing them to seamlessly integrate into an F20C bottom end. These K24 crankshafts are widely accessible, with wholesale prices ranging from $500 for Accord models to $1100 for TSX models, offering a significant stroke increase (99mm) to substantially enhance mid-range torque for the F20. To adapt a K24 crankshaft for the S2000 drivetrain sprocket and flywheel, a few hundred dollars of machining is required, bringing the total expense to between $1000 and $1700. However, the narrower rod journal width of the K series crankshaft, approximately 17% less than that of the F20C, could lead to heightened bearing pressures and accelerated wear, potentially shortening engine lifespan.

Given the ready availability of high-quality Honda OEM K24 crankshafts, the substantial cost savings of $1000-$2000, and the 2-3 month lead time for custom crankshaft production, we have tailored our stroker kit to be compatible with the Honda K24 99mm crankshafts. By sufficiently reducing reciprocating weight yet preserving strength, we have successfully mitigated the effects of the reduced rod journal width, maintaining bearing pressure and wear at levels comparable to the F20C.

10K Halo Crankshaft Options

The primary choice for the 10K Halo kit is the OEM AP2 crankshaft with a 90.7mm stroke. However, production of this crankshaft has ceased for several years, making our main source for these parts the salvage or refurbishment of used OEM crankshafts.

As an alternative, we consider the Callies F22C billet crankshaft, part number S22010, which serves as a direct replacement for the OEM version. This Callies crank is constructed from 4340 steel, undergoes nitriding for surface hardening, and offers an aerodynamic counterweight design. Priced around $2000, the Callies F22 crankshafts represent excellent value for a billet crankshaft, especially since they are produced in larger quantities and are typically readily available, contrasting with the more expensive custom-built billet cranks that can cost between $3000 and $4000.

Crankshaft Force

Understanding the impact of lightweight reciprocating engine components can be illuminated by examining peak forces exerted on the crankshaft. The greatest force experienced by the crankshaft occurs at the moment of maximum tension at top dead center. The URGE Designs Halo kit, with its reduced reciprocating mass and extended connecting rods, diminishes these forces by 24-29% compared to conventional stroker kits. This equates to a staggering reduction of 1900 to 2300 pounds of force per cylinder at 9,000 RPM.

Furthermore, despite incorporating a 15mm longer stroke, the URGE Designs Halo Stroker kit manages to exert over 1300 pounds less peak force per cylinder than the AP1 F20C, which has an 84mm stroke. This remarkable engineering feat results in an 18% longer stroke while simultaneously achieving an 18% decrease in peak forces on the crankshaft compared to the AP1 F20C. The benefits extend beyond mere numbers, translating into tangible improvements in engine smoothness and a reduction in vibration.

Comparison chart for peak crank forces between 99×9 URGE Halo Stroker kit, OEM AP1 and competitive stroker kits. (Lowest is best)

Bearing Pressure

Even more vital than crankshaft stresses and engine vibrations are the stresses on bearings. The F20C is renowned for its broad rod bearing width of 23.9mm, which provides a substantial surface area to handle the stresses associated with a 9000 RPM redline. However, the K24 crankshaft presents a limitation with its narrower 19.8mm bearing width, leading to a 21% increase in bearing pressures. Coupled with a longer stroke, stroker kits utilizing the K24 crankshaft will encounter 30 to 38% higher bearing pressures than the AP1 F20C at equivalent RPMs. Even when other K24 crankshaft stroker kits have their redlines capped at 8000 RPM, the bearing forces remain 5-10% higher than an AP1 F20C. Consequently, bearing wear is a common point of failure for K series-based 99mm stroker kits.

The URGE Designs Halo stroker kit, through its significant weight reduction efforts, allows the maintenance of the F20C’s 9000 RPM redline while actually reducing bearing pressures compared to the AP1 F20C. This is achieved despite an 18% increase in stroke and a 17% decrease in bearing width that comes with using a K-Series crankshaft.

peak bearing pressures on a crankshaft for OEM AP1 and typical stroker kits. (Lowest is best)

URGE designs 99×9 Halo Stroker Kit Value

Priced at $3200, we acknowledge that the URGE Designs Halo stroker kit is approximately $1000-$1500 more expensive compared to typical 99mm stroker kits that utilize OEM pistons, standard aftermarket pistons, and 4340 rods. However, the advantages provided by our kit extend far beyond mere cost considerations. Our meticulous approach to part design and material selection results in significantly lighter and stronger reciprocating parts. These enhancements yield a 24-29% decrease in crankshaft force and bearing pressures at comparable RPMs, which substantially minimizes vibration, crankshaft deflection, and overall engine stress, leading to greater engine durability and longevity.

Moreover, the URGE Designs Halo stroker kit is engineered to operate reliably at 1000 RPM higher than other kits, while also delivering lower vibration, and reduced crankshaft and bearing forces. This capability offers the potential for an approximate 10% increase in peak horsepower—roughly 30 HP—and in average operating power compared to stroker kits with a lower redline. There is scarcely any other modification available for the F20C that can offer a roughly 10% or 30 HP increase in naturally aspirated horsepower for an additional $1500.

Considering the comprehensive costs involved in engine modification, which include engine removal, sleeving or cylinder coating, and assembly and installation, totaling an additional $3000 to $4000, the average stroker kit yields a 20% horsepower gain, about 50 HP. Therefore, the reduced engine wear, enhanced engine life, and the higher redline, along with the potential for an extra 10% power increase offered by the Halo stroker kit, present the best value for horsepower gains compared to other performance enhancements at the additional $1500 investment.