Halo Stroker Kit

The URGE designs Halo Stroker kits were a 5+year journey of redesign and part optimization with a goal of maintaining the high revving spirt of the Honda S500/600/800 and S2000 Halo cars.  

The URGE designs Halo Stroker kits come in 2 options. 

1. The 99×9 Halo Stroker Kit utilizing a 99mm K-Series crankshaft significantly increasing torque over the OEM S2000 supporting the headline 9000 RPM redline of AP1 S2000 WHILE reducing OEM bearing pressures.

2. The 10K Halo Stroker Kit utilizing a 90.7 mm AP2 S2000 crankshaft while paying an homage to the S500, S600 and S800 supporting a 9500+ RPM redline WHILE reducing OEM bearing pressures. 

Extreme Reciprocating Weight Reduction is key

Whether spinning a 99mm stroke to 9000 RPM or a 90.7mm stroke to 10,000 RPM, inline four vibrations, harmonics and achieving OEM bearing pressures drove a maniacal focus of light weighting.  Every gram saved will reduce peak crankshaft tension force by a ~12 pounds per cylinder and ~8 PSI per bearing. 

Wrist Pins

It all starts with the wrist pins, creating a foundation for piston weight reduction that enables connecting rod weight reduction.  Wrist pins are the most stressed part of a stroker kit and must endure thousands of pounds of force across a few inches while keeping the piston from slamming into the head one millisecond then transmitting combustion force through the rod the next millisecond.  The wrist pin diameter is our most important design parameter as we want to make the pin as small as possible while maintaining the strength to support extreme pushing and pulling forces 75+ times per second. 

The smaller we can make the wrist pin, the more we can reduce the piston compression height, minimizing piston weight.  If you study modern high RPM superbike pistons like we do, you will notice they have short compression heights supported by a small diameter wrist pin.  In the ultra-competitive world of single piston motocross bikes, piston and rod kits focus on reducing pin diameter to reduce piston weight, freeing up as much power as possible.  An additional benefit of compression height reduction due to smaller piston pins are longer connecting rods reducing the increased cylinder friction typical of stroker kits. 

To design the smallest wrist pin possible, we needed to use the strongest and stiffest material readily available. C350 has twice the yield strength vs chromoly steel allowing us to reduce diameter, wall thickness and length vs OEM pins while significantly increasing strength.  Higher strength pins maintain better roundness, reduce deflection, and minimize wear in this most stressed stroker kit part. 

Additionally, C350 wrist pins are coated with Diamond Like Coating (DLC) which is the hardest and lowest friction pin coating available.  DLC reduces friction so much, engine builders have measured tens of degrees Fahrenheit reduction in average piston temperature and noted negligible wear with low oil pressure on the dyno.  URGE designs C350 piston, small diameter pins weigh 59 grams or a 45% reduction over 100+ gram OEM F20/22C pins. 

Pistons

Pistons are the largest percentage of reciprocating weight and the largest opportunity to save weight.  Our piston partner was able to shrink the piston design around the 21% smaller diameter and 45% lighter C350 pins reducing piston compression height by 20% vs OEM.  From there, our piston designer used Finite Element Analysis (FEA) to mill off any material that was not supporting a critical stress.  There was a lot of opportunity under the piston crown, around the small diameter wrist pin support and behind the piston skirts. 

The URGE designs 89mm Halo pistons weigh ~240 grams vs 340-360 grams for OEM F20C and F22C 87mm pistons.  Off the shelf aftermarket pistons typically weigh 310-350 grams and competitive lightweight pistons typically come in at 260-280 grams at 89mm. 

Connecting Rods

Like piston pins, a critical choice in connecting rod design is choosing the right material.  Titanium has superior strength to weight ratio vs steel but at 3-4X the price that would limit the affordability of the URGE designs Halo stroker kits. 

Normally, an OEM or aftermarket piston weight with 9000 RPM redline at 99mm stroke or a 10,000 RPM redline at a 90.7mm stroke would require a 7/16” rod bolt to keep the bottom end together.  However, 7/16” bolt would require more material to support it increasing the bottom end rod weight.  In keeping with our optimized material choices, we choose MP35N rod bolts with 280+ KSI tensile strength and high fatigue strength.  These properties are why MP35N is used in high RPM racing supporting multiple races per rebuild like Formula 1, NASCAR, and IndyCar.  The MP35N rod bolt ultra-strength combined with ultralight pistons and pins, allow us to reliably use smaller than typical diameter bolts, further reducing rod weight. 

Out ultralight weight design and material choices have a compounding effect on the entire system.  Starting with smaller diameter piston pins, allows us to reduce piston compression height, saving weight on pistons, which allows us to save weight on the top of rod, combined with using stronger bolts allows us to save weight on the bottom of the rod. 

Our rod manufacture uses a custom 4330M alloy for their base material with ~40% higher strength vs typical 4340 alloy rods at an affordable premium.  Using FEA, our connecting rod manufacture was able to design a 4330M rod that weigh ~ 440 grams for the 99×9 Halo kit or >30% less versus OEM and ~20% less than 4340 aftermarket rods. 

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

For our 10K Halo Kit rods, we wanted to shave another ~10% weight to support the higher RPM.  Reciprocating forces increase by a factor squared with increased RPM, so we targeted a titanium like weight without the titanium cost, so we opted use our rod partners C107 super steel option.  We were able to get the 10K Halo Kit rods down to 400 grams, even with the longer rod needed with a 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 does all this optimized design add up too?  Below is a comparative chart and as you can see, the URGE Halo kit has ~30% less reciprocating weight than OEM and >20% less reciprocating weight vs other stroker kits all while increasing strength with better materials like C350 wrist pins, 4330M or C107 connecting rods and MP35N 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 considered 2 options for the Halo stroker kit, OEM Honda K24 crankshaft and custom billet crankshaft. 

Honda K series crankshaft use the same bore spacing and main bearing journal dimensions as an F20C, so they drop into a F20C bottom end.  Readily available Honda K24 crankshafts wholesale for $500 for accords to $1100 for TSXs and provide enough stroke (99mm) to dramatically improve F20 mid-range torque.  K24 crankshafts require a few hundred dollars in machine work to fit S2000 drivetrain sprocket and flywheel so the total cost would be less than $100-$1700.  The main downside of using a K series crankshaft is that the rod journal width is about 17% narrower than an F20C increasing bearing pressures and wear potentially reducing engine life. 

Lower cost custom crankshafts retail for over $2000 with some material quality concerns that have seen some failures.  Custom crankshafts using high quality sourced materials start over $3000 and can range up to $4000 with optional upgrades.  We have specified and weighed several aftermarket crankshafts and were not able to get any weight reduction vs OEM Honda K-Series crankshafts while maintaining appropriate counterweight design.  The main benefit of using a custom crankshaft would be OEM F20C width rod journals to keeping bearing pressures and wear lower. 

Based on high availability to high quality Honda OEM K24 crankshafts, $1000-$2000 savings and 2–3 month lead time of custom crankshafts, we designed our stroker kit to work with Honda K24 99mm crankshafts.  We simply reduced reciprocating weight enough, while maintaining strength to overcome the reduction in rod journal width to keep bearing pressure and wear near F20C levels.  

10K Halo Crankshaft Options

The primary option for the 10K Halo kit is the OEM AP2 90.7mm crankshaft.  However, this crankshaft has been discontinued for several years, so the primary access to this crank is salvaging or reconditioning a used OEM crankshaft. 

The secondary option is to use a Callies F22C billet crankshaft, part# S22010, which is direct replacement for OEM.  The Callies crank uses 4340 steel, is nitrided and has an aero counter weight option.  Callies F22 cranks are just above $2000 which is a great price for a billet crank as they are produced in volume and typically in stock vs made custom and one off $3000-$4000 billet cranks. 

Crankshaft Force

One way to understand the value of light weighting reciprocating engine parts is calculating peak forces on the crankshaft.  The highest force on the crankshaft during a revolution is at full tension at top dead center.  The URGE designs Halo lower reciprocating weight combined with longer rods reduce the crankshaft forces by 24-29% vs typical stroker kits.  That is over 1900 to 2300 less pounds of force on the crankshaft PER CYLINDER at 9,000 RPM!! 

Even with a 15mm longer stroke, the URGE designs 99×9 Halo Stroker kit has >1300 lbs less peak force per cylinder than the AP1 F20C with an 84 mm stroke.  That is 18% increase in stroke with 18% reduction in peak crankshaft forces vs the AP1 F20C!!  This also translates into a noticeable reduction in vibration and smoother engine. 

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 critical than crankshaft forces and engine vibration are bearing stresses.  The F20C has a wider rod bearing width (23.9mm) vs other Honda 4 cylinders providing a larger surface area to support the stress of a 9000 RPM redline.  The main limitation with using a K24 crankshaft is 19.8mm bearing width which will increase bearing pressures by 21%.  Compounded with a longer stroke, K24 crankshaft stroker kits will have 30 to 38% higher bearing pressure than AP1 F20C at the same RPM.  If you limit the redline of other K24 crankshaft stroker kits to 8000 RPM, then the bearing forces are still 5-10% higher than an AP1 F20C.  Not surprisingly, the most common failure of K series based 99mm stroker kits are due to bearing wear.  Due to the extreme light weighting of URGE designs Halo stroker kit, you can maintain the F20C 9000 RPM redline and have lower bearing pressures than the AP1 F20C despite an 18% increase in stroke and 17% reduction in bearing width from using a K-Series crankshaft.   

Comparison of peak bearing pressures for OEM AP1 and typical stroker kits vs 99×9 Halo stroker kit (Lowest is best) 

URGE designs 99×9 Halo Stroker Kit Value

At $3200, we realize the URGE designs Halo stroker kit is around $1000-$1500 more expensive vs typical 99mm stroker kits using OEM pistons or stock aftermarket pistons and 4340 rods.  However, as we have mentioned throughout this story, our part design and material selection enable significantly lighter weight and higher strength reciprocating parts.  This translates into a 24-29% reduction in crankshaft force and bearing pressures at the same RPM significantly reducing vibration, crankshaft deflection and engine stresses increasing engine durability and lifetime. 

Additionally, the URGE designs Halo stroker kit can operate 1000 RPM higher vs other stroker kits with lower vibration, crankshaft forces and bearing pressures.  This provides a ~10% or ~30 HP potential and average operating power versus lower redline stroker kits.  There is not any modification you can make to an F20C for ~$1500 that can increase NA horsepower ~10% or ~30 HP. 

When factoring in other costs such as engine removal, cylinder plating, assembly and installation, there is another $3000 to $4000 in total costs for installing a stroker kit that will typically net about 20% HP gain or about 50 HP.  Thus, the reduced engine wear, increased engine life and higher redline with an additional ~10% power potential of the Halo stroker kit for ~$1500 more provides the best horsepower value vs other performance parts.