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518 HP 4Piston Racing Drag-Spec K24

Joining the 500 Club, the naturally aspirated way

Jeff Smith
Apr 18, 2017
Photographer: Mark Gearhart

The equation for horsepower is simple: torque x rpm / 5252. The plan is simple in concept, but demands the best when it comes to execution.

A friend calls that the equation for jungle love and he’s probably right. Small displacement, four-valve motors thrive on this equation because if you can maintain the torque while spinning the engine faster, horsepower will follow in satisfying denominations. But the price for spinning engines hard is steep both monetarily and in a commitment to building the best.

Luke Wilson and his partner, Josh Klein, are true engine builders. They’ve been cranking out Honda B- and K-series jewels for decades, and they know what it takes to make these babies sing. While much of the world’s attention seems to have dropped lately almost entirely on the doorstep of the turbo scene, there are still a number of true believers in naturally aspirated power. Rather than dial up more boost, the purist approach, if you will, has always been to seek out new ways of making more power without help from spinning pinwheels.

JE6 0718 Photo 2/20   |   Starting with a production K24 block, 4Piston installed a set of L.A. Sleeve ductile iron liners to push the bore out to 90 mm (3.54 inches) over the stock 87 mm (3.42 inches).

There have always been two big hurdles—the high watermarks of 500 hp and 10,000 rpm. Wilson and his gang at 4Piston have been building the K24 engine platform for multiple years, aiming at that goal. 4Piston actually offers a K440 engine, a 2.5L drag race engine at 15:1 compression on alcohol that can push the wheel horsepower numbers above 400 as a turnkey package.

JE6 0698 Photo 3/20   |   Wilson chose GRP’s aluminum, 6.125-inch-long rods to connect to the Wiseco forged aluminum pistons. For this naturally aspirated, high-rev application, lighter rods are essential.

With that in mind, 4Piston decided to build an enhanced version of that package to eclipse the 500 mark at the flywheel. This would come to fruition with help from another longtime member of the team, Matt Monday. The plan had to revolve around a production Honda K24 block, cylinder head, and 16:1 compression just to push it a little further.

JE6 0704 Photo 4/20   |   The pistons are Wiseco 2618 alloy forgings with a 90mm bore and only two-ring grooves with ArmorPlating hard-anodized coating on top along with Wiseco’s Armorglide skirt coating to reduce friction. Also note the vertically drilled gas ports that load the top ring.

While the production K24 offers decent displacement at 2.4L, Wilson and Klein knew they’d need more, so they jacked in a set of L.A. Sleeve ductile iron liners to push the bore to 90 mm (3.54 inches) and matched that with a custom Winberg crank that upped the stroke to 106 mm (4.173 inches) to produce 2,687 cc¬—we’ll just round that off to 2.7L.

JE6 0707 Photo 5/20   |   Checking bearing clearances is essential with any race engine and imperative at the speeds this engine will see. 4Piston prefers ACL tri-metal bearings for its K24 stroker.

It’s not a huge leap to understand that weight is everything when your goal is stratospheric rpm. Starting with the piston and rod combination, Wilson went with a set of 2618 forged Wiseco pistons that weigh a scant 265 grams. For a class-specific drag race engine, anything you can do to reduce piston friction pays off in cheap horsepower, so with help from Wiseco, they decided on a two-ring piston package that eliminates the ring in between. This places extra load on the oil ring to keep the cylinder wall clean of excess oil, but also reduces ring drag by a considerable margin.

JE6 0711 Photo 6/20   |   There are those who say Winberg sets the standard for quality in the race crankshaft business and 4Piston would agree, electing to bump up the stroke to 106 mm (3.90 inches) for this engine.

Wilson also offered some interesting comments on the 0.042-inch (1.06mm) top and 0.143-inch (3.63mm) oil ring package. “We have run an 0.025-inch top ring and it can make power, but it needs frequent service. We never put them in customer engines because they don’t rebuild frequently enough and the engines will degrade and in short time; they will make more power with the bigger ring. Also, a lot of guys are running aggressive fuels now that are a lot harder on rings—and everything else—than pure methanol.”

JE6 0709 Photo 7/20   |   Most ring manufacturers insist on a ring expander to install, especially with rings in the 1.0mm-or-thinner range.

The pistons are also coated with Wiseco’s Armorglide skirt coating to further the hunt for less drag, and after the valve reliefs and gas ports were finalized, Wiseco also applied the hard anodized ArmorCoat to the combustion surface. But an equally important aspect of the piston design must address the forces acting on the piston at extreme engine speeds. Even with a mere 265-gram piston, that still means a minimum of 1,000 pounds yanking on the wristpin at 10,000 rpm. That’s why the wristpin is supplied with a DLC (diamond-like coating) to withstand these excessive loads.

JE6 0723 Photo 8/20   |   Thin rings make installing the piston and rod assembly a bit challenging, but it’s easier with a Wiseco tapered ring compressor.

That calculation also takes into account the extra 0.270-inch greater distance the piston must travel when Wilson added the 4.170-inch (106mm) stroker crank. Winberg has a reputation for proper clocking of the counterweights to compensate for these monstrous forces even on a typical single-plane crankshaft. Starting with a 4330V aerospace forging with generous fillet radii, the crank is then nitrided and hardened to maximize the surface strength and improve the bearing life.

JE6 0734 Photo 9/20   |   Initial torque is backed up by checking the rod bolt stretch to ensure a proper clamp load. High rpm tries to pull the cap right off the rod, so an optimized clamping load is critical.

Weight is everything with high-rpm engines, and Wilson offered that a standard billet crank for these engines generally weighs around 39 pounds. But with the lightweight reciprocating pieces, 4Piston prefers to pull some mass out of this rotating package and have milled and drilled its current crank down to 33 pounds. The company has experience with a customer engine that is currently supercharged at 725 hp with one of these cranks and it is living just fine. It’s about living near the edge without falling off.

JE6 0741.JPG Photo 10/20   |   Measuring deck height is another crucial blueprinting spec, so 4Piston uses its own twin-indicator stand to finalize the measurement. Aluminum rods require more clearance to accommodate for the increased linear expansion. 4Piston generates a 0.060-inch piston-to-head clearance using a 0.020-inch deck and a 0.040-inch Cometic head gasket.

If the rotating assembly is all about strength and durability at these engine speeds, then the cylinder head has to be about airflow in order to meet the demands of filling the cylinder more than 83 times per second. Wilson and 4Piston have done all of their own cylinder head development and based on their 20 years of experience, have created some impressive flow numbers (see chart below). Anytime you can move 435 cfm past a couple of titanium valves that are only 1.510 inches in diameter, you have arrived.

Flow* - Intake - Exh. - I/E
0.100 - 83 - 32 - 38%
0.150 - 128 - 97 - 76%
0.200 - 167 - 136 - 81%
0.250 - 207 - 172 - 83%
0.300 - 247 - 202 - 82%
0.350 - 287 - 219 - 76%
0.400 - 324 - 231 - 71%
0.450 - 357 - 239 - 67%
0.500 - 383 - 245 - 64%
0.550 - 403 - 250 - 62%
0.600 - 417 - 252 - 60%
0.650 - 427 - 255 - 60%
0.700 - 435 - 257 - 59%

*These numbers were generated using 28 inches of water test depression with a 90mm test cylinder. The final column represents the exhaust-to-intake-flow relationship between the intake and exhaust side of the chamber. A low I/E percentage often indicates impressive intake port flow numbers.

Wilson is hesitant to delve much deeper into how they’ve achieved these numbers when it comes to seat angles and cross-sectional areas on the intake or exhaust ports. If you had invested as much time into this project, you’d be hesitant as well. But access to this flow for your heads is only a phone call away.

JE6 0757 Photo 11/20   |   Now we get to the fun stuff. The CNC-ported head is filled with Ferrea titanium 1.51-/1.181-inch valves and a 4Piston Pro Stock single valvespring kit. The valvetrain consists of aluminum Ferrea rockers that are half the weight of factory steel versions.

The valvetrain must also be durable, resistant to deflection, and capable of accelerating those valves at these stratospheric rpm numbers. Like the Ferrea titanium valves, 4Piston has chosen that company for the cam followers as well. For the camshaft, Wilson tapped Skunk2 and, again, is hesitant to delve much deeper than the 0.678/0.615-inch intake and exhaust lift numbers the cams deliver.

JE6 0746 Photo 12/20   |   The CNC porting pushes some truly impressive flow numbers of 400-plus cfm out of this production K24 casting. The port work is near jewel-like.

Much like the cylinder head development, part of this engine’s secret to power lies in the combination of compression—16:1—melded with engine-specific intake opening points that are closely dictated by such physical properties as piston-to-valve clearance. Much of what happens in the opening and closing points are dictated by that eternal real estate squabble between the pistons and the valves. It’s important that they never meet.

JE6 0751 Photo 13/20   |   It’s a straight shot from the intake right past the valves to the chamber, which is why the flow numbers are so good.

All of this came to its ultimate test on the dyno recently. 4Piston easily eclipsed its 500hp goal with a peak of 519 hp at 9,400 rpm. Peak torque chimed in with 297 lb-ft at 8,600 rpm, but perhaps more importantly, this little spinner managed more than 280 lb-ft of torque across a span from 6,800 to 9,600 rpm—nearly 3,000 rpm. In a world where high-horsepower NA engines are extremely peaky, this reveals a very impressive powerband. That’s what makes for a quick ride.

JE6 0779 Photo 14/20   |   Those are Skunk2 BMFx cams about to work their magic. Wilson won’t comment on duration or the lobe-separation angle, but he did tell us the valve lift is 0.678 inch while the exhaust valves open to 0.615 inch.

So Luke Wilson and his guys at 4Piston have proved that with attention to detail and the right parts, there are opportunities out there for naturally aspirated engines. It’s all about airflow and rpm.

JE6 0783 Photo 15/20   |   With the cams slid in place, Intake closing and overlap are essential specs, but the engine builder also has to be aware of valve-to-piston clearance. A minimum valve-to-piston clearance for a high-rpm engine is essential to maintain compression. The intake clears by a scant 0.030 inch while the exhaust is a touch wider at 0.055 inch.
JE6 0790 Photo 16/20   |   Final assembly includes a set of ARP L19 head studs and a Cometic 90.5mm MLS head gasket to ensure a proper seal. Cometic makes these gaskets specifically for 4Piston Racing.
JE6 0819 Photo 17/20   |   With the valves and springs in place with installed heights all equalized, the Ferrea rockers slide right in place.
JE6 0899 Photo 18/20   |   Induction duties fell to Kinsler with its 71.5mm throttle bodies and 1,600cc/min injectors (152 lb/hr). The large injectors are necessary for the VP M5 methanol fuel. 4Piston experimented with inlet tubing length but essentially the shortest will make the best peak horsepower.
JE6 0875 Photo 19/20   |   Wrapping up the install included adding the Dailey dry sump, Moroso oil pan, and the ATI balancer. Myers Competition supplied the stepped headers that end up measuring 2.250 inches on the primary tubes. Motec is responsible for exerting the fuel and spark control.
K24 dyno graph with power curve Photo 20/20   |   To truly appreciate this power curve, take a look at that nice flat torque curve where this motor delivers no less than 280 lb-ft of torque from 6,800 rpm to horsepower peak at 9,600. That’s an impressive 2,800-rpm powerband. Wilson commented, “That is a big header, and the engine can make a broader powerband with a smaller header and an additional inch or so with a little 2.5” collector. For our application, we are pulling power out in first gear, then the rest of the track is 8,300-10,600 rpm. We are OK giving up some low end in favor of carrying the top another hundred or more rpm.”

UP CLOSE
CRV K24A1; LA Sleeve Ductile Iron liners bore - 90mm = 3.543 inches/stroke 106mm/4.173, 164ci, 2687cc or 2.7L; Winberg crankshaft; GRP splayed beam aluminum connecting rod, 6.125 inches long, 340 grams; Wiseco 90mm bore, 265 grams, two-ring billet pistons with hard anodizing, top ring: 0.043-inch, oil ring: 0.143-inch, DLC coated wristpin; ACL Race Bearings; 4Piston Peacemaker cylinder head; Ferrea 1.510” titanium intake valves, 1.181” Titanium exhaust valves, valve locks; 4P RR8000 rocker arms; 4Piston PSI Pro Stock single valvespring kit; Skunk2 BMFx custom cams; Dailey Engineering dry sump; Kinsler 71.5mm (2.81 inches) ITBs; Myers Competition header, 2.00-2.125-2.250-inch, 18-inch primaries; Moroso oil pan; Motec M130; VP M5 methanol fuel

CONNECT
4Piston Racing 4pistonracing.com
Cometic Gaskets cometic.com
Dailey Engineering daileyengineering.com
Ferrea Racing Components ferrea.com
GRP Connecting Rods grpconrods.com
Kinsler Fuel Injection kinsler.com
L.A. Sleeve lasleeve.com
MoTeC Systems motec.com
Skunk2 Racing skunk2.com
Winberg Crankshafts winbergcrankshafts.com
Wiseco Pistons wiseco.com

By Jeff Smith
1 Articles

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