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.
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.
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.
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.
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.
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.”
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.
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.
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.
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.
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.
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.
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.
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.
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
Ferrea Racing Components
GRP Connecting Rods
Kinsler Fuel Injection