The 500 Horsepower Naturally Aspirated K24 Engine by 4Piston

May 2, 2017 / by Jeff Smith

We take an exclusive look inside 4Piston Racing's 500+ horsepower naturally aspirated K24 engine build.

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

HP = Torque x RPM / 5252

To learn more about Wiseco's Honda and Acura piston kits, click here.

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.

Starting with a production K24 block, 4Piston installed a set of LA Sleeve ductile iron liners to push the bore out to 90mm (3.54-inch) over the stock 87mm (3.42-inch).

Luke Wilson and his partner Josh Klein are true engine builders. They’ve been cranking out 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 few true believers in normal aspiration. 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 hurtles – the high water marks of 500 hp and 10,000 rpm. Wilson and his gang at 4Piston have been building the K24 Honda 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 turn-key package.

Wilson chose GRP’s aluminum, 6.125-inch long rods to connect to the Wiseco forged aluminum pistons. For this normally 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 long-time 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. But the ultimate goal was a horsepower number that started with the number five.

While the production K24 offers decent displacement at 2.4L, Wilson and Klein know that more is usually better, so they jacked in a set of LA 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-inch) to produce 2,687 cc’s that we can round off to 2.7L.

(Left) The pistons are a Wiseco 2618 alloy forgings, with a 90mm bore and only 2 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. (Right) Checking bearing clearances is essential with any race engine and imperative that the speeds this engine will see. 4Piston prefers ACL tri-metal bearings for their 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 weighs 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 2-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 (1.06 mm) top and 0.143-inch (3.63 mm) oil ring package. “We have run an 0.025 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.” 

(Left) There are those who say that Winberg sets the standard for quality in the race crankshaft business. 4Piston elected to bump the stroke up to 106mm (3.90-inch) for this engine. (Right) Most ring manufacturers insist on a ring expander to install, especially when the rings in the 1.0 mm 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 wrist pin at 10,000 rpm. That’s why the wrist pin is supplied with a DLC (diamond-like coating) to withstand these excessive loads.

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.

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 their current crank down to 33 pounds. They have 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.

(Left) 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-inchCometic head gasket. (Center) 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 valve spring kit. The valvetrain consists of aluminum Ferrea rockers that are half the weight of factory steel versions. (Right) 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. As near engine porn, there are countries where it may not be legal to publish this photo!

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 over 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. We’ve included the complete numbers in an accompanying chart, but anytime you can move 435 cfm past a couple of titanium valves that are only 1.510-inch in diameter, you have arrived.

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. 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.

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 for its dual overhead cam action. Again, he’s hesitant to delve much deeper than the 0.678/0.615-inch intake and exhaust lift numbers that the cams deliver.

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.

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.

With the valves and springs in place with installed heights all equalized, the Ferrea rockers slide right in place.

All of this came to its ultimate test on the dyno recently. As you can see from the accompanying dyno sheet and graph, 4Piston easily eclipsed their 500 hp goal with a peak of 519 hp at 9,400. 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 N.A. engines are extremely peaky, this reveals a very impressive power band. That’s what makes for a quick ride.

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

Wrapping up the install included adding the Daily dry sump, Moroso oil pan, and the ATI balancer. Myers Competition supplied the stepped headers that end up measuring 2.250-inch on the primary tubes. Motec is responsible for exerting the fuel and spark control. Induction duties fell to Kinsler with their 71.5mm throttle bodies and 1600 cc/min injectors (152 lbs/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.

By the Numbers

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.

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%

 Power Curve

RPM

TQ

HP

4,800

192

167

5,000

217

206

5,200

222

220

5,400

232

239

5,600

248

260

5,800

248

260

6,000

246

281

6,200

243

287

6,400

253

308

6,600

277

348

6,800

287

372

7,000

285

381

7,200

280

384

7,400

285

402

7,600

283

409

7,800

284

422

8,000

291

443

8,200

293

458

8,400

289

462

8,600

297

487

8,800

296

496

9,000

294

503

9,200

288

505

9,400

289

518

9,600

281

514

9,800

274

512

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 power band. Wilson commented 'That is a big header, and the engine can make a broader power band with a smaller header and an additional inch or so with a little 2.5-inch collector. For our application, we are pulling power out in 1st gear, then the rest of the track is 8,300-10,600 rpm. We are okay giving up some low end in favor of carrying the top another hundred or more rpm.'

Up Close

Honda K24A1 from a CRV
Production block with 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-inch long, 340 grams
Wiseco 90mm bore, 265 grams, 2 ring billet pistons with hard anodizing
Top Ring:  0.043-inch, Oil Ring: 0.143-inch
DLC coated wrist pin
ACL Race Bearings
4 Piston Peacemaker cylinder head
Ferrea 1.510” Titanium intake valves 
Ferrea 1.181” Titanium exhaust valves
4 Piston PSI Pro Stock single valve spring kit
Ferrea valve locks
Ferrea 4P RR8000 rocker arms
Skunk2 BMFx custom Camshafts
Dailey Engineering dry sump
Kinsler 71.5mm (2.81-inches) individual throttle bores
Myers Competition header, 2.00-2.125-2.250-inch, 18-inch primaries
Moroso oil pan
ECU is a Motec M130
VP M5 methanol fuel 

Topics: SPORT COMPACT, ENGINE BUILDS, featured, ENGINE TECH

Related posts

Jeff Smith

Written by Jeff Smith

NEVER MISS A POST