1,000 horsepower is a common goal these days. Besides being a nice, even number, it marks the threshold of going from a standard, everyday power number to something special, needing that fourth digit to the left of the decimal. There’s something special about the mystique of cracking that four-digit barrier.
While that number isn’t anywhere near as rare as it was a decade or so ago, it is no less impressive now, especially when it’s being done with only four cylinders, 2.4 liters (146-ish cubic inches) of displacement, and a single 72mm turbocharger, in an engine designed for legitimate street use. Enter 4 Piston Racing‘s Honda K24 build.
“We’ve been building the K24 engines for over a decade,” says Luke Wilson, one of the founders of 4 Piston Racing. “We really got into the naturally aspirated power, but most of our customer base is into turbocharged power. We’ve put a lot of engine development and thought processes into how to make the power so that people can enjoy it how they want to.”
The Short-Block That Could
This particular engine was designed to do a variety of things well. It’s not a dedicated drag race engine, but rather one that will live at elevated power levels wherever you happen to be driving the car, whether it’s to the grocery store or at the racetrack. “The idea is you can turn it down for a variety of uses, and then if you want to go to the track and turn it up, all you have to do is hit a couple of keys, turn a knob or hit a button and you’re making big power,” says Wilson.
As a base for the engine, the team at 4 Piston uses a very specific core – the K24 out of an Acura TSX. Once stripped down, the aluminum block is machined for aftermarket sleeves from Ramey Racing. The ductile iron sleeves have an extremely thick locating flange with water passages built in to create an extremely strong closed-deck engine block, with greatly reduced bore movement and distortion under extended hard use.
Surprisingly, the OEM forged crankshaft with the factory 99mm (3.898-inch) stroke is up to the task, and is used in the build, after having the oil holes chamfered and a high-pressure cleaning of the passages. Connected to the crank are a set of Wiseco BoostLine steel connecting rods, which are built to not only take the boost – as the name implies – but to do so with the least amount of added weight to the rod as possible.
“We used a big, heavy-duty BoostLine rod,” says Wilson. “It’s a steel rod that we can use on the street in daily use; it’s not like an aluminum rod setup. You can cold start [the engine] and go driving around and it can also take the big boost.”
Attached to the BoostLine rod are a set of custom-designed 87mm (3.425-inch) Wiseco pistons which have been specifically designed for heavy-duty boosted use. “It has a thick skirt with a big ring pack, and a thick crown,” Wilson explains. “Then, it uses a stout tool steel wrist pin with extra thickness in the pin boss area in it to make sure we can run long periods of high load with this thing at big boost levels.”
To keep everything running smoothly, 4 Piston Racing uses ACL Race Bearings set to their exacting main and rod bearing clearance specifications. While the OEM Type-S oil pump is said to be good up to 9,000 rpm, that’s not good enough, and the team at 4 Piston Racing installed their ported version, which they advertise as cavitation-free up to 11,000 rpm.
The team machines a groove in the deck surface for a copper-wire O-ring along with installing a set of ARP L-19 head studs, in order to handle the cylinder pressure, in addition to running a Cometic head gasket built specifically to 4P’s specifications to help seal the head to the block under intense cylinder pressures encountered by running 30-plus pounds of boost.
The TSX Top End
Matching the TSX block, 4P uses a OEM TSX K24 cylinder head casting as a base on which to perform their CNC port work. The K24A casting has the smallest as-cast port and not much room to work with, but they have built a porting program to not only improve flow and charge velocity, but leave enough wall-thickness to hold up to serious boost pressure.
Right off the bat, Wilson was clear that this cylinder head porting isn’t some monster-port designed only to make peak numbers. “It’s not a big giant drag race port in it,” he says. “It’s a port sized specifically for these turbo combinations so that the engine is responsive. On gear changes they come back into boost really fast. It keeps everything in the efficiency range of this turbo.”
To maintain an awesome surface finish within the ports, 4P employs what they call “slow-cut precision machining” which focuses on quality above speed. The new intake port design adds approximately 20-percent more flow, according to the flow bench, with final numbers in the 340 cfm range.
In the chamber, new valve seats are installed and finished with a 45-degree valve job for the 1mm oversize intake valves. Ferrea Super Alloy valves are utilized to be able to handle the extreme heat encountered in the turbocharged application, with 36mm (1.417-inch) intake and 30mm (1.181-inch) exhaust valves used. To control those valves, Ferrea valvesprings are used, combined with titanium retainers to be able to handle the 9,000-plus rpm the engine will turn.
On top of the head, 4P used their billet tool steel Outlaw Turbo iVTEC camshafts which feature three lobes and adjustable gears. “The engine retains VTEC capability so that it can idle and will behave like stock on the street,” says Wilson. “It also has variable intake cam timing, so that we can control the intake centerline angle from 104 degrees to 132 degrees.”
The bell of the ball, and the source of boost, is a Precision Turbo PT 7285 CEA turbocharger. The 72mm CEA compressor wheel is machined from a 2618 aluminum forging, and an air-cooled dual ceramic ball bearing center housing offers extremely high efficiency and fast transient response. “It’s a big turbo, but, this engine is a big displacement engine. It can handle it,” Wilson says.
The turbo is mounted to a Speed Factory turbo manifold, and the compressed air travels through a Speed Factory air-to-air intercooler, into a Skunk2 5.5-liter plenum intake manifold. “This combo produces big horsepower at low boost. Even on just the wastegate spring, it’s making well over 700 horsepower. At 30 psi we’re right at about 1,000 horsepower, and this turbo can make 55-60 pounds of boost on this setup,” claims Wilson.
However, the proof is in the dyno numbers, and at a little over 30 pounds of boost, this engine produced 1,038 horsepower at 9,760 rpm. Not bad at all for a 2.4-liter engine designed to be driven on both the street and the track.