The newest of new hybrid Honda motors mates a 2.4-liter bottom end from an Element, current-gen CR-V or Accord, with the cylinder head that sits atop the K20A2, the plant found in the RSX Type-S. Some time has passed since we first addressed this combo in our October '04 issue, and new tricks for maximizing power and speeding up the build have been discovered since.
First, some background (probably a rehash for many of you). Honda uses two different VTEC systems on its K-series powerplants and both happen to be called i-VTEC. The first setup, built into the K20A2 (RSX Type-S) and K24A2 (TSX), has three cam lobes for each two valves and looks like the VTEC most of us are familiar with.
The second configuration, found on the K24 (Element, CR-V, Accord) and the K20A3 (base RSX, Civic Si), is designed to minimize emissions and maximize driveability and gas mileage. In this system, there are two cam lobes, a normal one and a dinky one, for each pair of intake valves (nothing happens to the exhaust side). We want the real deal VTEC, so the A2 heads are it for us.
Honda engineered the additional 400cc into the K24 by both enlarging bore from 86mm to 87mm and stroke from 86mm to 99mm. The increase in stroke adds 19.7mm to deck height, which could become a problem when it's time to shut the hood. The added height also means the exhaust manifold shifts up 19.7mm, so prepare to make adjustments if you look to do something similar.
Interestingly, the 2.4 in the TSX (the K24A2) is something of a compromise between the two motors. It features a 10.5:1 compression ratio, vs. the 9.6:1 for the CR-V, 9.7:1 for the Element and Accord, and 11:1 for the RSX-S. It's also got a 7100-rpm redline, and Acura says it makes 200 hp. The higher revving engine also has stronger rods and crank and seems to have basically the same head we're swapping here. This begs the question, why not just transplant a complete TSX engine?
First of all, they're hard to find and not cheap. Once you source an engine, you have to deal with its throttle-by-wire and an ECU that's not compatible with other K-series vehicles. To address these compatibility issues you'll need to install an RSX-S or Civic Si intake manifold and throttle body. You'll also have to find or make a plug for the EGR port on the TSX head, since it'll be exposed with the new intake manifold.
Mike LaPier at Autowave in Huntington Beach, Calif., is building our Franken-motor. The owner of the powerplant initially brought the 2.4-liter to him because it was knocking severely, but after pulling off the head and removing the problem, a small piece of slag in the chamber that did little damage, LaPier convinced the owner to build something bigger and better.
The block was sent to Benson's Machine in Santa Ana where it was prepped and re-sleeved to 0.020 over. Pauter rods were sourced for the project, as well as 11.5:1 CP pistons, the same compression ratio that a JDM Integra Type-R boasts. LaPier will use a TSX crankshaft and re-use other parts from the K24 (the engine was practically new when he tore it down). Once complete, the motor is headed for the engine bay of a 2002 Civic sedan.
One last thing about putting together the bottom end: the K20A2 comes with an oil cooler while the K24 does not. There are three popular ways to remedy this. Some simply don't run the cooler, which seems foolish given that Honda thought it was a good idea for the K20A2. The next option is to plumb the oil to an outboard oil cooler, probably the easier of the two oil cooler methods. The last choice is to have the K24 block machined for the coolant return line, which is right above the oil filter boss on the K20A2. You will need to use the K20A2 water pump since it has the coolant supply fitting for the oil cooler.
The cylinder head gets fresh camshafts from IPS, a set of Eibach's new valve springs, and an ITR intake manifold. LaPier cobbles it together, then tunes and dynos the hybrid beast.