This may be already be common knowledge, but it's a good blurp on the technical differences between the A2 and A3 heads, vtec etc.
"The K20A2 in the Type-S works the way you expect VTEC to work. The two camshafts are equipped with three cam lobes and rocker arms for each cylinder's pair of intake and exhaust valves. At 5800 rpm, oil pressure activates pins that lock the outer rocker arms to the center arm. This forces both valves to use the higher lift, longer duration center camshaft profile. However, this is augmented by VTC on the intake side, which manipulates the timing of the cam itself. This can be used to augment torque, reduce emissions or a variety of different things depending on what the computer thinks is best at the time. The RSX's K20A3, the Si's K20A and the CR-V's K24A1 use i-VTEC differently. First, it only operates on the intake valves. But even then, the philosophy is changed. Until the VTEC threshold is reached, the lesser K engines essentiality only use one intake valve per cylinder. The other is opened just a crack, enough to keep fuel from pooling behind the valve, but that's about it. In addition, the VTC is tuned primarily to keep emissions as low as possible. All this weirdness results in excellent swirl inside the combustion chamber and very efficient combustion. It's great for fuel efficiency and low emissions. However, it isn't so great for driving fun, as the engine inhales less deeply and revs lower."
A blurp on K series frankenstein motors.
"The K24A is more closely related to the K20A3 and K20A. While it uses the same i-VTEC tuning as those engines, it's the long stroke design that's intriguing. The difference is in the block. The K24's deck height is roughly 19 mm higher than its smaller siblings. It's also slightly bored, with 1 mm larger cylinders. The compression ratio is also down slightly from the non-Type-S engines, 9.6.1 vs. 9.8:1. So what? Well, the natural temptation is to throw the K20A2's efficient head onto the K24A1 block, raise the redline and have a torquey, ultra-powerful i-VTEC stroker Frankenstein monster engine.
The actual bolting on part wouldn't be too difficult, as the heads should mount right up. However, you do run into an issue with piston speed. At its 7900-rpm redline, the K20A2 in the Type-S has a piston speed of 4464 feet per minute (fpm). Thanks to its long stroke, the K24A1 comes close to that, running at 4225 fpm at its much lower redline of 6500 rpm. By the time you've spun your K24 up to just 6900 rpm, you're already at 4485 fpm, and at the 7900 rpm redline of the K20A2, you're at a crazy 5135 fpm. For comparison, even the hyperkinetic S2000 with its 9000 rpm redline doesn't exceed 5000 fpm (it maxes out at 49% fpm). And the Integra's B18C1 only reached 4573 fpm. Translation: If you're going to plunk a K20A2 head on a K24A1 block and redline the concoction to 7900 rpm without seriously building up the bottom end.. duck."
I thought you guys might be interested.
Source:
Information on the K-series engines THIS IS A MUST MUST READ - Page 2 - K20A.org .:. The K Series Source . Honda / Acura K20a k24a Engine Forum