The k20a2 and the k20a3 share the same 2.0-liters of displacement, bore/stroke, block, head, valves, and intake manifold. However, pretty much the entire internals and external bolt ons couldn't be more different.
The k20a2 has higher compression pistons, more agressive cams, dual valve springs, and a tri-Y header with larger primaries than the k20a3. If that wasn't enough, the K20a2 has a completely different - and more advanced - method of iVTEC than the a3.
On both motors, i-VTEC really consists of two features: VTC (Variable valve Timing Control) and VTEC (Valve Timing and lift Electronically Controlled).
VTC, in both cases, is simply a hydraulically controlled adjustable cam sprocket that varies intake cam timing on the fly depending on engine speed and load. VTC does not affect the exhaust cam.
However, It's the VTEC portion of these systems that differs.
On the K20A2, VTEC is old school. Like a B-series, the cam has 2 profiles on 1 shaft. At about 6000 rpm, the middle rocker stops resting on the lost motion assembly, and picks up to follow the large lobe. Nothing new.
On the other hand, the K20A3 uses a completely different method, but is still called VTEC. At low rpm, the two intake valves operate with different cam lobes. One valve gets the primary lobe (the taller lobe) and the other gets the secondary lobe (shorter). The exhaust valves always get shoved open by a single lobe with no tricks at all. When VTEC kicks in at 2300 rpm, both valves start following the primary lobe. That's it. There's no 5500 rpm kick in the seat.
Other than this use of a shorter valve at low rpm, its much like a SOHC VTEC D16Z6: VTEC only on the intake side. But even then, can you really call <that> VTEC?
All of that is enough for the A2 to boast a 40 horsepower and nearly 20 ft-lbs of torque advantage over the A3.
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