2006 R18A
3-stage VTEC
Examine the SOHC VTEC and SOHC VTEC-E implementations. The clever Honda engineers saw that it is a logical step to merge the two implementations into one. This is in essence the 3-stage VTEC implementation. 3-stage VTEC is implemented on the R18A 1.8 SOHC engine in which the VTEC-E mechanism is combined with the power VTEC mechanism.
Many of us probably has laughed at the poor ignorant layman who said "I want power AND economy from my Honda". We know of course that power and economy are mutually exclusive implementations. Honda decided not to abide by this rule. Now, with 3-stage VTEC, we get BOTH power and economy !.
The diagram below illustrates the 3-stage VTEC implementation. The intake rocker arms have two VTEC pin actuation mechanisms. The VTEC-E actuation assembly is located above the camshaft while the VTEC (power) actuation assembly is the standard wild-cam lobe and rocker assembly.


Below 3500rpm and with gentle accelerator pressure, neither pin gets actuated. The engine operates in 12V mode with very good fuel combustion efficiency. When the right foot gets more urgent and/or above 3500rpm, the upper pin gets actuated. This is the VTEC-E mechanism at work and the engine effectively enters into the '2nd stage'. Now R18A 3-stage works in 16V mode (both intake valves works from the same mild cam-lobe).
Stage 2 operates from around 3500rpm to 5200rpm. When the rpm exceeds 5200rpm, the VTEC mechanism activates the wild cam-lobe pushing the engine into the '3rd stage', the power stage. Now the engine gives us the full benefit of its 140ps potential !
The 3-stage I-VTEC R18A engine is used on the current 8th generation Civic. The 3500rpm cutover from lean-burn to normal 16V operation in fact varies according to load and driver requirements. With gentle driving, lean-burn can operate up to 3500rpm or higher. The essence of 3-stage I-VTEC is power AND economy implemented on a 1.8l SOHC PGM-Fi engine. Many people mistake 3-stage I-VTEC as a "superior" evolution of the power oriented DOHC VTEC implementation, describing DOHC VTEC as "the older 2-stage VTEC" and implying an inferior relationship. This is totally wrong because DOHC VTEC is tuned purely for high specific output and sports/racing requirements. 3-stage I-VTEC is in truth an evolution of SOHC VTEC and VTEC-E, merging the two implementations into one.