LOL ... Same discussion which we had few years back.
Actually toyota VVT-i and daihatsu DVVT is not develop by their own. The vvt founder was YAMAHA. Toyota buys the technology from YAHAMA and comply in their latest. Earlier daihatsu engine wasn't feature with VVT system. The VVT is part of the feature after toyota acquire daihatsu many yrs back. Both system is similar in mechanism and functionality.
Why vvt happen to have one side at intake ? For fuel efficient purpose , continuosly cam phase shifting need to happen at intake alone. The cam phase shifting will be advance at low revving and low load. The cam phase shifting is continuosly retard with every increment of RPM. As we know , by advance the cam angle of intake cam will help to improve pick up at low end. But vvt system is designed to advance 50 degree max at initial. The design was aimed to draw back the burnt gas into intake port and purge back to cylinder when intake open later. The burnt gas will mixed fresh mixture and burn again. As claimed , burnt gas still consist with some unburn fuel element and draw back to burn once again. With this concept , the less fresh mixture needed for every suction stroke.
Dual vvt-i is happened to see in 3sgte or Altezza engine. That is performance setup and not really fuel save setup .
In the world emission controlled is being urge to take serious act in japan and euro . V-tec system at intake exhaust is not longer apply unless vvt system use along with this system. Same goes to other variable valve timing system develop by lotus which used in Porsche and current CPS. Due to emission controlled , proton only allow to have intake VTC . They had the VTC system for both intake and exhaust during development . They can't comply them if they need to pass the emission test.
