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What VTec basically does it allow the valves on a car to open for longer periods of time during specific intervals. So during high revs allows more intake/exhaust to enter the engine block.
SOHC engines only have VTec on the intake side while DOHC engines have it on both intake and exhaust valves.
If I remember correcty, there's a VTec solenoid right by the distributor which controls a seperate camshaft. This shaft has the same 2 intake/exhaust lobes but has a 3rd one that has a larger and longer lift than the other 2. So during the higher rpm range the 1st and 2nd valves run off the 3rd (the 1st and 2nd dont contact there lobes until the 3rd is disabled).
There are many differences between turbo and vtec. So many that they usually can't be compared to one another. Turbo can be set to an unlimited amount until your engine explodes while VTec can be set to engage/disengage at different points. The VTec system will give you the extra torque/hp without sacrificing fuel economy.
Post if you have more specific questions because there is such a broad range of answers I just can't cover them all.
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