variable valve timing

The variable valve timing used the best of two camshaft profiles. Some of the most common abbreviations for this are the Honda vtec and Toyota vvt-i valvetrain. Although I believe the Acura nsx was one of the first to use it. Other systems are now being used by all manufacturers. Some systems actually use two separate camshafts to benefit from low end torque, good emissions, and top end power with the other cam. The vtec uses a second lobe on the same cam with another profile. Others use a wide cam lobe with a different profile on each side of it. They then move the cam side to side. The problem with using one cam is compromise. Most single or dual overhead cam vehicles are tuned for mid range power and low emissions. This means reasonable lift numbers, but not optimal. Lift is simply how high the cam lifts the valve into the head chamber, to let air pass through it. The overlap is the amount of time that the intake and exhaust valves are opened into the chamber at the same time.

 

The longer the overlap, within reason, the more top end power you can make. This is what gives muscle cars the (harley) lumpy idle sound. As the exhaust is still open it creates a vacuum and helps pull fresh air into the cylinder through the intake valve. The problem with this is that it runs poorly at idle, makes less torque and would not pass emissions tests. By being able to switch between cams, you can use the conservative cam cruising down the road getting 30+ miles per gallon. Then as the engine reaches a certain rpm it engages the other cam lobe which allows alot more air to pass through and makes maximum power when you need it. This way you have a daily driver that has the power of a high performance tuned car. One other system just controls the timing of the cam opening by rotating the cam at higher speeds. This dosen’t affect the lift but is simple and provides power gains.Another type of variable valve technology, is one that uses hydraulics and sometimes electronics. They essentially use a lifter that can be pumped up and shortened to act like a different height cam. At low speeds it does not pump up and uses the stock profile. At higher rpm’s the lifters pump up and release to act like a taller camshaft. This produces more lift at the valve. This is a look at the future of valvetrains as it is not yet easy to do.