Lemme Splain Somethin To Ya

First you must know that when we are talking about Timing were talking about Variable Valve Timing we aren't talking about simply Ignition Timing. Much like Honda's V-TEC Subaru's with VVT rely on an oil pressure actuator to shift the cam's position and engage another set of lobes effectively changing how far the valve opens and for how long. Most V-TEC/VVT application use this on only the intake cam or cams.

The tuning of the VVT system are drastically different when looking at NA cars compared to Turbo applications.

NA cars use precise amounts of Valve Overlap (This is where the exhaust valves stay open while the intake valves are begining to open. Therefore the time that the Intake and Exhaust valves are open overlaps) to make intake manifold vaccum to pull enough air and fuel into the combustion chamber to make power.

Forced induction applications need to have nearly zero overlap in their valve timing to maximize the charge air coming in from the turbo. In other words you don't want any of that prescious compressed air from the turbo to leak out of the combustion chamber because the exhaust valves are open at the same time the intake valves are allowing air in.

We all know that turbos aren't on boost at all times. Infact at low RPM levels most stock turbo cars are like badly tuned NA pigs. To aid this situation VVT can be used in conjuction with other engine management to spur low RPM power until boost kicks in. During low RPMs the cam should be set up with the right amount of overlap to make power with intake manifold vaccum. Then when the turbo is spooled the cam is shifted to engage lobes that translate into little to no overlap.

All in all the intent of Variable Valve Timing on a Turbo car isn't necessarily to make more Max Power but more to increase low end power that isn't present in turbo cars.

Brad