Exhaust Valve Opening (EVO) usually occurs 50 to 60 before BDC and is a tradeoff between pushing the exhaust gases out in time and extracting the work from the burning gases. For example, with an early Lancer Evolution, the high-pressure exhaust gases take advantage of blow down and then have extra time to rush out of the exhaust valve and equalize with the exhaust back pressure. Thus, when the piston rises back up from BDC, it has to do less work to push the remaining burned gases out. However, reducing the pumping work comes at the expense of extracting work from the burned gases. Exhaust valve closing typically falls between 8 to 20 after TDC. It should occur after TDC so that cylinder pressures do not rise and prevent fresh mixture from coming in. A late EVC favors high power since it ensures a large valve overlap (both valves open at the same time) allowing as much burned gases to exhaust while bringing in fresh air and fuel.
Following the preceding discussion, there is no one set valve timing that is right for an engine. Since engine loads and speeds are constantly changing, valve timing requirements also change. This is why race cars that are tuned for peak power and constant high rpm operation typically struggle at lower engine speeds.
Being a newer engine, the VQ35DE has the additional benefit of using Nissan's Continuously Variable Valve Timing Control System (CVTCS), which will help mitigate the "short circuiting" at low rpm. With CVTCS available on both the intake and exhaust camshafts, continuously changing the valve event phasing during engine operations will allow us to optimize the VQ35DE over a large operating range, increasing the area under the horsepower curve. Valve event phasing will also allow us to compensate for lowrpm overlap, valve to valve, and valve to piston clearance issues that come with using high lift and duration cams.
Putting It All Together
Designing a cam that meets all the above conditions is the entire challenge in cam design. Cams with high lift provide more flow, but have low rpm limitations, more wear and inertial loads as well as potential valve contact issues. Using longer duration valve events requires cam profiles that maximize time at peak lift. These aggressive ramp profiles have more severe inertial loads as well as valve contact issues. Add on top the possibility that cam phasing can potentially be tuned so that the valve might hit the piston during overlap and you have a very complex problem.
Since Cosworth didn't build the motor for just the Castrol Syntec Top Shop Challenge, several different cams were used during development. Ultimately, this motor will be destined for use in the European motorsport market. Cosworth engineers tested a few different profiles and intake and exhaust cam combinations with higher lift and longer durations. Their verdict was that non-staggered intake and exhaust cams with near identical durations worked best for this engine. Our cams, not shown on the dyno and cam graphs will be a prototype set with 300 degrees of duration and a peak lift of 12.0mm on the intake side and 296 degrees and 12.0mm peak lift on the exhaust side. Almost 50 degrees more duration than the stock 256 degree intake and exhaust cams and up to 3mm more peak lift.
Other Valvetrain Considerations
Aggressive cams typically require rpm to work better. Wringing out every last bit of horsepower from the VQ35DE will require spinning the motor to higher rpms (up to about 10,500 rpm). As a consequence of running more valve lift and duration, compounded by a higher rpm ceiling, valvetrain stresses and dynamics will change. This is why the Cosworth CNC head also has lighter weight titanium retainers that reduce inertial loads and improve dynamic response of the valve train. Stiffer dual valve springs are also necessary to prevent valve float at the engine speeds that the original valve train was not designed to operate at.