Since we won’t be seeing a camless engine in the average consumer’s hands anytime soon, we will have to continue adapting the age old camshaft to keep up with our ever-increasing horsepower needs.
Today’s various dual overhead cam (DOHC) valvetrain technology encompasses several different types of variable valve timing (VVT) systems, each displaying a different strategy of altering the timing of the valve events, with the complexity and dynamics of each varying based on the engine platform and its intended application.
In general, modern variable valve timing can adjust the volumetric efficiency of your engine by RPM and load. The most basic form of VVT used today has an RPM crossover point to hydraulically switch between two or three static timing profiles that advance or retard a multi-valve system, and the most advanced using continuous variable valve timing (CVVT) while also incorporating variable valve lift — allowing for independent manipulation of the intake and exhaust valve event timing, in addition to variable cam lift and duration, available at any RPM and load.
To a varying degree, this advanced level of valve timing control allows for precise adjustments that help shape the powerband in a way that has replaced the need to swap cams as often, and achieve the same power goals with a less aggressive cam grind. This system is also used by car manufacturers to improve drivability, emissions, and fuel economy in the idle and cruise cells (as an EGR component) while also improving mid range and top end performance.
With factory turbocharged DOHC applications becoming the norm, and entry level VVT technology improving leaps and bounds in the last decade, the relationship between variable valve timing and turbo performance is definitely one of great joy (or migraines) for many tuners.
There is a common misconception that all turbo power plants need as little valve overlap as possible. This still holds true with older turbochargers and to an extent with smaller fast spooling turbos, this is due to the smaller and less efficient turbine housing causing the exhaust gases to stack up against at the turbine housing, causing exhaust reversion at full and part throttle and reducing performance. But, with newer turbochargers that are more efficient and have bigger turbine housings, it is possible to tune VVT in a similar way as you would a naturally aspirated engine.
This effect will vary based on your engine platform and turbocharger efficiency, but we will be using Mitsubishi’s latest cam phasing MIVEC (CVVT) iteration found in the 2.0-liter turbo 4B11T engine as an example. With MIVEC, we are able to independently control the intake and exhaust valve events (this does not affect lift or duration), giving the tuner control of overlap, advance, and retard at any RPM and load. MIVEC allows for the intake cam to be adjusted anywhere between 10 degree minimum advance to a maximum of 35 degrees, while the exhaust side can be phased to anywhere between 0 and -35 degrees of retard angle (limited mechanically).
In 2009, Bryan Medway (formerly of GST Motorsports), a well known Mitsubishi ECU calibrator from the Bay Area of San Francisco, was one of the first to begin openly testing the new MIVEC system using various amounts of overlap and openly posting the results online. He tuned a completely stock Evo X and had a little extra time to test his recalibrated MIVEC maps (VVT settings) directly against the factory values. Medway took his recalibrated tune and pasted the factory MIVEC maps back into it and reflashed the ECU, he then compared the dyno results to his recalibrated VVT tables and then posted the results on several online forums.
From the factory, Mitsubishi seems to focus overlap during mid to high RPM cruise and light throttle cells, with its main function as an internal EGR component, and then fully advancing the exhaust cam at moderate loads and anywhere beyond that.
Compared to stock, Medway’s revised VVT maps retarded the intake cam slightly in low load cruise cells and was left about the same as stock in higher load cruise areas. In mid to high load spool up cells from 2,750 RPM to 4,500 RPM, the intake was heavily advanced, before retarding back to the floor by redline. The exhaust cam was retarded aggressively in cruise cells and quickly advanced as load and RPM increased.
The overlap produced during cruise acts as an internal EGR system and reduces the pumping loss of the engine to improve fuel economy, while also creating a peakier torque curve down low. By advancing the exhaust and intake during spool up, peak torque is moved to a lower RPM that is closer to the spool range. The advanced intake allows for more air to be ingested on the intake stroke, and advancing the exhaust provides a faster spool and even more torque by taking some of the still expanding exhaust gasses from the combustion chamber and feeding it into the turbine wheel. When using the stock turbo, there is very little for the 4B11T to gain from overlap at high RPM.
Your engines VVT settings will vary depending on your engine platform, turbo choice, manifold runner design, exhaust backpressure, etc. Newer and more efficient turbocharges will happily improve performance with added overlap during spool up, cruise, and even full throttle with some combinations. In general, turbocharged engines don’t require as much of a scavenging effect at a high RPM as a naturally aspirated engine.
Adding overlap at high RPM will generally end up robbing performance due to the turbochargers natural scavenging effect pulling the cool air/fuel mixture out through the exhaust before it has a chance to combust, in comparison to more advanced exhaust valve timing using some of the hot and still expanding gasses from the end of the power stroke to spool the turbo quicker.