Driving Sports

Lag. Despite only containing three letters, it really is a four-letter word in the world of turbocharged engine tuning. Lag is basically the time delay from when you hit the loud pedal to when the turbocharger begins making useable boost. Lag is not a fixed time delay; how much lag a given car has depends on a few variables, like displacement of the engine, size of the turbo and even the cam timing. While the first two variables tend to be fixed, Subaru has developed a system to vary cam timing, known as Active Valve Control System, or AVCS.

But first we need a quick primer on how a cam works. Basically, a cam is a rod with bumps on it. As the cam rotates, the bumps, or cam profiles, act on the valves in your engine to open and close them. Normally, the order in which the valves open and close, also known as cam timing, is fixed. The timing belt rotates the cam, which spins and opens and closes the valves as determined by the position of the cam lobes on the cam shaft. This system works very well, but it presents the tuner with an absolutely fixed variable in cam timing. AVCS changes this.

AVCS allows the cam shaft to advance or retard cam timing, on the fly. It does this through a series of oil pressure switches and solenoids. Essentially, instead of rotating around a fixed axis, the cam lobes can be made to open valves earlier or later in the combustion cycle. But why would we want to do this?

The simple reason is that your cams determine where your engine makes power. They determine where and when the valves open to let fuel and air in, and exhaust gasses out. In a turbocharged car, power is made when exhaust gasses spin a turbine, which shares a shaft with a compressor, which then feeds high-pressure air and fuel into the engine. If we can alter how and when exhaust gas reaches the turbine, we can alter turbo spool threshold.

On past Subaru engines, Subaru has made AVCS available only on the intake cam. Through careful tuning of the AVCS, tuners are able to decrease turbo spool threshold in an effort to make power delivery more linear.   From an emissions standpoint, changes to the AVCS allow a cleaner mixture out the tailpipe as well.

On the new generation of EJ257, the engine found in the 2008 STI, Subaru has included AVCS on both intake and exhaust cams, allowing tuners to greatly manipulate the way in which the engine delivers power.

The previous generation AVCS allows up to 40 degrees of change in the advance of the intake cam, which combined with the fixed exhaust cam gives a variable overlap anywhere from 0 to 40 degrees.  In the stock ECU map, this overlap is ramped in during spool-up, and tapers off at high RPM. The new Dual AVCS system allows a more restricted 30 degrees of intake cam advance, but also throws in 40 degrees of exhaust cam retard. With the combination of the two, valve overlap can be dialed from -17 degrees (no overlap) all the way up to 53 degrees of overlap.  Not only do you get more overlap, but for a given overlap you also have the ability to move the overall cam timing.

What does this mean in a real world application? How about near-normally aspirated throttle response, even on large-turbo cars? Take, for instance, our Perrin/Surgeline Tuning-tuned, 2008 STI. This car is running a largish Garrett GT3076R with the larger .82 A/R exhaust housing. This is a fairly standard upgrade for past STIs. While this turbo does make great top-end power, it tends to be a bit “laggy” for the street, with power not coming on until after 4,000rpm. However, with careful AVCS tuning, this same turbocharger on essentially the same engine now makes 15lbs of boost before 3,500rpm. This means that the turbo is making more boost lower in the rpm band, so that even at tip-in, 3-5lbs of boost is available, which means better power out of the corners and off-the-line acceleration.

Another application of AVCS is the ability to make more use of “wilder” cams. Normally, to make more power on the top, one must sacrifice power at the bottom end. This effect happens with turbochargers, as we have shown, and bigger cams with more cam profile as well. With the ability to alter cam timing on the fly, tuners can now mitigate the effect of wilder cams at the bottom end, while being able to exploit their benefits across the rest of the rpm band.

The best aspect of AVCS tuning is that it is completely adjustable and functions on the fly, so that the cam can effectively be altered for every point on the RPM band. Another aspect of AVCS tuning is its ability to alter engine economy as well. Coupled with SI-Drive, these two technologies can allow the tuner to program into the ECU a super-economy mode, and with the flick of the SI-Dial, the car can be back in ultra-power, race mode.  Keep an eye out for an upcoming feature with real world examples, dyno plots, and data showing how these variables come together.

As the Subaru ECU becomes more complex, and the variables for ECU tuning become more varied, it is important to find a tuner that fully grasps and understands the applications of these concepts so that you can get the most for your modification dollar. Why pay for more turbo and more lag?

Article and Illustrations by Jeff Sponaugle