Subiesport Magazine - April/May 2008 - (Page 46)

TUNING THE 2008 STI AVCS Tuning, Continued from Page 45. 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. 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. 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 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? 46 MAY 2008 http://www.flatironstuning.com

Table of Contents Feed for the Digital Edition of Subiesport Magazine - April/May 2008

Subiesport Magazine - April/May 2008 Contents Travis Geny Letters Readers' Rides Subaru News Wicked in White Pink Rules Harman Motive Nouveau Tuning the 2008 STI 2008 Rally Guide Alcan 5000 Rally Alcan Unleashed WRC Rally Monte Carlo WRC Rally Sweden WRC Rally Mexico 100 Acre Wood Kumakubo! D1 Drifting Where to Buy Subiesport 2008 Impreza WRX 2007 Legacy GT 1993 Impreza Wagon Fueled Up Ad Directory Spotted!

Subiesport Magazine - April/May 2008

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