Over the course of about a dozen dyno pulls, the most efficient peak boost and "ramp up" levels were found. At the stock boost level, A/f peaked to 13.5:1 at about 3,000 rpm and steadily fell to 12.1:1 by redline. Turning up the boost didn't seem to affect A/f ratios in the higher rpm's, but our 3,000rpm peak started to climb. At our most powerful run-246.0 whp and 247.8 lb-ft of torque at 11psi-it reached a scary 14.3:1. We decided to back boost off by one pound and allow the wastegate to open a hair more across the board for safety's sake, reinstalled the cat and ended up with 241.8 whp and 240.9 lb-ft of torque at 10psi, with the A/f ratio reaching a momentary peak lean point of 14.0. Could we have made more power? Yes. We could've turned up the fuel pressure manually, allowing us to boost another pound or two and see a little more peak output, but increasing the fuel supply across the board would've decreased power in the low and high rpm's, causing a rich condition and hurting drivability.
Increasing boost showed an inescapable drawback to pushing the limits of a small turbo. As boost levels rose, so did power output-but more so in the mid range than at top-end. As engine speed increased, the turbo's ability to supply it with air proportionately began to fall; signaling its efficiency was being overshot and chances are it was being stressed to levels it wasn't designed to endure. Coaxing more boost out of it would only add diminishing returns in the lower rpm's and near redline-decreasing the "area under the curve" of our powerband, and eventually killing our turbo.
A look at our ending graph will show the little T25 turbo can pack a pretty mean Punch, when pushed around. Next month we'll add a few more goodies to our stock turbo before installing it in g-dimension's dumpster, switching to the S15 SR's bigger T28 hybrid brother, and eventually to garrett's infamous gt2871-R slug. With the conclusion our final round of upgrades (...promises Carter), we will have not only tested the limits of three popular turbo upgrades for the SR, but those of a reliable, fast-as-hell, daily driven "street car" altogether. Adding some genuine road testing to the mix, we'll explore the benefits and drawbacks of making more power and see if "too much" can ever really be "too much".