Richard Holdener
October 18, 2010

For our "stock" test motor, we changed just about every parameter, including running a revised inlet system (open filter and section of 3-inch tube); a free-flowing exhaust, consisting of Hooker headers feeding 3-inch collector extensions; and absolutely no accessories (only an electric water pump). We also optimized the air/fuel and timing curves using the FAST management system (the factory tune was much more conservative) for performance, without concern for emissions.

Applying the basic bolt-on route, we replaced the factory power-producing components with more efficient aftermarket versions. Off came the stock '97 non-PI heads, cams, and intake manifold; on went CNC-ported heads from Total Engine Airflow (PN XE274H), Comp cams, and the late-model Power-Improved (PI) intake manifold. The late-model PI heads (ported by TEA) not only offer additional head flow (roughly 45 cfm per runner), they also increase the static compression thanks to smaller combustion chambers.

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The combination of the additional compression, head (and intake) flow, and cam timing had a positive effect on the power output of our '97 GT test motor. Using the stock short-block with the TEA heads, Comp cams, and PI intake (and AccuFab TB and elbow), the 4.6L produced 390 hp at 5,900 rpm and 384 lb-ft of torque at 4,800 rpm.

Given the torquey nature of the stock non-PI motor, we expected the modified motor to sacrifice torque down low. After all, we did add much wilder cam timing. Lucky for us, the trade off in low-speed power was minimal. Credit the additional static compression for minimizing the low-speed losses. Torque production from the all-motor combination exceeded 350 lb-ft from 3,800 rpm to 5,800 rpm, making for one sweet powerband. In terms of idle quality, the XE274H cams were certainly lumpy, and if we had them available in time for testing, we'd opt for the (higher-lift) PI versions of the XE cams.

If you go looking to run this combination, the non-PI motor is actually a better starting point, as the PI short-block will offer lower compression when run with the PI heads. If you already own the PI motor and are changing the pistons or stepping up to a stroker, this is a moot point.

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