Intake System

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The stock air intake system is still restrictive even without the precatalytic converter (flows only 550cfm @ 28" H2O depression) so there's plenty of room for improvement. Unsurprisingly the most popular entry level performance upgrade is an aftermarket cold air intake kit with handheld PCM programmer and I also followed that path by selecting the JLT series 3 CAI with SCT X3 tuner. Downloading the Brenspeed 91 octane tune into the car's PCM and installing the JLT CAI was a breeze apart from some minor trimming of the heatshield which was required for the hood to close properly.

The engine immediately felt more responsive and the low rpm "lag" of the drive-by-wire throttle was gone. There was a noticeable induction roar when pressing on the throttle which, when combined with the soundtrack of the engine, was very intoxicating! Acceleration through the gears was noticeably stronger, especially in 5th gear driving on the highway.

JLT Series 3 Cold Air Intake

Chassis dyno tests of otherwise stock '05-'09 S197 GTs with the same cold air intake & 91 octane tune yield an average of 289hp & 303lbft. These figures translate into 329hp & 341lbft at the crank. Peak HP is a plateau from 5200-6200rpm and peak TQ arrives at 4300rpm. The stock 6250rpm rev limiter is conservative and it could easily be raised to 6500rpm.

Ported Throttle Body

The stock dual 55mm throttle body flows a generous 953cfm @ 28" H20 depression, which is theoretically enough to feed an engine putting out 635hp or roughly twice the output of a N/A 4.6L 3V engine. However the center ridge of the throttle body is wide, with sharp edges at the top of each bore that induce airflow turbulence. I feathered the edges at the top of each bore and blended them into the center ridge, streamlining the airflow path down the center of the throttle body thus allowing more laminar flow. The increased air velocity sharpens throttle response and produces a small HP/TQ gain.
If you're handy using a Dremel, this is an easy modification to perform, and it's free if you already have a carbide cutter and sanding rolls.