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.
Since we're upgrading the...
Since we're upgrading the non-PI heads with PI versions, we elected to install the factory PI intake manifold. Look for us to run an aftermarket PI intake on our stroker motor.
Accufab also supplied a 75mm...
Accufab also supplied a 75mm intake elbow. Testing has shown the intake and throttle body to be worth as much as 10 to 15 hp on the right application.
Not content to install stock...
Not content to install stock PI heads, we enlisted the aide of Total Engine Airflow to port the PIs. It supplied a set of Stage 2 heads that included new Ferrea valves, Comp Cams valvesprings, and complete porting to maximize flow.
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.
The CNC porting significantly...
The CNC porting significantly improved the flow rate of the intake ports. These intake ports flowed 221 cfm at 0.600-inch lift (up from roughly 152 cfm in stock configuration).
The exhaust flow was equally...
The exhaust flow was equally impressive, as the Stage 2 heads flowed 203 cfm at the same 0.600 lift.
The late-model PI heads also...
The late-model PI heads also feature smaller combustion chambers than the stock '97 non-PI heads. The change in combustion chamber increased the static compression to 10.4:1. Note the Ferrea valves and detail work to the combustion chambers.
The cylinder head upgrade...
The cylinder head upgrade for the non-PI motor includes Fel-Pro MLS head gaskets and ARP head studs.
The TEA heads were installed...
The TEA heads were installed onto the awaiting high-mileage (but still in excellent running condition) '97 4.6L short-block.
We replaced the factory non-PI...
We replaced the factory non-PI cams with these XE274H cams from Comp Cams. The (PN XE274H) cams, which offer 0.500 lift, a 236/240-duration split, and a 114-degree lobe separation, worked well. The PI cams from Comp might actually be a better choice as they offer 0.550 lift, but we only had the non-PI versions at our disposal in time to meet our editorial deadline.
We took the time to degree...
We took the time to degree the cams to ensure both were installed accurately. We wanted the two sides of the motor optimized for power production at different engine speeds.
Run on the engine dyno with...
Run on the engine dyno with our new TEA heads, Comp cams, and PI induction system, the modified Two-Valve motor produced 390 hp and 384 lb-ft of torque. Add a small shot of nitrous and you are looking at an easy 500 hp.
'97 4.6L Two-Valve W/Zex Nitrous...
'97 4.6L Two-Valve W/Zex Nitrous
It should be obvious from the supplied graphs that the stock 4.6L responded very well to CNC-ported heads, the Comp cams, and PI induction system. The ported PI heads not only improved airflow, but the static compression as well, thanks to a reduction in combustion chamber volume (the PI heads have smaller chambers that the non-PI versions). The Comp XE274H cams stepped up the cam timing significantly (0.500 lift and 236/240 duration at 0.050), while the late-model PI intake (with AccuFab 75mm throttle body) offered a substantial flow increase. The combination upped the power output of the early 4.6L Two-Valve from 266 hp and 341 lb-ft of torque to 390 hp and 384 lb-ft of torque. Torque production from the modified motor exceeded 350 lb-ft from 3,800 rpm to 5,800 rpm.