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Filling The Glass - Early Two-Valve Mods
Taking The Optimist's View With The Early Two-Valve 4.6's Performance Potential
When reviewing the specs of the '96-'98 4.6 Two-Valve motor offered in the Mustang GT, it's easy to dismiss the original mod motor as nothing more than a stopgap replacement for the legendary 5.0. As you might imagine, following a motor that spawned an entire industry was a difficult task. History has shown that filling those shoes-even half full-took more bullets than the new-fangled SOHC motor had in the clip. Mustang enthusiasts know the history of the mod motor and not three years after its introduction, the original Two-Valve was replaced by a more powerful version.
The Power Improved (PI) Two-Valve motor offered a serious hike in performance, finally pushing the GT motor beyond the power and performance offered by the original 5.0. Sure, the official 215hp rating of the '96-'97 motors was improved-or re-rated-to 225 hp in 1998, but the early Two-Valves never caught on with enthusiasts the way the 5.0 did. Even with the promise of improved efficiency offered by aluminum heads and an overhead cam configuration, a drop in displacement from the torquey 5.0 seemed a hard pill to swallow.
Though bemoaned by enthusiasts who tend to see the performance glass of these early GTs as half empty, a second glance might actually reveal the glass half full. Sure, these early motors offered a disappointing combination of lower compression, milder cam timing, and an intake and head package that flowed considerably less than the '99-and-up PI motors. Rest assured: power is hiding in the early combination, begging to be unleashed. Oddly enough, it's this relative lack of performance that makes these early GTs so attractive. With the later PI Mustangs at a premium, the '96-'98 GTs can be had for a bargain.
Part of the bargain is the early 4.6's compression ratio. The '96-'98 motors relied on pistons with an 11cc dish to work with 51cc combustion chambers, producing a static compression of 9.3:1. The PI motors combined a 17cc dish with a smaller 42cc combustion chamber to produce 9.7:1. As luck would have it, the later PI heads are a direct bolt-on replacement, providing the late-model PI intake is included as well. While these heads offer a flow improvement compared to the early versions, a substantial portion of the power gain offered by this swap comes from increasing the static compression from 9.3:1 using the 51cc chambers to 10.6:1. In addition to the hike in static compression, the PI heads also provide a much needed squish area, enhancing mixture motion to improve power and fuel economy, a larger exhaust valve, and the ability to flow even more with proper porting.
To maximize the performance potential of our PI head upgrade, we shipped the stock castings to Total Engine Airflow for the Stage 2 performance upgrade. The Stage 2 porting process was able to increase the flow rate of the stock PI heads from 176 cfm to 226 cfm, a gain of 50 cfm at a 0.600 valve lift. The exhaust flow was likewise improved, from 125 cfm to 209 cfm, a gain of 84 cfm.
Naturally, the PI heads required the use of the matching PI intake. The intake swap was not only required due to the difference in intake port design between the heads, but it was actually beneficial, as the PI intake flowed significantly better than the '96-'98 version. Credit ports with a larger cross section and decreased runner length for the improvements, though the changes result in a slight drop in low-speed power compared to the original non-PI manifold. This is a minor trade off, given the impressive power available in the mid- to high-rpm ranges (up to 6,000).
In truth, the PI head swap was part of the upgrade package we had in store for the '97 motor, which we pulled from the engine bay of a mangled Mustang. The high-mileage modular was anything but spectacular, offering nothing more than even compression on all cylinders. In short, the motor was what you would expect to find underhood in almost any potential project Pony.
In addition to the PI induction system, we took the opportunity to bless the stepchild Two-Valve with a set of performance cams. Straight from the Comp Cams catalog, the XE270AH versions offered a 0.550 lift, dual-pattern duration specs of 234/238 degrees at 0.050, and a 113-degree lobe separation angle. The high lift allowed us to take full advantage of the airflow offered by the ported PI heads, while the duration change shifted the torque curve to greatly improve the peak power numbers.
While the Comp Xtreme Energy cams, TEA-ported PI heads, and matching PI intake were the highlights of the performance upgrades, a few other modifications were present to further improve the power output over stock. On the induction side, we relied on a 75mm throttle body and inlet elbow from Accufab. Since the motor was run on the engine dyno, no accessories or further induction system were necessary. On the dyno, the motor was programmed with a FAST engine management system, allowing us to eliminate the induction system from the airbox back to the throttle body. In the end, we relied on a simple induction tube with a radiused entry on the stock and modified motor configurations.
Horse Sense: Early Two-Valve motors actually offer a benefit over the later PI motors in the form of static compression. While the later PI motors offered more static compression from the factory (9.7:1 versus 9.3:1), the difference in the combustion chamber/piston configuration makes upgrading the early non-PI motors with late-model PI heads a double performance gain.