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Four-Valve Modular Engine Upgrades - Cobra Crankup
Testing ported heads, cams, and intake upgrade—plus nitrous—on a 4.6L Cobra Mod mill.
Originally introduced in the Mustang in 1996, the Four-Valve modular motor recently gained a new lease on life when Ford introduced the latest and greatest 5.0L version. With the new Mustang sporting a 5.0L and the Shelby offering the supercharged 5.8L, you have to ask—where does that leave the old 4.6L?
Actually, the 4.6L Cobra motor is alive and well, for good reason. Though no aftermarket heads have become available, the factory stuff can be made to flow pretty well. With the proper porting, big cams, and a ported intake, you have the makings for one healthy beast—add cubes, like we did with our recent DOHC build, and you can crack 500 hp quite easily. Back that up with nitrous or boost and you're talking about really serious power.
One need only look at Accufab's John Mihovetz to see just how far you can take a 4.6L. With twin-turbo mod motors exceeding 2,000 hp, the 4.6L can still hold its own with the very best performance motors. And to illustrate what is possible on a normally aspirated 4.6L Cobra motor, we decided to subject one to a series of bolt-ons.
The idea was to first test a stock motor and then add performance components to illustrate how well the Cobra responded. The quad-cam arrangement on the 4.6L meant this would be best achieved on an engine dyno, though the head, cam, and intake swap could be accomplished in the car as well.
While a stock NA 4.6L Cobra motor would suffice, we already had a 4.6L test mule courtesy of Sean Hyland Motorports. The motor had seen plenty of abuse, having previously been subjected to all manners of normally aspirated and boosted modifications. Combine that with extended periods of inactive duty, and we decided the motor deserved at least a light hone and fresh set of rings.
L&R brought the short-block back to life, as well as sourcing us a set of stock Four-Valve heads to serve as our baseline. We already had stock cams from a previous test, so after the hone and new rings, the motor was reassembled using Fel Pro MLS head gaskets and ARP head studs. Odds are that this mod motor will eventually see boost, so the head gaskets and ARP hardware was money well spent.
The stock heads came off a low-mileage '01 Cobra and required only a clean-up before installation. Add in the stock intake and exhaust manifolds, and toss on a Meziere electric water pump, and we were in business.
Run with a FAST XFI, it produced 336 hp and 332 lb-ft of torque. Often considered a high-rpm motor, the Cobra produced peak power at just 6,000 rpm, while peak torque came at 4,900 rpm.
Having established a baseline, off came the stock heads, cams, and intake to make room for our upgrades. First on the list was a set of ported stock heads from Sean Hyland. In addition to full porting on the intake and exhaust, the heads were also treated to slightly larger valves and a valvespring upgrade that provided sufficient coil-bind clearance for the Comp cams we had planned. The spring upgrade also allowed the motor to rev safely to 7,500 rpm. The ported heads were combined with Comp XE262AH cams. The dual-pattern XE262AH cams offered 226 degrees of intake duration, 222 degrees of exhaust duration, and a 114-degree lobe separation angle to go along with the 0.425 lift (both intake and exhaust).
A huge thanks goes to John Mihovetz for providing the necessary tools for performing the cam surgery and assistance with dialing them in. Without the valvespring compressor, custom piston stop (to precisely locate TDC), and associated components, we would never have been able to degree the cams properly.
While the main components are very similar to the Two-Valve GT motor, the Four-Valve also featured a secondary drive assembly from the exhaust cam to the intake. This secondary chain drive required a second pair of cam sprockets and a dedicated chain tensioner. The cam installation also required removal and compression of all 32 of the hydraulic lifters. In the end, the Four-Valve 4.6L cam installation was not terribly difficult, just time consuming.
The ported heads and XE262AH cams were combined with a ported stock intake, also from Sean Hyland, but no changes were made to alter the runner length of the factory intake. We also tried a larger throttle body, but at this power level, the stock throttle body seemed more than adequate.
The final performance upgrade included Hooker headers to replace the factory exhaust manifolds. Since we were looking for all the extra power we could find, the headers were a natural upgrade. After the upgrades and a tweak in the tune, output jumped to 435 hp and 409 lb-ft of torque. Not only did the upgrades improve the power output by just over 100 hp, but the torque peak was up 80 lb-ft. In fact, the power output was up through the entire rev range.
Having made all of the changes at once, we still had some daylight to burn and a healthy Four-Valve just begging for more mods. Despite not being offered by Ford Racing any longer, we decided to pull out an FR500 intake manifold we had stashed away. The FR500 is unique—not only is it made of lightweight magnesium, but it also offers dual runners. The design combined one set of long runners to bolster low-speed power with a set of short runners to enhance the top end. Unfortunately, we didn't have the actuators to switch between the long and short runners, and were instead forced to run the intake with the short runners activated. Looking primarily for peak power numbers anyway, the FR500 increased the output of the 4.6L to 483 hp at a lofty 7,400 rpm.
For the last hurrah, we added nitrous. Though a simple wet fogger system would suffice, we dusted off an old Nozzle system from NOS. We like the equal distribution offered by the individual-port system, as well as how cool the thing looks once installed. The system features individual nozzles that are positioned between the injectors and receiver holes in the intake manifold. For this test, we removed the FR500 and reinstalled the ported stock intake.
Like most systems, the NOS kit is adjustable using different nitrous and fuel jets. With jetting designed to supply an additional 100 hp, we pushed the button and were instantly rewarded with peak numbers of 547 hp and 526 lb-ft of torque. Big numbers with nitrous are so easy, just make sure you have plenty of fuel and retard the timing as we did by 3-4 degrees at this power level. Looking back at how easy it was to increase the power output of the Four-Valve, we'd say the 4.6L has a solid future in the performance industry.