Common sense would dictate that when choosing cams for a supercharged Cobra motor, we would gravitate to those cams under the heading XE-R Supercharged and Nitrous cams. Unfortunately, these cam profiles required a valvespring change. Instead, we stuck with something from the Xtreme RPM category that featured only 0.425 lift. The low lift figures allowed us to retain the stock valvesprings without fear of coil bind.
9 The KB blower upgrade retained...
9 The KB blower upgrade retained use of the factory lower intake manifold. Note the design featured ultra-short runners.
After looking over the four Xtreme RPM offerings, we chose the XE262AH-14 cams (PN 106100). According to the catalog, the XE262AH cams will provide great street performance, while offering significant horsepower and torque gains (without computer modifications). Basically, the XE262AH cams seemed to be a perfect performance plug-n-play proposition. For our FAST engine management, the plug-n-play feature wasn't as important (on the dyno), but it would make in-car tuning much easier. 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).
Sharp MM&FF readers who have perused the spec box may have noticed that the supercharged cams were dual-pattern as well, but between the Xtreme RPM and XE-R Supercharged cams, the supercharged versions featured more exhaust duration. Our XE262AH cams offered 4 degrees more intake duration (226 degrees versus 222 degrees), but the comparable supercharged cam featured a 2-degree split (224 degrees versus 222 degrees) skewed in favor of the exhaust. How well would the cams designed for a normally aspirated engine work on our supercharged one? Would the decreased exhaust duration hurt power?
In truth, we already knew the answer to these questions, having run these cams many times before. The old adage that if the cams add power normally aspirated, they will add power under boost holds true even for Four-Valve mod motors. The cam swap required removal of both main and secondary timing chains, and compression of all 32 of the hydraulic lifters. If this is your first time, we recommend taking a photo for reference when putting it all back together. Just make sure to line up the dots on the sprockets with the colored links on the chains. As indicated previously, the Four-Valve 4.6L cam swap was not technically difficult, just time consuming.
Having run the '03 Cobra motor with the stock cams, we performed the cam swap on the dyno and had the new configuration up and running in (relatively) no time. The idle quality changed only slightly (by roughly 2 inches), not enough to be concerned about tuning and nowhere near the maximum-effort grinds. Since we did not dial in the cams, we figure there were even better manners and possibly more low-speed power had we elected to advance the cam profiles. Equipped with the XE262AH cams, the supercharged Cobra produced peak numbers of 572 hp and 533 lb-ft of torque. As expected, the peak boost pressure reading dropped by roughly 0.5 psi.
10 Though we ran the twin-screw...
10 Though we ran the twin-screw with the same pulley ratios as stock (to directly compare them), we were more interested in cranking up the boost on pump gas. The stock crank pulley was replaced with a system that allowed us to increase the size of the crank pulley up to 9.5 inches.
11 Here is a shot of the...
11 Here is a shot of the 2.8L supercharger installed on the '03 Cobra engine.
12 We ran a number of different...
12 We ran a number of different blower pulleys to adjust the boost pressure. We eventually settled on a maximum of just over 15 psi on pump gas.
We've run this cam test previously on a normally aspirated combination to the tune of 50 hp. On this supercharged application, the cams were worth 34 hp measured peak to peak. Impressively, there was very little trade off in power lower in the rev range. Again, we suspect additional power was available through the entire rev range with advancing the cam profiles, but time did not allow this procedure.
With our Eaton M112 testing completed, it was time for more boost. Waiting in the wings was a serious blower upgrade in the form of the Kenne Bell 2.8L Twin-Screw supercharger. The twin-screw design offers a number of benefits, such as a reduction in parasitic losses associated with driving the blower. Though the factory air-to-water intercooler was on hand to keep things cool, the limiting factor was ultimately the 91-octane, premium unleaded fuel used for testing. With race fuel, the 2.8L Kenne Bell was capable of supporting over 1,000 hp. Configured to provide a peak of 15.2 psi and with a safe (pump-gas) tune, the Cobra engine belted out the 741 hp and 622 lb-ft of torque. Note from the graph that the power was still climbing rapidly, but in deference to the new owner, we purposely limited engine speed during testing.
Stock '03 Cobra vs. Comp XE262AH Cam
Equipped to produce a peak boost pressure of 11.8 psi, the supercharged '03 Cobra engine was run first with stock cams, and then after swapping in the XE262AH grinds from Comp Cams. Equipped with the stock cams, the Cobra motor produced 538 hp at 6,500 rpm and 541 lb-ft of torque at 3,600 rpm. After swapping in the cams, the peak numbers jumped to 572 hp at 6,600 rpm and 533 lb-ft at 4,400 rpm. The cams increased the power output starting at 4,100 rpm, but lost out slightly below that point. We suspect additional gains could be had by taking the time to degree (and advance) the cam timing.
Stock '03 Cobra vs. Kenne Bell Blower Upgrade
The cam swap netted impressive power gains, but we were just getting started with the Four-Valve mod motor. Oddly enough, the liming factor in terms of supercharged performance on the '03 Cobra motor was actually the factory supercharger. Upgrading the Eaton Roots blower with a larger and more efficient twin-screw design from Kenne Bell will yield massive power gains. Even limited by pump gas (and a concerned owner), the Kenne Bell blower upgrade increased the power output from 572 hp and 533 lb-ft of torque to 741 hp and 622 lb-ft of torque. The peak boost jumped from 11.8 psi to 15.2 psi.