Richard Holdener
May 1, 2002

This is one story I've wanted to do for years, but just never seemed to get around to. I have tested camshafts on both normally aspirated and supercharged motors, but my supercharged cam testing has been limited to centrifugal superchargers. As luck would have it, 5.0 owners have been blessed with the fact that performance cams that offer power improvements for their normally aspirated motors will do so on supercharged applications, at least those equipped with centrifugal superchargers. Basically what this means is that the performance cam you installed in your 5.0 will probably work well after you add that Paxton, Vortech or PowerDyne centrifugal supercharger. There are of course some cam profiles that work better with superchargers than others, but for the most part, cam choices for centrifugal supercharged motors mirror those for normally aspirated applications.

To establish a baseline, we decided to run the test motor first with a stock 5.0 H.O. cam.

The question now is how does this cam information apply to positive displacement superchargers? Is there really a difference between positive displacement superchargers and centrifugal superchargers? Don't they both just supply more air to the motor? The answer to this last question is yes, sort of. True enough that any type of forced induction supplies additional air to a motor that it could not otherwise ingest of its own accord.

In the very near future, we will be experimenting with cam timing for turbo applications, which is definitely different than those required for a supercharger, but back to the differences between the positive displacement and the centrifugal. While both types of superchargers supply excess air to the motor, their method of supply differs greatly. Before getting to the cam test, let's take a brief look at how they differ.

After establishing a baseline, we planned to swap in an Xtreme Energy 258HR hydraulic roller cam. The dual-pattern XE258HR offered a .533/.544 lift split and a 208/216 split in duration. Note that the lift and duration were skewed in favor of the exhaust. Dual pattern cams seem to work best on blower motors.

The positive displacement supercharger offers a fixed amount of air per revolution. The actual amount of air supplied per revolution is determined by the size or more accurately the displacement of the supercharger. The twin-screw Autorotor superchargers offered by Kenne Bell are available in a wide variety of different displacements to meet the needs of different engine sizes and efficiencies. The airflow to the motor is determined by the size of the blower and the number of revolutions in relation to engine speed. Basically, spinning the blower faster will provide more revolutions of that fixed amount of air. A paddle wheel on a Mississippi river boat is a good example. Each revolution pushes a given amount of water based on the size of the individual paddle. The only way to increase the speed of the boat is to increase the speed of the wheel as the displacement of each paddle is fixed. Such is the case of a positive displacement supercharger.

Being much smaller physically, the impeller of a centrifugal supercharger requires much more speed to produce a similar amount of airflow. One revolution of the impeller will not provide the same amount of air as one revolution of a comparable positive displacement supercharger. Accordingly, centrifugal superchargers require significant rpm in order to function properly. While the efficient operating (rotor) speed of a typical positive displacement supercharger is around 12,000 rpm, the impeller on a centrifugal supercharger might need 40,000, 50,000 or even 60,000 rpm to produce a like amount of flow. In order to achieve this high impeller speed, centrifugal superchargers rely on an internal step ratio (the ratio is 3.45:1 on most Vortech models). This step speeds the blower up relative to the engine. Additional ratio is obviously needed to reach 40,000 rpm or more, so the centrifugal superchargers rely on a drive pulley ratio to further speed the impeller. Installing a larger crank pulley relative to the blower pulley greatly increases the impeller speed. This method is also employed by manufacturers of positive displacement superchargers.

Another advantage of the Flowzilla was that it was machined to accept this billet aluminum 90mm throttle body from AccuFab. Every effort was taken to minimize inlet restrictions.

This physical size combined with the difference in operating speed produces dramatically different efficiencies and power curves. A positive displacement supercharger usually provides near-instant boost production and a ton of low-speed torque along with it. The high-rpm nature of a centrifugal supercharger means it is best at supplying big horsepower increases. The trade off for all that top-end efficiency is less than ideal low-rpm boost production. The typical boost curve on a properly sized positive displacement supercharger is a straight line. As soon as the throttle is mashed, the blower provides peak boost and should be able to carry it all the way to redline. Such was the case during our cam tests. Centrifugal superchargers tend to make boost and power in relation to engine speed, with power coming on hard in the mid-range and especially at the top end. Boost for boost, the centrifugal will make more power due to its inherent efficiency (being a true compressor). The positive displacement supercharger however, will almost always provide more average torque throughout the rev range. Does this difference in efficiency and power production dictate different cam timing? That's what we aimed to find out.

The best way to find out what cam profile works well with a positive displacement supercharger is to try a few. That's exactly what we did here at MM&FF. The first step in our cam testing was to build a suitable test motor. There is obviously no "ideal" test motor, but we did want one that was representative of what might be found in a typical street Mustang. To that end, we built a 302 capable of withstanding the rigors of forced induction, without going overboard on the buildup. The basis of our test motor was nothing more exotic than a simple 5.0 short-block. Why not start with what nearly everyone else already has in their Mustang? This early 5.0 short-block was equipped with factory forged pistons. These pistons were notched in anticipation of the high-lift Xtreme Energy cams and larger-than-stock valve sizes in our aftermarket cylinder heads. Other than the valve reliefs, the short-block was kept bone stock.

The mass air meter is every bit as critical as the throttle body, so we chose a 77mm unit from Pro-M. In addition to offering plenty of airflow, the Pro-M meter was calibrated to match our 36-lbs./hr. injectors.

Knowing that we planned to run a pair of cams offering well over .500 lift, we opted to install a set of free-flowing cylinder heads that could take advantage of the additional cam lift. Though many different heads are available for the 302, we chose a set of AFR 185 heads for our test motor. We have had excellent results with these AFR heads in previous tests and wanted to maximize the airflow potential of our supercharged test motor. The AFR 185 heads were installed on the awaiting short-bock using a set of Fel Pro 1011-2 head gaskets and ARP (7/16) head studs. Head studs are a good idea when running any type of forced induction. Topping the AFR 185 aluminum heads was an Extrude-Hone ported GT-40 lower intake. Since the Kenne Bell supercharger kit employed a dedicated upper intake casting, no GT-40 upper was necessary.

The centerpiece of our positive displacement supercharged test motor was the Kenne Bell (Autorotor) supercharger. Though the blower kits come complete, we opted for a few upgrades for our test motor. The first upgrade was the supercharger itself. Instead of the standard 1.5 1500, we installed the larger 2.2 Blowzilla supercharger. This Blowzilla offered 50 percent more power potential than the standard 1500. Since we expected the supercharger motor to exceed 400 hp, we wanted to make sure we had enough supercharger to properly test the cams. In addition to the Blowzilla blower, we also installed the Flowzilla inlet manifold. Not wanting to restrict the breathing of our test motor, we employed one of the new Kenne Bell inlet castings to minimize restrictions. In previous tests, the Flowzilla has been shown to be worth as much as 40 hp over the standard inlet casting. The combination of Blowzilla and Flowzilla gave us plenty of power potential.

RC Engineering supplied a set of 36 lbs./hr. injectors for our test motor. The larger injectors were a must since we planned to creep up on the 500-hp mark with the larger of the two Xtreme Energy cams.

The larger inlet manifold also allowed us to run a 90mm throttle body, as the inlet was machined to accept the big-bore throttle body. We employed a billet version from AccuFab for our testing. Additional intake mods included a 77mm mass air meter from Pro-M. The meter was calibrated for the 36-lbs./hr. injectors sourced from RC Engineering. The 36-pound injectors were good for over 500 horsepower, or more than enough for our test motor. In addition to the recalibrated meter and larger injectors, we employed an adjustable fuel pressure regulator and custom chip from Kenne Bell to meet our fuel and timing needs. After setting the billet MSD distributor at 8 degrees initial timing and making minor adjustments to the static fuel pressure, the motor ran perfectly every pass. The final performance component was a set of 15/8-inch Hooker long-tube headers feeding a pair of 3-inch exhaust pipes and Flowmaster mufflers. With everything finally hooked up, we were ready to test the supercharger combination with the stock H.O. cam.

Blower Cam Shootout--HorsepowerStock H.O. vs XE258HR vs XE274HR
Obviously even positive displacement superchargers like aggressive (but not too aggressive) cam timing. The XE258HR increased power from 438 hp at 6000 rpm to 456 hp at 5700 rpm. The difference in the peak power rpm was valve float. It is likely that the peak power might have exceeded 460 hp with the XE258HR cam had the stock hydraulic roller lifters not given up early. Adding new lifters allowed the XE274HR cam to rev freely to 6000 rpm (but not beyond). The supercharged motor pumped out 485 hp at 6000 rpm with the XE274HR cam. The boost pressure dropped from 8.5 psi with the stock cam to 7.5 psi with either of the two Comp Cams. Oddly enough, there was no drop in boost pressure between the two Comp Cams.

It is worth mentioning that the test motor was equipped with all the accessories and a complete exhaust. The supercharger was equipped with a 31/8-inch drive pulley which produced 8.5 psi from 3500 rpm all the way to 6000 rpm. With the stock cam in place, the supercharged 302 produced 437 lbs.-ft. of torque at 3900 rpm and 438 hp at 6000 rpm. The rather high power peak can be attributed to the lack of upper intake runners in the Kenne Bell casting. The short runners produced peak power all the way out at 6000 rpm, much higher than the 4800-5000 rpm shown for a stock 5.0. The positive displacement supercharged motor had no trouble demonstrating its torque producing capabilities, thumping out over 400 lbs.-ft from 3500 rpm all the way to 5600 rpm. The horsepower curve was impressive as well, topping the 400 horsepower mark from 5000 rpm to 6000 rpm. The output was pretty impressive considering the mild stock cam timing.

Now it was time to see if the positive displacement supercharger worked well with the same cam timing as a centrifugal. I selected a pair of cams that have worked well in the past, namely a pair from the Comp Cams Xtreme Energy line. Both of these grinds have proven effective on normally aspirated and centrifugal supercharged motors in the past, so I was curious to see how they would perform on this Kenne Bell-equipped 302. First up was the milder XE258HR, a dual-pattern cam offering a .533/.544 lift split along with a 208/216 split in duration. Note that the dual-pattern cam favored the exhaust, something I have found to work well especially on supercharged applications. This cam was tested previously for MM&FF and found to increase the power of a normally aspirated 302 by as much as 30 horsepower. How would it fare on the supercharged motor?

Blower Cam Shootout--Torque Stock H.O. vs XE258HR vs XE274HR
Perhaps the most telling evidence of the desire for more aggressive cam timing is in the torque production. More often than not, we see a trade off between low-speed power and high-end power. A larger cam generally trades horsepower for torque, but no so in this case. Both of the Xtreme Energy cams increased power across the board compared to the stock cam. The 302 was just begging for more lift and duration. Equipped with the XE274HR cam, torque production of the supercharged test motor never dropped below 424 lbs.-ft. That's one impressive 302.

Installing the new cam on the dyno took a couple of hours. After we had the supercharged motor buttoned back up, we warmed everything up and were ready to rock. After putting the hammer to it, the first thing we noticed was that the boost pressure was down by 1 psi (to 7.5 psi). The next thing we noticed was that even though the boost pressure had dropped with the new cam, the power was up, everywhere. The XE258HR cam improved power across the board. The motor really responded to the new cam timing, as the peak torque was now up to 449 lbs.-ft. at a slightly higher 4100 rpm and peak power was up to 456 hp at 5700 rpm. We noticed during the pull that the motor experienced valve float around 5700 rpm--not a desirable situation. The valve float was the reason for the lower 5700-rpm power peak. Had the motor been able to rev freely, we see no reason why it would not have exceeded 460 hp at 6000 rpm. Even with the slight valve float problem, the Xtreme Energy cam provided a significant power gain. The cam was worth as much as 24 additional horsepower.

Next up was one of my favorite 5-liter cam grinds, the venerable XE274HR. With 224 degrees of intake duration and 232 degrees of exhaust duration, the XE274HR is what might be considered a sizable cam for a street 302. The truth is that this profile works great on street motors, both supercharged and normally aspirated. After another mad thrash, the new XE274HR, was installed in place of the XE258HR. We took the opportunity to install a set of new Comp Cams roller lifters in place of the tired factory units. We wanted to give the new cam every opportunity to shine.

Boy, did it shine. How does 485 hp at 6000 rpm and 456 lbs.-ft. at 4100 rpm sound? That translates to a gain of 47 horsepower from a simple cam swap. Once again, the boost pressure was down to 7.5 psi, but the power was up everywhere. The XE274HR cam increased the power output across the board, from 3500 rpm to 6000 rpm. Though not definitive by any means, this test indicates that your Kenne Bell-supercharged 5.0 will respond favorably to the same cams that work so well with centrifugal superchargers.