One of the common misconceptions about cam profiles for forced induction applications is that you have to do everything you can to minimize the dreaded overlap. In fact, some experts go so far as to say you should run stock cam profiles in blower applications.
Real-world dyno testing, of course, indicates otherwise as blower motors respond well to a variety of cam profiles that were originally designed for normally aspirated applications. The argument in favor of the stock cam is that the short duration and wide lobe separation angle eliminates overlap. Overlap is thought to be the enemy of boosted applications, as (so goes the thinking) that boost is allowed to escape out the exhaust.
In reality, cam duration beyond stock specifications is used in a wide variety of high-performance blower applications to determine the effective engine speed. Just as with a normally aspirated motor, the cam specs (primarily duration) can be used (along with intake runner length) to determine where the motor will produce peak power and torque. Shifting the torque curve higher in the rev range will usually produce a higher peak power level, but usually low-speed power will suffer in the trade off. You might suspect that this trade off has a negative effect on power once you add boost, but our testing on this supercharged small-block indicated otherwise.
The idea behind this test was to dispel the notion that mild cam timing is best for supercharged applications. To properly test this, we ran a pair of Xtreme Energy cams in our small-block test engine. To provide complete information to the readers, we ran the motor in both normally aspirated trim and equipped with a Vortech supercharger (with both cams). This way we could illustrate the power gains offered by the change in cam timing in both normally aspirated and supercharged trim. Would the gains be greater N/A or after we added boost? Questions like these are why we spend so much time on the dyno.
Our test motor was an iron-headed, L98 350 originally plucked from the local wrecking yard. When we built it, the goal was 400 hp on the cheap so we added a set of Pro Comp aluminum cylinder heads, a Comp Xtreme Energy cam and single-plane induction system, all for around $1,100. Since we had it at our disposal, we decided to utilize it in this supercharged cam shootout. The side benefit to this motor was that the larger of the two cams we intended to test was already installed.
First let's take a look at the cams we intended to run in this comparison. In the mild corner was the Xtreme Energy XR258HR. Basically a step or two above the stock L98 cam, the XR258HR offered a lift split of 0.480 in/0.487 ex, a duration split at 0.050 of 206/212 degrees and a 110-degree lobe separation angle. This was a mild cam designed to offer good idle quality and plenty of low-speed torque. Obviously these specs also minimized overlap.
By contrast, the larger XR282HR cam was a real performance piece. The XR282HR featured both more lift and duration, checking in with a lift split of 0.510/0.520, a duration split (@0.050) of 230/236 degrees and the same 110-degree lobe separation angle. Now the question was which one would provide the best overall power curve. Naturally we suspected the motor would make more power with the larger cam, but would the gains offered continue once we added boost? What we were looking to find out was which one made the best blower cam.
Before running, our test mule required some minor work. The first order of business was boost. The super for our supercharged mill came from Vortech Engineering in the form of its Universal Carbureted small-block kit. Originally designed for a blow-through carbureted application, we used this system with electronic fuel injection. This was a simple matter of running an Edelbrock Victor Jr. intake that had been retrofitted with a set of injector bungs. Use of the converted carbureted intake allowed us to bolt on the carburetor enclosure and 4-hole throttle body from Accufab.
Unfortunately, interference between the fuel rails and bottom of the carb enclosure necessitated that we run a 2-inch carb spacer. This change further necessitated the use of an additional spacer to properly locate the junction between the discharge of the V1, S-trim supercharger and the inlet elbow for the carb enclosure. Once again, it was Accufab to the rescue in the form of a 90mm EGR spacer for a 5.0L Ford. Since the bolt holes and 90mm opening matched the Vortech enclosure already, all that was necessary was to mill the spacer to the proper thickness (1.23 inches). With the spacer in place, everything lined up and we were in business.
The (now) aluminum-headed L98 was run with an MSD distributor, a set of 1.75-inch dyno headers and the FAST XFI engine management system. Also present was a set of 36-pound injectors, which were a tad on the small side for our supercharged motor, but we managed to just sneak by using 50 psi of fuel pressure. The first order of business was to run the motor with the XR258HR cam in normally aspirated trim. Equipped with the Pro Comp aluminum heads, converted Victor Jr. EFI intake and XR258HR cam, the 350 produced 358 hp at 5,400 rpm and 396 lb-ft at 3,800 rpm. Torque production exceeded 375 lb-ft from 3,000 rpm to 4,900 rpm, but power fell off rapidly after 5,500 rpm.
After the N/A runs, we added boost from the Vortech supercharger. The blower was equipped with a 3.33-inch blower pulley spun by a 6.0-inch crank pulley. Fed through the carburetor enclosure, the Vortech produced a peak boost pressure of 9 psi at 6,200 rpm. Equipped with the blower, the motor produced 454 hp at 6,200 rpm and 450 lb-ft of torque at 4,500 rpm.
After running few back-up runs, we tore into the engine to replace the XR258HR cam with the wilder XR282HR grind. Off came the induction system, rockers and pushrods, followed by the front cover, timing chain and Xtreme Energy cam. The XR282HR cam was run with the factory lifters. After everything was buttoned back up, we ran the combination in normally aspirated trim. Equipped with the larger XR282HR cam, the motor produced 407 hp at 5,800 rpm and 405 lb-ft at 4,000 rpm. This represented a jump of nearly 50 hp over the smaller cam. While the smaller cam offered more torque up to 3,500 rpm, the larger cam pulled away thereafter.
Adding the blower to the equation really showed the worth of the larger cam. Running the XR282HR cam increased the peak power numbers to 561 hp at 6,200 rpm and 501 lb-ft at 5,000 rpm. Oddly enough, the peak boost dropped down from 9 psi to 8.1 psi--a sure sign that the breathing potential of the motor had improved.
Comparing the results, we see that the cam change offered a gain of roughly 50 hp in normally aspirated trim, but over 100 hp once we added boost. It is also important to point out that the while the smaller cam improved torque production below 3,500 rpm in normally aspirated trim, this situation changed once we added the blower. Even down at 3,000 rpm, the larger cam made more power than the smaller one when equipped with the supercharger. Naturally all testing was run with the same blower and crank pulleys, the same timing and air/fuel ratios as well as the same air and water temperatures. This test illustrated that not only do forced induction motors tolerate wilder cam timing, but they (in fact) thrive with the right cam specs.
It bears mentioning that the 560 hp power level achieved with the XR282HR cam and Vortech pushed the internals of the stock short-block right to the ragged edge. At this power output, the cast pistons may well fold under the pressure, this despite the use of race fuel and conservative tuning levels. If you plan on running this much power, be prepared to step up to forged pistons at the very minimum, but forged rods would also be highly recommended. The stock cast crank will likely tolerate the abuse, but the rods and pistons are certainly suspect.
Just make sure to pick the right cam for your blower motor.