Supercharged Chevy Small Block - Boosted Bumpsticks

We Compare Cams On A Blown & N/A Small-Block.

Richard Holdener Aug 1, 2009 0 Comment(s)
Sucp_0908_01_z Supercharged_chevy_small_block Cam 2/26

Choosing the right cam for your supercharged small-block makes all the difference in the world.

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.

Sucp_0908_03_z Supercharged_chevy_small_block Long_block 3/26

You'll be able to read about the build-up of this low-buck long-block in a future issue.

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.




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