The Hot hydraulic roller cam actually comes in several different configurations. You can purchase the cam separately or in a kit complete with the springs, retainers, keepers, and rockers. This is the kit we opted for along with the roller lifter kit that included all the retainers and guides, plus a separate retainer plate to locate the cam in the block. All these parts are necessary to install the roller cam in the 350 H.O. block.
We also included a couple of other revisions to this combination by using a set of Comp Cams beehive style valvesprings instead of the LT4-style springs included in the kit. The main reason for this is that the beehive springs will bolt right in place of the stock springs, while the larger 1.32-inch-diameter LT4 springs require significant and expensive machining to the valvespring pockets. However, we did have to remove the heads to elongate the pushrod holes on the inboard side to prevent rubbing the pushrods on the heads while using the 1.6 roller rockers. The high-ratio rockers move the pushrod cup closer to the trunion, which moves the pushrod closer to the end of the pushrod slot in the Vortec heads, necessitating the light drilling with a 31/48-inch drill bit.
Taylor performed all these basic modifications to the 350 H.O. and then bolted our freshly upgraded small-block back on the dyno to evaluate the results. Roller cams do not require the break-in period that flat tappet cams do, so once Taylor had normalized temperatures, we started beating on the 350 once more. With the timing and jetting experiments accomplished, the combination of the roller cam and its additional rocker ratio revealed that these changes added significant power, making 401 hp at 5,600 rpm. This test illustrates that Vortec heads are certainly capable of a streetable 400 hp as well as excellent low-speed and midrange with 428 lb-ft at 4,200. With 15.5 inches of manifold vacuum at a 950 rpm idle, this is a very streetable package.
Once we began reviewing all the numbers, it seemed curious that the Comp Cams 268 Xtreme Energy cam didn't fare better. After looking closely at the cam specs, we noticed that both the stock 350 H.O. cam and the Hot hydraulic roller cams offered a 10-degree longer exhaust duration compared to the intake while the Xtreme Energy cam only offered 6 degrees of separation. We know that the stock exhaust port on the Vortec heads is weak in comparison to the intake, which might explain why the factory H.O. and the Hot cam performed as well as they did, especially at higher engine speeds. We think we could have made more power with a 268 Xtreme cam with an exhaust lobe measuring around 234 degrees at 0.050. The closest Xtreme exhaust lobe is the one used on the Xtreme Energy 274H cam at 236 degrees. Unfortunately, we didn't have time to test this concept. Also note that both GMPP cams used a slightly wider lobe separation angle of around 112 degrees.
ConclusionRight out of the box, the GM Performance Parts 350 H.O. is an outstanding street engine, and while the cam specs may look conservative, with the addition of a larger 750-cfm carburetor and an Edelbrock Performer RPM Air Gap intake, we've shown that this package is capable of an outstanding 370 hp and almost 410 lb-ft of torque. Add the GM Performance Parts hydraulic roller Hot cam and package, and this rascal will thump with a reasonable 400 hp and do it on pump gas. While we could always add more camshaft, we're heading toward a little different tack, but we'll save that for a later issue.