GM Performance Parts 350 H.O. Crate Engine - Project Humble Pie, Part II

Last month, we introduced you to the GM Performance Parts 350 H.O. crate engine. We began testing using the Deluxe model that comes completely outfitted with a water pump, HEI distributor, spark plugs, and a 600-cfm vacuum secondary carburetor. In search of more power, we added Hedman headers, an Edelbrock Performer RPM Air Gap dual-plane intake, and a 750-cfm Holley double-pumper carburetor.

Those simple bolt-on changes pumped the 350 up from its stock power rating of 330 hp and 380 lb-ft of torque to a stout 377 hp and 415 lb-ft of torque. While we were impressed with the small-block's significant gains, we felt that a cam change would be worth even more. That's what we're going to dive into this month.

Mild Hydraulic

The 350 H.O. engine comes with a mild yet even-balanced dual-pattern flat-tappet hydraulic camshaft. Based on the lift and duration numbers, we expected that even a slightly larger performance hydraulic would make much bigger numbers. The first cam we tried was a Comp Cams Xtreme Energy 262 cam. Our goal with this selection was to see how much power we could make while maintaining near-stock idle quality. Ed Taylor installed the Comp Cams hydraulic and soon had the 350 up and running. While power did improve, the gains were so minimal that we have not listed them in the dyno chart numbers. Instead, we decided to jump right into the next larger cam in the Xtreme Energy stable--the 268.

Again, Ed swapped sticks with the help of a Comp Cams two-piece timing chain cover he installed just for this purpose. If you intend on doing multiple cam swaps, this heavy aluminum unit is an outstanding time-saver. With a couple of hours of labor behind him, Taylor was again making noise, this time with the more aggressive Comp XE268 cam. After a quick 20-minute break-in session and a few pulls to establish the proper timing and recheck the proper air-fuel ratio, Taylor made a series of full-throttle runs to collect the numbers. Even with the much longer duration, the 268 cam made more torque down low than the stock 350 H.O. cam, but not as much as we had hoped. Peak horsepower also picked up 7 more hp than the original cam.

At this point, the little 350 H.O. is making 377 hp and an amazing 415 lb-ft of torque with a relatively mild cam. The larger 268 didn't make dramatic increases in power, which points out just how good the original 350 H.O. cam really is, since these minor increases in power would be difficult to see in the quarter-mile. A longer-duration flat-tappet hydraulic cam would obviously make more peak power but would also sacrifice low-speed torque to accomplish this, so we decided to go a different route and jump up to a hydraulic roller cam instead.

Roll On

Listed in the GM Performance Parts catalog is a relatively obscure hydraulic roller cam package originally designed for off-road LT4 fuel-injected engines that offers a mild intake lobe using stock roller lifters tag-teamed with a set of GM Performance Parts 1.6:1 aluminum roller rockers. We've had previous experience with this cam and decided it was a perfect match for the 350 H.O. engine package and the Vortec heads. The beauty of this system is that you can purchase a complete cam, lifter, pushrod, and rocker arm set from GM Performance Parts that will bolt right into the 350 H.O. because it's a one-piece rear main seal engine fully machined to accept a hydraulic roller cam package. GM Performance Parts chose to originally equip the H.O. with a flat-tappet cam to keep the engine affordable. But we wanted more power, so the swap was a natural.

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 3/8-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.

Conclusion

Right 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.