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 HydraulicThe 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 OnListed 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 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.
Cam Specs Cam Adv. Dur. @ Lift Lobe Dur. 0.050 Angle Sep. Stock 350 H.O. Int. 320 212 0.435 112.5 Exh. 324 222 0.460 Comp XE268 Int. 268 224 0.477 110.0 Exh. 280 230 0.480 GMPP Hot Int. 279 218 0.525 112.0 Hyd. Roller Exh. 287 228 0.525Note that the stock cam appears to have a longer advertised duration than the Comp Cams. This is merely the difference in where GM decided to establish the opening point of the lobe. If the stock cam were checked at Comp's checking height of 0.006 inch, the duration numbers would be substantially smaller. Also, the Hot cam uses 1.6:1 roller rockers while the other two cams utilized 1.5 rockers.
Power ResultsTest 1 is the best overall power the 350 H.O. made from last month's tests. This configuration included an Edelbrock Performer RPM Air Gap manifold, Holley 750-cfm 0-4779 mechanical secondary carburetor, Hedman 151/48-inch headers, and Borla Pro XS 211/43-inch muffled exhaust. Idle vacuum measured 16.5 inches at 850 rpm.
Test 2 charts the results from the Xtreme Energy 268 hydraulic flat-tappet cam. While most longer-duration cams usually sacrifice torque at the bottom to gain peak hp increases, this combination improved everywhere over the smaller stock cam. The 268 cam also extended the power up to 6,000 rpm where the smaller cam fell off radically after 5,600 creating a wider 1,400-rpm power band between 3,800 and 5,400. Idle vacuum measured 14 inches at 900 rpm.
Test 3 illustrates the advantages of a hydraulic roller camshaft with its more aggressive lift curve. The roller offers shorter duration at 0.050 yet makes significantly more power than the flat-tappet cam making an honest 400-plus hp with stock port Vortec iron heads. This mild cam makes for a great everyday driver combination with 15 inches of vacuum at idle at 900 rpm.
RPM TEST 1 TEST 2 TEST 3 DIFF TQ HP TQ HP TQ HP TQ HP 2,400 357 163 366 167 - - - - 2,{{{600}}} 367 182 372 184 385 191 18 9 2,800 374 199 377 201 392 209 18 10 3,000 382 218 385 220 399 228 17 10 3,{{{200}}} 392 239 397 242 409 249 17 10 3,400 400 259 408 264 419 272 19 13 3,600 {{{405}}} 278 414 284 425 291 20 13 3,800 407 295 415 {{{300}}} 425 307 18 12 4,000 408 311 414 315 425 {{{323}}} 17 12 4,{{{200}}} 409 327 413 331 428 342 19 15 4,400 406 340 412 345 425 356 19 16 4,{{{600}}} 402 352 408 357 420 368 18 16 4,800 394 360 399 364 415 380 21 20 5,000 385 366 392 373 407 387 22 21 5,200 375 371 379 375 397 393 22 22 5,400 361 371 366 377 385 396 24 25 5,600 346 369 352 375 375 401 29 32 5,800 - - 336 371 360 397 - - 6,000 - - 327 371 338 386 - -Parts ListAll prices were taken from GM Performance Parts dealer Scoggin-Dickey Parts Center in Lubbock, Texas. We've listed a few other GM Performance Parts dealers that carry these parts. We've also listed some parts like the roller lifters and the valvesprings part numbers separately in case you need separate parts. Keep in mind that the overall Hot cam kit comes with the cam, rocker arms, springs, retainers, keepers, and locks. All you have to add are the roller lifters and the pushrods. The Comp Cams prices were pulled from www.summitracing.com. All prices are accurate for December 1, 2003.
Component Source PN Price 350 H.O. short-block GMPP 12556121 $1,500 Long-block, 350 H.O. GMPP 12486041 2,{{{300}}} Deluxe 350 H.O. engine GMPP 12496968 3,485 Hyd. roller Hot cam kit GMPP 12480002 500 Hyd. roller lift kit GMPP 12371042 {{{200}}} Pushrod kit for hyd. rollers GMPP 12371041 30 Cam retainer plate GMPP 10168501 8 Roller rocker arms, 1.6:1 GMPP 12370839 280 Valvesprings, LT4 style GMPP 12495494 33 Comp Cams 268H Xtreme hyd. Comp Cams 12-242-2 98 Comp Cams beehive springs Comp Cams 26918 NA Comp Cams beehive retainers Comp Cams 774 NA Timing set, roller cam Comp Cams 3136 74