It's inevitable. As soon as we start to make respectable power with our General Motors Performance Parts 350 H.O. crate engine, all reason goes out the window and we are seduced by the lure of bigger horsepower numbers. The little power devil who sits on our shoulder is the one whispering in our ear, "Go ahead, do it! Plug in that bad-boy roller cam. Go ahead-you'll love it!" Through last month, we had managed to stay on the conservative side of things and we left you with the results of testing a GM Performance Parts Hot hydraulic roller cam with 1.6 roller rockers. That mild hydraulic roller and rocker system made a respectable 401 hp and 428 lb-ft of torque. Keep in mind, this is with a bone-stock set of iron Vortec heads. But we wanted more.
Cam It UpWe started by rooting through the hydraulic roller side of the cam catalogs, but decided that if we were serious about power, the only choice was a mechanical street roller. We talked to the guys at Comp Cams, and for the same price as a standard off-the-shelf cam, they created a cam that spec'd out at 280/288 with 0.538-inch lift with 1.6 rockers, or slightly more lift than the previous GM Hot cam. Unfortunately, even this cam exceeded the capability of the beehive springs, and we ended up using larger dual valvesprings
Converting to a mechanical roller cam also requires a pile of new parts. While all of Comp's Street Rollers now come with an iron gear for the distributor drive, you will still need new lifters, pushrods, roller rockers (if you don't already have them), some type of cam button, a new timing set, and a specific roller cam fuel pump pushrod. We've outlined all the parts necessary for this conversion in the Parts List chart, but some details are worth mentioning.
Perhaps the most important point for any roller cam is ensuring that you limit the cam's fore-aft movement. This requires a cam button placed on the snout of the cam that limits movement through contact with the timing chain cover. We're using one of Comp's cast-aluminum two-piece cam covers, but a reinforced stock cover will also work. We like the rollerized cam button since it is not subject to wear and is also adjustable to accurately set the camshaft endplay.
Using a more aggressive camshaft also meant that we needed a stronger valvespring. We decided to use the Comp 977 spring, which is a larger 1.430-inch diameter dual spring that specs out with 155 pounds of load on the seat at 1.850-inch installed height and 419 pounds at 1.250 (or 0.600-inch lift). This is a good dual spring, designed for lots of rpm yet not so big that it would pull the pressed-in studs out of the heads. This obviously meant that we also had to remove the heads and machine the spring pads from the stock 1.250-inch diameter to the larger 1.430-inch diameter. The best plan would have been to install ARP screw-in studs at the same time we cut the springs pads, but we were in a hurry and didn't make that change-shame on us.
The TestAfter our dyno flogger Ed Taylor completed the cam swap into our GM Performance Parts 350 H.O. engine and as soon as the engine warmed up, he cranked the throttle wide open. Because we added 22 degrees of duration to the intake side, we expected to lose some low-speed torque, which did occur between 2,600 and 3,600 rpm. Comparing the torque to last month's best run using the much shorter-duration GM Performance Parts Hot hydraulic roller cam, the torque loss did occur. But by 4,000 rpm, we were gaining power with a 420 peak horsepower number compared to last month's 401. With the larger cam, we didn't expect peak torque to increase by much, and it didn't. But what was surprising was that the larger cam's peak torque occurred at 4,400 rpm while the much smaller Hot cam's peak torque chimed in at 4,200. We expected the peak torque to be at a slightly higher rpm, but for a street car this is still very manageable.
But we don't want to muddy the waters too much here. This is a big cam for a 9:1 compression 350ci small-block. The key indicator here is idle vacuum, which measures a measly 7 inches at 850 rpm. This compares with more like 11 inches of vacuum with the milder hydraulic roller combination. What this max throttle dyno test doesn't illustrate is how crisp (or soggy) the engine would be at part throttle. While the idle vacuum numbers indicate a weak part throttle, the vacuum numbers are actually better than we anticipated given the cam's long duration numbers.
Frankly, we were hoping to see 430 with a cam change, which turned out to be optimistic. As it was, we saw a solid 19hp increase, and had we spent another couple of days flogging rocker ratios, lash changes, and perhaps a single-plane intake manifold, it's possible we could have achieved our goal.
ConclusionWith this test, we've now tested this same long-block combination with four different camshafts with the same heads, intake, and carburetor combination. The beauty of this is that you can see how the additional cam timing affects the power curve. Big cams basically move the torque curve up in the rpm band, hurting the bottom-end torque while improving top-end horsepower. It's as simple as that. But for a street engine that spends most of its time at part-throttle, a big cam may not always be the best choice. But with all these tests, you now have a better idea of the style of cam that is right for your next small-block buildup.
Power CurvesTest 1 is the maximum power generated last month using the GM Performance Parts Hot hydraulic roller cam and 1.6:1 GMPP roller rockers. This test also used an Edelbrock Performer RPM Air Gap intake, Holley 0-4779 750-cfm carburetor, an HEI distributor, a set of 151/48-inch Hedman headers, and a pair of 211/42-inch Borla Pro X/S mufflers.
For Test 2, we swapped in the custom Comp Cams 280/288 Street Roller, lifters, and rocker arms as well as a stronger MSD-6A ignition system with 36 degrees of total timing. The mechanical roller camshaft offered 22 degrees more duration at 0.050-inch tappet lift, which is why we lost torque below 3,600 rpm. Idle vacuum is 7 inches at 750 rpm.
RPM TEST 1 TEST 2 DIFF TQ HP TQ HP TQ HP 2,{{{600}}} 385 191 379 188 - 6 - 3 2,800 392 209 388 207 - 4 - 2 3,000 399 228 398 227 - 1 - 1 3,{{{200}}} 409 249 408 249 - 1 - 3,400 419 272 419 271 - - 1 3,600 425 291 425 291 - - 3,800 425 307 428 309 + 3 + 2 4,000 425 {{{323}}} 429 327 + 4 + 4 4,{{{200}}} 428 342 432 346 + 4 + 4 4,400 425 356 433 363 + 8 + 7 4,{{{600}}} 420 368 429 375 + 9 + 7 4,800 415 380 426 389 +11 + 9 5,000 407 387 420 400 +13 +13 5,200 397 393 411 407 +14 +14 5,400 385 396 401 412 +16 +16 5,600 375 401 392 418 +17 +17 5,800 360 397 380 420 +20 +23 6,000 338 386 368 420 +30 +34 6,200 - - 355 419 - - 6,400 - - 339 412 - - Avg. 401 326 403 342Cam SpecsWe've included the specs on the stock 350 H.O. cam, the GMPP Hot hydraulic roller, and the custom Comp 280 mechanical Street Roller. It's not accurate to compare advertised duration flat tappet or hydraulic roller specs to mechanical roller cams, but at least the duration at 0.050 numbers can be used. Keep in mind that the mechanical roller lash must be subtracted from the valve lift specs to get an accurate idea of net valve lift for mechanical roller cams. We've included those net lift numbers in parenthesis.
Cam Adv. Dur. @ Lift Lobe Sep. Dur. 0.050 Angle GMPP Hot Hyd. Roller Int. 279 218 0.525 112.0 Exh. 287 228 0.525 Comp XR 280R Xtreme St. Roller Int. 280 {{{240}}} 0.504 (0.488) 110.0 Exh. 288 248 0.504 (0.488) w/1.6 rockers 0.538 (0.522) Lash-0.016 intake and exhaustParts ListThe GMPP prices listed here were generated through Scoggin-Dickey while the remaining components were priced through www.summitracing.com.
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 Performer RPM Air Gap Edelbrock 7516 220 Carb, 750 mech. sec. Holley 0-4779 325 Headers, 151/48-inch, four-tube Hedman see catalog NA Comp XR280R, roller cam Comp 12-771-8 {{{240}}} Comp mechanical rollers Comp 888-1 473 Comp XR280R cam and kit Comp K12-771-8 874 Pro {{{Magnum}}} rail rockers, 1.6 Comp 1318-16 325 Pushrods, Hi-Tech Comp 7993-16 130 Valvesprings, dual Comp 977-16 132 Retainers Comp {{{740}}}-16 60 Keepers Comp 611-16 26 Roller thrust button Comp {{{200}}} 10 Roller timing set Comp 3100 50 Fuel pump pushrod, bronze tip Comp 4607 14
