It’s time for egghead theory and speculation to put up or shut up. Elsewhere in this issue, you’ll find pages upon pages of scintillating tech theory explaining the fundamentals of camshaft dynamics. Even so, how factors like lift, duration, and lobe-separation angle should affect the power curve are often quite different from how they actually impact engine output in real life. Techno babble is fine for message board know-it-alls, but the ultimate truth serum for any camshaft is the liquid that flows through a dyno’s water brake. As engine expert and PHR contributor David Vizard often says: “I don’t have an opinion, I have a dyno.” With the help of the School of Automotive Machinists (www.samracing.com), we tested three of COMP Cams’ most popular small-block Ford hydraulic roller grinds on a street 347 stroker to gauge how different lobe profiles impact engine output and driveability characteristics in the real world.
To maximize the relevance of our quaint dyno thrash, we needed a test engine that represented what a large cross-section of the hot rodding public would build. As such, our test subject is about as mainstream as you can get, a 302-based small-block Ford stroked to 347 ci with a projected output of 450 hp. Living in magazineland for too long would lead you to believe that everyone is building 900hp big-blocks or twin-turbo LS1s, but that simply isn’t the case. Not only is a more pedestrian small-block like our 347 much easier on the budget, it’s also more representative of the kind of motor found in your typical street machine. Furthermore, since engines are merely glorified air pumps, the results gleaned from this cam test could just as well apply to a similarly spec’d 350 small-block Chevy.
The short-block specs on our test mule are rather ordinary, featuring a factory block bored to 4.030 inches, an Eagle 3.400-inch steel crankshaft and rods, and pump-gas–friendly 9.6:1 JE pistons. The 347’s primary breathing apparatus, on the other hand, is quite a bit more impressive. Using this dyno session as a good excuse to test out the newest addition to the Brodix SBF cylinder head catalog, we ordered a set of its new LH-series 17-degree aluminum castings. With ports designed by famed Ford engine builder Keith Craft, the heads flow an advertised 300 cfm at .700-inch lift. Feeding the 195cc ports is an Edelbrock Victor Jr. intake manifold and a Holley 750-cfm carb.
Two of COMP Cams’ most popular SBF camshafts are PN 35-518-8 and PN 35-522-8. Both are Xtr
A Tale of Three Cams
As no surprise, a quick call to COMP Cams’ Trent Goodwin confirmed that the most popular grinds in the SBF catalog are relatively mild designs that offer a great balance between sub-6,500 rpm power and low-speed driveability. The challenge was picking three cams that were different enough to show a measurable difference on the dyno, yet similar enough to where variations in horsepower and torque output could be pinpointed to specific differences in their lobe profiles. The smallest of the lot, which we’ll call Cam A for the sake of simplicity, is a 224/232-at-.050 unit with .555/.565-inch lift and a 112-degree lobe-separation angle. Cam B, the largest of the bunch, features 232/240 degrees of duration at .050, and .565/.574-inch lift, also with a 112-degree LSA. The wild card of the bunch is Cam C, which measures in at 227/241 degrees of duration at .050, with .531/.515-inch lift and a 107-degree LSA. As one of COMP’s Thumpr grinds, Cam C was designed with a lopey, aggressive-sounding idle in mind, and as such, it has a tighter LSA and an earlier exhaust valve opening point.
Based on specs alone, Cam B would seem to have the advantage on paper since it packs the most duration and lift, but that’s not how things played out on the dyno. Cam A was good for 437 hp at 6,000 rpm, and 409 lb-ft of torque at 4,900 rpm. As expected, Cam B improved upon those figures dramatically, kicking out 464 hp at 6,200 rpm and 426 lb-ft of torque at 5,100 rpm. Longer-duration cams typically sacrifice low-end torque for top end power, and predictably, Cam A held an advantage of 5 to 10 lb-ft of torque from 3,600 to 4,200 rpm. After that point, Cam B pulls ahead big time, holding a 15-25hp advantage throughout the rest of the power curve.
While the SAM crew slid Cam C into the block, since its duration specs fell in between Cams A and B, we expected horsepower output to fall somewhere in the middle as well. Boy, were we wrong. Cam C proved to be the most potent of them all despite giving up 5 degrees of intake duration to Cam B, and .034- and .059-inch of lift on the intake and exhaust valves, respectively. In fact, the Thumpr produced more torque down low than the smallest cam in the test, in addition to posting the highest peak output of 466 hp at 6,300 rpm. Peak output aside, the Thumpr trounced the 232/240-at-.050 grind by a large margin in the area-under-the-curve department, holding a 10-15 hp advantage throughout the majority of the power curve. The Thumpr proved to be the most rev-happy as well, pulling hard to 6,600 rpm, while Cam B was out of breath by 6,200 rpm.
To ensure valvetrain stability and quick-and-easy timing changes, COMP set us up with a se
There is a lot to like about Brodix’s new LH 17-degree small-block Ford cylinder heads (PN
The Brodix castings feature CNC-blended 60cc combustion chambers fitted with 2.020/1.600-i