Testing Three Cams on a 347 Ford - Camshaft Three-Way

Test Mule
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.

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.

Seeking the expertise of someone much smarter than the typical journalist, we ran the results by SAM’s Head Instructor Judson Massingill for a more educated opinion. As it turns out, it’s all about cylinder pressure. “The intake duration of the Thumpr cam falls right in between Cam A and Cam B, so duration isn’t what’s accounting for the power difference. Plus, the Thumpr has the least amount of lift, so you can throw that out of the equation as well,” he says. “What’s going on in this motor is that the tighter LSA and increased overlap of the Thumpr is helping pull more air through the intake valve. The improvements in intake charge filling and cylinder pressure is what’s bumping up both torque and horsepower. Tight LSA cams usually drop off at high rpm, but the reason that’s not happening in this instance has to do with cylinder pressure. On a relatively low compression motor like this 347, you don’t have much cylinder pressure to begin with, so having a wide LSA really hurts you. With a tighter 107 LSA, the increase in cylinder pressure helps make up for the low compression ratio of the motor. If this 347 had a higher compression ratio, the tighter LSA of the Thumpr would have made less of a difference on the dyno.”

Ultimately, no amount of speculating is an adequate substitute for good old-fashioned dyno testing. Since speculation doesn’t always hold up on the dyno, it’s not a bad idea to dyno test multiple cams in your engine combo of choice before deciding on the perfect grind for your application. Compared to what it costs to build a motor, cams are relatively inexpensive, so it’s cheap experimentation.