Holley supplied a 750 HP series...
Holley supplied a 750 HP series carburetor for our testing. The 750 was sized perfectly for both the 306 and 347.
First up on the dyno was the 306. Since both the 306 and 347 had run previously, there was no need for any break-in procedure. After minor jetting and an 8-degree timing sweep (from 28 degrees to 36 degrees), we were rewarded with peak numbers of 412 hp at 6,500 rpm and 396 lb-ft at 4,900 rpm. Torque production exceeded 350 lb-ft from 3,500 rpm to 6,100 rpm. We were thankful the Track Heat heads had plenty of valve spring pressure to allow our hydraulic roller motors to rev cleanly to 6,500 rpm.
As expected, the 347 offered not just more power, but more power everywhere, from 3,000 rpm to 6,500 rpm, but the power curves deserve further scrutiny. After tuning, the 10.0:1 347 offered 456 hp at 6,000 rpm and 435 lb-ft of torque at 4,500 rpm. Note that both of these peaks occurred lower in the rev range than the smaller 306. While the power curve of the 347 rolled over past 6,000 rpm, the 306 continued to climb right to 6,500 rpm. Effectively, the heads, cam, and intake represented a slightly wilder (though still perfectly streetable) combination on the 306 than the larger 347.
Before we sign off, let's take a closer look at the results and apply some simple math to see how the 347 really compared to the smaller 306. Simple math tells us that the 306 produced a specific output of 1.346 hp per cubic inch. By contrast, the 347 checked in at a slightly lower 1.314 hp per cubic inch. Why the drop in specific output despite the increase in peak power?
We ran both motors with a...
We ran both motors with a set of 13/4-inch Hooker headers feeding 18-inch collector extensions.
The culprit is probably the camshaft. The XE274HR was an excellent choice (who would argue with 456 hp from their 347?), but in terms of specific output, the larger motor actually requires wilder cam timing to keep pace with the smaller one. The 347 requires cam timing that will allow it to produce peak power near 6,500 rpm (like the 306). The smaller of the two XFI stroker cams offered by Comp Cams (0.579 lift, 236/248 duration, 114 LSA) would be ideal on the 347 if you are looking to improve the specific output. If the 347 matched the specific output of the 306, you'd be looking at 467 hp. The benefit of running the XE274HR cam in the larger 347 is (of course) improved idle quality and driveability, though it works great even in a low-compression (supercharged) 302.
Though the 347 lost out to the 306 in terms of horsepower per cubic inch, it fared much better in the torque department. Producing 395 lb-ft, the 306 checked in at 1.287 lb-ft/cid, while the larger 347 offered 1.253 lb-ft/cid with its 435 lb-ft. The reason the specific torque production was so much closer was that the milder cam timing did not hurt the 347 (to the same extent) lower in the rev range, where peak torque occurs. In fact, the 347 matched the specific torque output of the smaller 306 up to roughly 4,500 rpm, where the (relatively) milder cam timing started to make itself known.
It must also be pointed out that the dual-plane intake will alter the effective operating range of the motor when the displacement changes. Having exceeded 500 hp with this ProComp manifold, we know it offers the airflow necessary for our combos, but the combination of runner length and cross section were tuned to a specific operating range based (to some extent) on the displacement of the motor.
This test once again proved the old adage that bigger really is better, but so too did it illustrate that bigger motors actually need bigger cams, heads and intake manifolds to maintain a specific output. The choice (as always) comes down to where in the rpm range you want your power to be.
Effect Of Displacement: 306 Vs. 347
The power numbers demonstrated that the 347 offers more power throughout the tested rev range. The 306 produced 412 hp and 396 lb-ft of torque, compared to 456 hp and 435 lb-ft for the 347. Despite the lower peak numbers, the smaller 306 was king in terms of specific output, with 1.346 hp per cubic inch (compared to 1.314 for the 347). The 347 actually matched the specific output of the 306 up to 4,500 rpm, but requires slightly wilder cam timing to escalate the power production higher in the rev range.