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306 Small-Block Engine vs 347 Small Block Engine
Is Bigger Better? We Test A 306 Against A Like 347 To Find Out.
For our two test motors, we were looking to keep the static compression ratio at 10.0:1. This required a change in either combustion chamber or piston volume of roughly 9 cc. Thus, keeping the static compression ratio the same actually meant altering yet another variable (albeit a minor one compared to the change offered by compression ratio). In the end, we assembled both motors with (4.030 bore) flat-top pistons and milled the Twisted Wedge heads to achieve the desired compression ratio. The reason for the rise in compression in a stroker that uses the same heads is increased displacement without increasing the combustion chamber size. So, more air and fuel can be compressed into the same area in the chamber.
With our short-blocks, both sporting 4.030 bores and 10.0:1 compression, it was time to top them with suitable heads, cam, and intake. To keep things simple, we ran these motors carbureted, and made sure to choose heads and a cam that are suitable for both. In terms of camshafts, obviously a stock 5.0L cam favors the smaller 306, while a wild roller cam might work best in the larger 347, so we were forced to choose a cam suitable for both displacements. We went with one of the author's favorite 5.0L cams, the XE274HR cam from Comp Cams, which offers a 0.555/0.565-lift split, a 224/232-duration split at 0.050, and a 112-degree lobe-separation angle.
Since displacement tames the cam timing, this cam was effectively smaller on the 347 than the 306. This means things like idle vacuum and carburetor signal will be greater in the 347 compared to the 306, despite sharing the same camshaft. By contrast, the same cam will make peak power higher in the rev range on the smaller 306, all things being equal. This (as always) assumes the head flow and intake/carb combo will support the intended power output and rpm range.
As with wilder cam timing, increased displacement can be combined successfully with cylinder heads featuring increased port volume. We demonstrated previously that just about any set of aftermarket heads will show marked improvements over the stock 5.0L heads. Knowing the production E7TE heads will be a major restriction even on the smaller 306, we chose to top our test motors with a set of Trick Flow Twisted Wedge Track Heat 185 heads.
The Track Heat 185 heads featured full CNC porting to significantly improve the flow rate of the as-cast heads. Though the peak flow numbers (301 cfm intake, 231 cfm exhaust) looked impressive, every bit as important are the low and mid-lift flow numbers. The 185cc intake ports are small enough to work well on the 306, yet large enough for the 347.
Remember, back in 1969-1970, Ford saw fit to top the diminutive Boss 302 with what were basically 351 Cleveland 4V heads that featured intake port volumes of 248 cc. Though the Boss 302 was often criticized for offering little or no torque down low, recent testing demonstrated that it offered every bit as much low-speed torque as the Chevy DZ302 (which ran significantly small intake ports). We ran heads on a 5.0L application that exceeded 200 cc and they offered better power than the stock E7TE heads, even down at 2,500 rpm. The point here is don't get too wrapped up in port volumes (but do look for a test on that subject in the very near future).
To match the compression ratio on the two motors, we had to run two pair of identical Track Heat heads. The single difference between the heads is that one set (for the 347) featured 66cc combustion chambers, while the set for the 306 retained the 57cc chambers. The heads were teamed with a dual-plane Qualifier Plus intake manifold from ProComp. The high-rise aluminum intake features an air gap that separates the runners to cool the charge air for improved power. Testing has shown that this intake works well on both 306 and 347 street/strip applications where peak power comes below 6,500 rpm.
The ProComp intake was teamed with a Holley 750 HP carburetor. Naturally, the jetting was optimized for each combination. It is worth mentioning that the carburetor size was more than adequate for both combinations, but care must be taken when comparing major differences in displacement, as the additional power will require a larger carburetor. Additional components employed on our pair of test motors include an MSD billet distributor and wires, 13/4-inch Hooker headers, and a Meziere electric water pump. Both motors were run with Lucas 5W-30 synthetic oil, Denso Irridium plugs, and a K&N oil filter.
The water, oil, and air temperature were equalized, as were the timing and air/fuel curves. It bears mentioning that a stroker motor may respond to different timing values than the 302, but both motors ran best with 34 degrees of total timing.