306 & 347 Engine Comparison: Is A Bigger Engine A Better Engine?

Though the edict "there is no replacement for displacement" is probably as old as the internal-combustion engine itself (no doubt coined by a racer), the basic philosophy goes back even further. At the root of the internal combustion-oriented phrase is the simple idea that bigger is better. Long before any racer smashed a gas pedal in anger and came back to the pits asking the crew chief for more power, bigger things have somehow been better than little ones.

The bigger-is-better theme is not one concocted by humans, as nature is filled to the brim with examples, some dating back well before humans entered the picture. Larger trees get more rain and sunshine, larger dinosaurs ate smaller ones, and larger cave men beat up smaller ones for their pick of cave women.

Like it or not, "bigger is better" is one of the laws of nature.

This natural law carried over to horsepower once we humans put down our stone tools and began working with steel. That a bigger engine can make more power than a smaller one should not be any great revelation. The big three recognized this, and each came out with its version of the big-block for applications that required more power than their existing small-block motors could reliably produce. When a 5.0L V-8 wouldn't get the job done, one of the best options at the time was to replace it with a 7.0L V-8.

In fact, this was the basic philosophy that produced the musclecar era. Take a (relatively for the time) lightweight, midsized sedan usually powered by a small-block V-8 and add a more powerful big-block V-8. In the case of Ford, many 302- and 351-powered machines received 427s, 428s, or 429s. The added power offered by the big motors transformed the ordinary performance machines into extraordinary ones.

Though bigger is better still applies to engine displacement and power potential, a number of other variables must now be considered. The reason we don't see any more big-block Mustangs or Camaros is that though powerful, the big-blocks of yesteryear were simply not efficient. In the late '60s and pre-gas-crunch early '70s, it was OK for a motor to be thirsty as long as it made lots of power. While offering plenty of horsepower compared to a similar-era small-block, most big-blocks were somewhat less than fuel efficient, and they certainly weren't environmentally friendly. Single-digit fuel mileage might have been acceptable back then, but it isn't now--especially when it comes with excessive emissions. Making 400 hp is great, but doing so with 7 liters of displacement is simply not efficient, not in light of the current small-displacement motors being offered by the big three.

Why the trip down big-block memory lane when the focus of this article is on building a bigger small-block motor? The big-block lesson is important here, as the buildup and subsequent testing of the stroker small-block was done for a number of reasons. A stroker is obviously built to make more power, but the comparison between a 302 and 347 was also designed to test efficiency. Where the big-block motors were somewhat less than efficient in power production per displacement, our 347 stroker was anything but. Our dyno testing would demonstrate that not only did the stroker offer more power, but it also offered power at an identical efficiency rate as the smaller 302.

Confused? Stick with us, as all will be explained in detail.

The premise of these dyno tests was quite simple. What is displacement worth in terms of power? More specifically, what effect does adding additional cubic inches to a motor combination have when all the other variables are constant? The variables we're referring to for this test included the cylinder heads, the camshaft, the intake manifold, the carburetor, the headers, the ignition, and the total timing. The comparison was run between a modified 306 (0.030-over 302) and a 347 stroker from Coast High Performance. We fit the 306 with a number of aftermarket components that increased the power substantially over the stock output. These same components were then removed and installed on the 347 to determine the power output with the added displacement.

As it turned out, the test was a success, as the 347 responded well to the performance components used on the 306. As expected, the 347 made not only more peak horsepower and torque, but it also made more power everywhere. The longer stroke really added torque down low, where the 347 registered 360 lb-ft to the 306's 288 lb-ft. The 347 maintained the 70-80 lb-ft advantage until 5,400 rpm. Peak horsepower was up to 464 hp at a lower 5,800 rpm. The added displacement reduced the peak-power engine speed by 300 rpm.

This is a perfect example of the greater displacement taming the cam specs and requiring more total airflow to support the additional power output. The power chart clearly illustrates that increased displacement produced a big jump in power all the way through the rpm. It was necessary to swap out the 650-cfm Speed Demon carburetor in favor of the larger 750-cfm model on the 347. While the 302 made best power with the smaller 650-cfm carburetor, the 347 required the larger model to reach the 464hp level.

Obviously the heads, intake, and carburetor worked well on the 347, as the efficiency actually increased slightly to 1.34 hp per cubic inch. Want to know what a 347 stroker would be worth with your current 302 combination? Simply take your current power output and divide it by the displacement to figure the efficiency. If your 302 produces 350 hp, that's 1.16 hp per cubic inch. Take that number and multiply it by 347 ci and you get just a tad more than 400 hp. Even more important than the extra 52 hp is the 60-plus lb-ft of torque the stroker will give you down low. If you have the right combination of components, a 347 stroker makes for a great street/strip combination.

Horse Sense:
The author ran a similar rear-wheel comparison test between a 347 and a 302 using the stock 5.0 (EFI) components on a Dynojet. In that test, the 347 short-block was the only change--the heads, intake, and other power components remained stock. Installation of the 347 netted only an additional 20 hp and 40 lb-ft of torque. The reason for the minimal power gain in that test was that the 302 short-block was already restricted by the stock induction and exhaust components. Adding the 347 stroker short-block only compounded the problem.