Muscle Mustangs & Fast FordsHow To Engine
306 Small-Block Engine vs 347 Small Block Engine
Is Bigger Better? We Test A 306 Against A Like 347 To Find Out.
There is an old saying that states there is no replacement for displacement. Basically this boils down to the simple fact that bigger motors make more power than smaller ones. While this seems logical on the surface, there are a number of considerations when comparing two motors of different displacements.
To facilitate the change in displacement, other changes will be necessary that may (or may not) ultimately affect the outcome. Things like changes in bore and stroke, which alter the bore-to-stroke and rod-to-stroke ratios, can affect the power curve irrespective of the change in displacement. The same can be said for compression ratio, or the changes in piston design or combustion chamber size, to achieve the same static compression. Even the cam timing, head flow, and induction system will operate differently on motors of differing displacements. Thus the change in cubic cinches brings about a whole slew of other changes that can have a sizable effect on power production.
Despite these variables, we decided to forge ahead and run a test of our own by comparing a 306 to a 347 with identical bolt-on components. The 347 is one of the most commonly available stroker assemblies currently offered for the 8.2-inch-deck-height, 302-based motors. In terms of engine specs, the 5.0L (302) achieves its displacement with a combination of a 4.00-inch bore and a 3.00-inch stroke. By comparison, the 347 combines a slightly larger 4.030-inch bore with a 3.40-inch stroke.
Traditionally, motors with larger bores relative to stroke are considered better for high-rpm power, however, proper cam timing, head flow, and the induction system must be chosen to take advantage of the favorable bore-to-stroke ratio. The stroker assembly also alters the rod-to-stroke ratio by changing the length of both the connecting rod and stroke. The typical 5.0L 302 offers a 3.00-inch stroke with a 5.090-inch-long connecting rod. By contrast, the 347 increases the length of both, with a 3.40-inch stroke and a 5.40-inch rod (both supplied by ProComp, in this case). This drops the rod ratio from 1.696:1 for the 302 to 1.588:1 for the 347.
Like the bore-to-stroke ratio, longer rod-to-stroke ratios favor high-rpm power production by minimizing rod angularity and altering the dwell time at and acceleration rate away from TDC. The simple change in displacement from a 302 to a 347 has already produced a combination that (on paper) should favor power production lower in the rev range than the smaller 302. As we shall see, this trend continues with cam timing, cylinder head flow, and even the induction system. But first, let's take a look at what happens to the compression ratio in a stroker.
If we're looking to test the effect of the displacement alone, we need to keep all other variables the same. As we have already discovered, changing the displacement has altered basic ratios associated with changing the bore and stroke, but things like heads, cam, and intake can all remain the same. This is also true of the static compression ratio, though keeping the compression ratio constant between the two different displacements requires changing either the piston or combustion-chamber design. There are other ways to alter the static compression ratio, like changing the deck height (how far down the piston is in the hole) or head-gasket thickness, but these have negative side effects as well.