Richard Holdener
August 11, 2010

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.

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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.

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Dyno Time
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?

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

306347Difference
RPMHPTQHPTQHPTQ
3,0001893312133732442
3,5002333502674013451
4,0002873773264283951
4,5003283833734354551
5,0003763954084283233
5,5003993814374173836
6,0004113724563994527
6,5004123324493633731

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.