306 & 347 Engine Comparison: Is A Bigger Engine A Better Engine?
Everyone's Building 347s These Days. Are They Really Better Than 306s?
From the April, 2001 issue of 5.0 Mustang & Super Fords
By Richard Holdener
Photography by Richard Holdener
To see what more displacement...
To see what more displacement was worth, we assembled both a 302 short-block and a 347 short-block and took them to the Westech dyno facility.
Both the 302 and the 347 were...
Both the 302 and the 347 were equipped with the same heads, intake, and carburetor in order to demonstrate the effect of the additional 41 ci. The 302 had previously been bored 0.030 over to reach a displacement of 306 ci. The 347 was likewise bored 0.030 over, a fact that eliminated one variablenamely better breathing from the larger bore unshrouding the valves.
The cam used in both the 306...
The cam used in both the 306 and 347 was from Comp Cams. As both test motors were built using late-model 5.0 blocks, we utilized a hydraulic-roller profile. The Xtreme Energy series was chosen for maximum power and driveability. The dual-pattern XE282HR offered a 0.565/0.574-lift split and 232/240-duration split. Lobe center checked in at 112 degrees. The cam was run with matching 1.6 ratio roller rockers to arrive at the designated lift values. The pistons in both motors were notched in anticipation of the near-0.600 valve-lift numbers.
First on the dyno was the...
First on the dyno was the 306, but not before a few notable modifications. The 5.0/302 short-block was equipped with the stock cast crank, 5.09-inch rods, and factory 9.2:1 forged pistons, but the block deck had been milled to position the pistons 0.005 out of the hole.
We decreased the World Castings...
We decreased the World Castings Windsor heads combustion-chamber size by milling the deck surface of the heads. This raised the compression by one full point, from 9.2:1 to 10.5:1.
The World Castings heads were...
The World Castings heads were also treated to Extrude Hone porting and were equipped with a 2.02/1.60 valve combination. The ported heads flowed 275 cfm at 0.600 valve lift through each intake port and 235 cfm at 0.600 valve lift through each exhaust port. The ported heads offered plenty of flow for both the 302 and the 347. By running the comparison with a ported head, we made sure the head flow would not restrict the larger motor and skew the test results.
In addition to the Extrude...
In addition to the Extrude Honeported World Castings heads, we fit the 306 with an Edelbrock Victor Jr. intake. Since the 306 and 347 would be receiving a healthy Xtreme Energy cam, the Victor Jr. intake got the nod over the dual-plane Performer RPM. Every effort was made to maximize the power potential of the test motors. The Victor Jr. was installed out of the box with no porting or even gasket matching. Topping the Victor Jr. intake was a 650-cfm Speed Demon carburetor. As we saw, the 306 worked best with the smaller 650-cfm carb (the 750 was tried), while the added displacement and power output of the 347 required the larger 750-cfm Speed Demon. Naturally, each motor required minor jetting to maximize power and stabilize the air/fuel mixture during the dyno pulls.
Each motor was also run with...
Each motor was also run with a Wilson Manifolds 2-inch, open-carb spacer between the Victor Jr. intake and Speed Demon carburetor. The spacer added as much as 5 extra horsepower to each test motor, so it was included in the test. The final element in common between each motor was a CSI electric water pump. Neither motor was run with accessories. The electric water pump was used to circulate water through the motor during the dyno pulls, but no alternator, water pump, or power-steering pump was employed for this series of tests.
Each motor was run with the...
Each motor was run with the same 1.6-ratio roller rockers from Comp Cams.
Additional mods to each test...
Additional mods to each test motor included a complete MSD Digital 7 ignition including a billet distributor, plug wires, and a Blaster coil. The high-capacity ignition offered plenty of spark for our hot test motors. Dyno runs were made with different timing, but both motors seemed to run best with 36 degrees of total timing.
We installed the 306 on the...
We installed the 306 on the engine dyno using a set of 1-3/4-inch, full-length Hooker headers. Both motors were run without mufflers or any type of tailpipe but with an 18-inch-long, 3-inch-diameter extension attached to each header collector. The collector extensions offered an increase in low-speed torque production compared to running the open header without restricting the exhaust flow.
The 306 topped the 400hp mark...
The 306 topped the 400hp mark with a peak reading of 404 hp at 6,100 rpm. The little 306 pumped out some impressive torque numbers as well, registering 369 lb-ft of torque at 5,400 rpm. Though hindered by a short stroke, the 306 managed to reach 300 lb-ft of torque production at 3,100 rpm and maintained that figure to the 6,200-rpm shut-off point of our test. At 404 hp, the 306ci test motor produced 1.32 hp per cubic inch. Thats a fairly respectable power output, given the relatively mild combination. Fords comparison, a stock 225 hp 5.0/302, manages only 0.75 hp per cubic inch. Even the Cobras mighty 4.6 Four-Valve motor can muster only 1.14 hp per cubic inch. Producing 1 hp per cubic inch has always been the yardstick by which performance motors are measured. This 404hp 306 showed up fairly well. We were anxious to see how the 347 would fare.
After removing the 306 from...
After removing the 306 from the dyno, we set to work dismantling the test motor in order to swap the performance components onto the 347 short-block. It should be noted here that the 306/347 test did have one variable that obviously skewed the test results somewhat. In building the short-blocks, we had to make a choice between running the same flat-top piston or running different piston configurations in order to maintain the same static compression ratio. Everything else being equal, the added displacement of the 347 would increase the static compression ratio.
Our 347 stroker kit (test...
Our 347 stroker kit (test motor) came from Coast High Performance. The 347 stroker kit included a crank, rods, pistons, pins, rings, and bearings. We opted to keep the flat-top pistons and incur the extra point of compression. Thus, the 347 ran with almost one extra point of static compression. Figure about 3 percent extra power for each compression point, so the additional compression was worth about 10-12 hp according to our calculations. Given that displacement effectively tames the cam specs and the bigger motor really needs more head flow, we werent too worried about the minor compression change hurting the test results.
The heart of the stroker kit...
The heart of the stroker kit was the 3.4-inch stroker crank.
Forged pistons were installed...
Forged pistons were installed on the 5.315 rods.
The large stroke required...
The large stroke required notching the bottom of the cylinder bores on the 5.0 block.
New rings and bearings were...
New rings and bearings were also installed on the 347 test motor.
Once the short-block was assembled...
Once the short-block was assembled and the XE282 Comp cam installed, the World Castings Windsor heads were installed and torqued in place.
Unlike the 306, which ran...
Unlike the 306, which ran best with a 650-cfm Speed Demon carburetor, the 347 required a larger 750-cfm Speed Demon.
Our finished 347 was all ready...
Our finished 347 was all ready for the dyno testing.
We utilized the air/fuel monitor...
We utilized the air/fuel monitor at Westech to ensure a consistent air/fuel ratio. We ran each motor at the same air/fuel mixture to make the test results as accurate as possible. Both motors ran best (on 92-octane) at 13.2:1 air/fuel.
With Westechs Steve...
With Westechs Steve Brule at the helm, the 347 thumped out 464 hp at 5,800 rpm and 440 lb-ft of torque at 5,200 rpm.
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.
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.
Coast High Performance
1650 W. 228th St.
1 Industry Blvd.
Demon Carburetion/Grant Performance Technologies
1450 McDonald Rd.
2700 California St.