Chris Hemer
February 1, 2008
Photos By: From the 5.0 Mustang archives

If you're considering extra cubes, then chances are you're wrestling with a fairly common dilemma: Do you build a 347 out of your 5.0 or step all the way up to a 351? Perhaps the following info will help you decide.

351W: Advantages
* Strength: The 351W block is stronger than the production 5.0 by a long shot. Thicker walls, a 3-inch main, and 2.311-inch rod journals (versus 2.248/2.123 for the 5.0) are contributing factors.

* Taller Deck: The 351W features a deck height of 9.503 inches versus the 5.0's 8.206 inches. This means a longer rod can be used for even more cubes--up to 435 with a production two-bolt block, and 454 cubes with a four-bolt FRPP block, according to George Klass at Coast High Performance.

* Rod-to-Stroke Ratio: In stock configuration, the 351 has a better rod-to-stroke ratio than a 347 (1.70:1 for the 351W versus 1.58:1 for the 347) by virtue of its longer rods (5.956 versus 5.400 for the typical 347 rod). The 351 also features 1/2-inch head bolts instead of the 302 block's 7/16-inch bolts.

351W: Disadvantages
* Size: The 351W is 2.250 inches wider than a 302, necessitating a number of changes (at additional cost) to make the swap possible. Hood clearance can become an issue, and there will be less room to service the plugs.

* Weight: As discussed in the Keep the Muscle, Lose the Fat sidebar, the 351W is beefier and is typically more than 100 pounds heavier than a 302-based engine.

347: Advantages
* 302-Based: The 347 is created when a 302 block is bored 0.030 and fitted with a 3.400-stroke crank and custom rods/pistons. This means a 347 has the extra cubes you desire, yet it can still use the same headers, manifolds, brackets, and so on as a stock 302.

* Weight: The 302-based engine is more than 100 pounds lighter than the 351 in stock form, and it can be made downright feathery with a few aluminum components.

347: Disadvantages (and perceived disadvantages)
* Limited Growth: While 347 ci is certainly a respectable number, it's the practical displacement limit for a two-bolt-main production block.

* Limited Strength: A two-bolt-main production block is typically capable of withstanding up to 600 hp, and that's with a girdle, studded mains, and so on. Even a mildly built 347 with a supercharger can bust that figure. An R302 block will solve the problem, but there goes your budget.

* Poor Rod-to-Stroke Ratio (perceived): There has been a lot of talk about the 347's rod-to-stroke (R/S) ratio. Simply stated, the R/S ratio is the length of the connecting rod (center-to-center) versus stroke of the engine. A higher ratio means the piston stays at top dead center longer, promoting better combustion and, theoretically, more power. Compared to the 351W's 1.70:1 R/S ratio, the typical 347's R/S ratio of 1.58:1 doesn't look good, but it's actually better than a lot of other noted performance engines, including the 454 big-block Chevy (1.53:1) and 400 small-block Chevy (1.48:1). Even the legendary 428 CJ was only marginally better than a 347 at 1.63:1. Unless you're building an engine to compete with Billy Glidden, R/S ratio really doesn't add up to much in an otherwise well-built engine.

* Oil Burner (perceived): Piston design is critical to the success of any 347 kit, according to George Klass at CHP. Trying to improve upon the 347's R/S ratio only moves the pin further up into the piston. If the pin is moved up into the oil ring land, the top of the pin will be located above the oil ring, allowing more oil to get past the oil rings and into the combus- tion chamber. This is how the 347 got a reputation as an oil burner. However, George says, many kits--including CHP's--place the pin below the oil ring, so oil consumption is not a problem.

Cost Factor
The next thing you'd probably like to know is, which would be cheaper, a 347 or a 351W? As discussed, the 351W requires numerous extra parts to accomplish the swap, while the 347 doesn't. However, depending on what 347 kit you purchase, the initial short-block may be more expensive than a 351W. Since we can't know what combo you have planned, the best idea is to add up the cost of a 351W, factor in the extra parts, and then compare that to the cost of the 347, keeping in mind both engines can use the same heads. Our guess is that the 347 will probably be less, but depending on how serious you plan to get, a 351W swap could be equal to, or less than, the cost of a truly serious 347.