Displacement Replacement: Swapping Your Mustang's 5.0L for a 351W

Since we're assuming you've started out with a new or rebuilt 351 short-block, we're not going to talk about camshaft or timing-chain choices, but there is one important detail that bears mentioning. Early 351W blocks are not designed for use with roller hydraulic cams. This leaves you with two choices: Run a flat-tappet cam, or make some changes so you can run a hydraulic roller. "We find that about 50 percent of our customers stick with the flat-tappet cam," George Klass says. "But that limits you to less-aggressive profiles because of the ramp rates [of a flat-tappet grind]." George says if you want to use a hydraulic-roller cam, you can use stock 5.0 roller lifters as long as you run a small-base-circle cam (also known as a retrofit cam), available through a variety of cam manufacturers.

The next step up the ladder is the cylinder head choice. There's no difference here--in aftermarket heads, that is. Any popular aftermarket head designed for the 302 will also fit the 351W, provided the bolt holes in the heads are stepped and can be drilled out for 1/2-inch fasteners. Using junkyard heads and then porting/rebuilding them is another option, but this won't really save you much money over a set of assembled aftermarket heads unless you do most or all of the work yourself. Junkyard heads also won't flow as well as a pair of aftermarket heads.

Again, because the 351W has taller decks than the 302, this places the cylinder heads further apart, which precludes the usage of a fuel-injected 302 lower manifold or any 302 intake manifold for that matter. If you're going with fuel injection, you have only a few manifold choices, according to Ed Marsh at Windsor-Fox Performance Engineering. You can use the FRPP GT-40 setup used on the Ford Lightning or the Lightning lower and a Cobra upper. Other options include Edelbrock's 351W truck lower (PN 3884) with a 5.0 passenger car upper (Performer PN 3822 or Performer RPM PN 7125), and Trick Flow's 351 EFI Manifold (PN 5150000-4), which includes the upper and lower (also available separately). Yes, the 351W truck upper will bolt up to the engine, but it's not a performance piece and hood clearance problems would be extreme, Ed explains.

For racing purposes, Coast High Performance offers its 351W EFI Spyder intake, which is a four-barrel Victor Jr. manifold with 1/2-inch fuel rails and a 90-degree Power Elbow that accepts up to a 90mm throttle body.

Now that we've talked about what's necessary to bolt the 351W into the engine bay of your Mustang, we'll discuss the stuff you're gonna bolt to the engine--namely the accessories and the exhaust system.

Your stock 302 power-steering and air-conditioning (if so equipped) brackets will not work on the 351W, but there's good news. FRPP offers a 351W Engine Swap Accessory Drive Kit for the '85-'93 Mustang that is available with the power-steering bracket only (M-8511-A351) or with both the power-steering and air-conditioning brackets (M-8511-B351).

Whatever exhaust system you currently have will work, but the headers you have (factory or otherwise) will not, again because the 351W is so much taller. MAC offers short-tube 351W swap headers (PN E358692) that will bolt up to the stock or stock configuration aftermarket H-pipe, as does FRPP (PN M-9430-A58 for '86-'93; PN M-9430 R58 for '94-'95). MAC, Hooker, Hedman, Coast High Performance, and a host of other manufacturers offer long-tube headers.

While the stock fuel system will sustain a stock 351W, odds are the 351W you have planned won't be stock--so the fuel system will likely require some modification. Beginning at the fuel tank, replace the stock pump with a larger unit, such as FRPP 150- or 190-lph pumps. The 150 is good for about 400-plus horsepower; the 190 is good for about 500 using the stock lines and 351W rails. Higher horsepower levels will likely require a custom fuel system with a larger pump, lines, and aftermarket rails.

Choosing the correct injector for the horsepower level you plan to attain is critical. A mildly built 351W can use 24-lb/hr injectors, although it's more likely you'll have to use 30- to 36-lb/hr injectors (36-lb/hr units are no longer available via FRPP). Whether you're building a more normally aspirated engine or a serious supercharged engine, you'll need a mass air meter calibrated for your new injector size, such as those offered by Pro-M and FRPP. Finally, in order to run fuel injection on a 351W, Ed Marsh says you'll have to get a distributor from an EFI 351 truck (5.8L), again because of the difference in deck height. Find one at the junkyard, or buy one new from Ford (PN E7TZ-12127-D). If you're using a later, roller-cam-equipped engine ('93-up), you'll also want to replace the cast-iron distributor gear with a steel gear.