1 302 Strokers Vs 351W Conversions

The one constant that can be seen throughout the performance industry is that fast is never fast enough. It’s all too common for an enthusiast to add all the bolt-ons, pulleys, gears headers etc. Then add better breathing cylinder heads, an intake and a supercharger, turbocharger or nitrous. Now, they’re standing around scratching their heads saying “What next?” As the old adage goes, “There’s no replacement for displacement” and suddenly an engine with more cubic inches is the answer for the horsepower addict.

The two most popular methods for increasing the displacement of the late model Mustang is a stroked 302, usually displacing somewhere between 321 and 347 cubes or a 351W conversion. The purpose of this installment of the Water Box is to examine both options and offer some insight into which is the better choice for the average enthusiast.

Increasing the stroke of the stock 302 can yield substantial performance increases without affecting overall derivability or the car’s current manners. Since it’s visually impossible to tell that your 302 may actually be a 347, the extra stroke offers an element of surprise to unsuspecting competitors.

While stroking a 302 has many benefits, it also has some shortcomings. Perhaps the most important point to consider is that the late model 302 block is structurally weak and prone to substantial flexing. In mild applications, block strength is not as important a concern as it is with higher horsepower levels (425 or more). To build a high horsepower stroked 302 that will not suffer constant head gasket failure and the threat of coming apart, a Motorsport A4 block is a must. The A4 block is the end all for head gasket problems and can safely channel 1000+ horsepower, it’s the kind of block that engine builders dream about. The problem is cost. Add $500 – $750 in machining to the $1600 price tag of the bare block and it quickly becomes cost prohibitive for the average enthusiast. A stroked 302 can be reliable using a Mexican 302 block (which is much stronger than late model 302 blocks) but they’re becoming more and more difficult find.

Now let’s focus on 351W conversions. We’ve built and installed dozens of 351W’s in late model Mustangs and the results are very positive. The 351W is the answer to many of the 302’s weaknesses. Even a late model 351W block (’79 – present), which is the weakest of all 351W castings, is significantly stronger than any production 302 including pre ’81 302 blocks. The blocks are so much more rigid than 302’s that while trying to squeeze every last hp out of our 351W ’88 LX at Maple Grove, we detonated the engine hard enough to brake 2 pistons. Breaking pistons may sound like a tragedy, but it showed how well the 351W keeps head gaskets in place, all we were using were Fel-Pro 1011-1’s, they weren’t high tech head gaskets. If it had been a late model 302 block, we would have easily blown 4 sets of head gaskets during the weeks leading up to Maple Grove.