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Part 1: Introduction and Prep

New Project, New Goals
In the past several years small block Ford stroker engines have become all the rage. Every machine shop and specialty parts house advertises some sort of stroker kit selection, the most common being the 302 based 347 kits, but even 427 cubes are now available from the 351 Windsor blocks.

So when time came to decide on a new engine project for our resident '67 Mustang (see Project Cars), we naturally took the stroker option into consideration. Of course the suggestion was made to simply step up to a "big block", or at the very least a 351 Windsor, and possibly even a stroked one. However, considering our projects are always limited in the funds department, moving to a new engine class would mean extra expenditures such as headers, exhaust, oil pan, distributor, intake, and the list goes on.

The 302 based stroker simply looked better and better as the cheap way to gain displacement. We ultimately decided on a 331 cid motor, for reasons we'll explain in this article.

We knew for certain that we wanted more displacement. The 302 which we had built (see 400 Horsepower 302) had indeed met it's goal of running 11.90's, however left us desiring for more streetability. The big solid-roller cam and 750cfm carb atop the single plane intake meant a high rpm power band and poor low speed manners.

More displacement will give us a good increase in torque all across the board, thus allowing us to tone down the cam. Ultimately our goal is bring the '67 back as first and foremost a street car... a fast street car. To that regard we expect it to run similar ET's, but this time it will be accomplished by displacement and torque, not over camming and high-revving.

How much stroke is enough?
Undeniably the most popular 302 based stroker on the market is the 347. Originally created using a 351 Cleveland crank cut down to 3.400" stroke and a 5.400" Eagle rod, the 347 is about as much displacement you can squeeze out of a 302 block without running into severe strain or accelerated wear. Technically it is possible to get as much as 355 cubes from a 302, but this requires custom machining and the life span of such a motor is very short, making it a seldom seen, race-only application.

The Scat 331 kit uses forged SRP pistons. Note the pin hole is just below the oil ring groove. On 347 pistons the pin intersects the groove, resulting in increased oil consumption.

Most 347 kits require the piston to be so short that the piston pin intersects the oil rings. While a nickel-sized gap behind the ring on each side of the piston may not seem like a huge issue, in fact such a gap will increase the oil consumption of the motor to some level. We spoke to four reputable machine shops to get their opinions on using the 347 in a street application, and their responses were equally divided. It seems as if half of the Ford crowd is convinced the 347 is not an ideal street motor due to rapid wear on the rings (from the relatively low rod ratio) and increased oil burning.. The other half feels the piston pin issue is a non-issue, and refers to the number of satisfied 347 owners using them in daily drivers.

Several companies are offering kits with custom pistons which raise the oil ring grooves above the pin hole. The Coast High Performance 347 kits accomplish this with a 0.085" shorter rod (5.315")). Naturally these 347 kits are more expensive, and critics say area above the piston crown becomes so thin that the piston is prone to breaking at the ring lands upon the slightest detonation. We've actually seen this very thing in a supercharged 347.

Shortly after the 347 became popular, the 331 kits started hitting the popularity charts. Using the same 5.400" rod, but less stroke, results in an ideal 302 stroker. The rod ratio is nearly the same as the stock 302, meaning the side loads from a 331 rotating assembly is not aversely different than that of a stock motor. The 0.150" shorter stroke means the piston pin is well below the oil rings.

All in all the 331 has recently become very popular amongst those not wanting to engage in the "how long will a 347 last" debate. While it did make us uneasy to give up 16 cubes, we're hopeful that we can gain the power back from the 331's better rod to stroke ratio. Continue

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In This Article:
We detail the step by step buildup of a 331 cubic-inch stroker engine. The 302 based powerplant will be the new engine for our Project '67 Mustang.

Also See:
Part II: Assembly
331 Stroker Track Results

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