By moving the rod journals further away from the crank centerline, the pistons can be pushed higher and pulled lower in the bore, subsequently increasing the swept area in the cylinder and increasing the displacement of the engine.
Another way to increase stroke is to offset-grind the crank. "This requires that the rod journals be ground down to a slightly smaller diameter, and also moved to one side (further out) from the centerline of the original rod journal. This is like putting a smaller circle inside a larger circle, and having the smaller circle set off to one side," explained George Klass of Coast High Performance.
Earlier, we mentioned that the stock stroke for a 302 is 3.0-inches and 3.50-inches for a 351. Common stroker crank feature strokes of 3.25-inches for a 331; 3.40-inches for a 347; 3.68-inches in a Windsor 377; 3.90-inches in a 392 and 4.00-inches in a 408. Once you've determined the stroke, you will need to select a crank material. Common crank materials include cast iron, high nodular cast iron, forged steel and billet steel.
There are a variety of companies selling stroker cranks for Ford engines. The type of crank you select should be matched to the horsepower level. However, it's important to note that supercharged engines place a heavy load on the crank snout so a stronger forged crank may be necessary. Cast iron cranks are a good choice for street stroker applications, but a forged or billet crank will offer the most strength. Again, let price and horsepower level be your guide.
Connecting Rods & Pistons
For your stroker to fit neatly inside the block, it is necessary to install longer-than-stock connecting rods and aftermarket pistons. Connecting rods are measured from the center of the small end to the center of the big end. Extended rod length is important in a stroker because it moves the pistons further away from the counterweights on the crank. However, lengthening the connecting rods means the pistons must be shorter in height or they will stick out of the bores.
Most of the stroker kits offered for the small-block Ford come with a set of quality"I" or "H" beam rods and pistons with the correct center-to-center dimensions. In addition, there are some stock-type rods that will fit dimensionally, but won't be as strong. Since you have to change the piston design, there's no question that the pistons you choose will play a critical role in the performance and longevity of the engine. As I just mentioned, it's necessary to shorten the pistons to make them fit in a stroker engine. Shortening pistons in a stroker application is nothing new, but there are things to consider when ordering pistons for your stroker. We recommend using a forged piston in all stroker applications though the design (dome, dish or flat top) will vary based on your application. Naturally, the most important dimension in a stroker piston is the wrist pin location.
Since the piston must be built shorter to fit inside the engine, the wrist pin must be moved up, which can cause it to interfere with the oil ring and cause an oil consumption problem. This will not be a concern in a drag race engine, but it might be if you want to get 100,000 miles from your street stroker. Always ask your engine builder about pin location so he can determine the right combination of parts for your engine.
Often when speaking of stroker engines, the topic of rod ratio comes up. And when the talk turns to rod ratio, you'll find many opinions. Rod ratio is the figured out by dividing the connecting rod length by stroke. A stock 302 has a rod ratio of 1.69. Generally, a rod ratio in that range is very good. In contrast, the 4.6 modular engine has an almost square bore and stroke (about 3.55-inches per) with a 5.933-inch rod and a rod ratio of 1.67 ratio. The 5.4 mod engine has a long 4.165 stroke, a long 6.65-inch rod length and a 1.59 rod ratio, which isn't considered to be optimum, but those engines run quite well.
In reality, changing rod ratio by small amounts usually only results in small increases or decreases in the percentage of power that any given engine can make. For now we'll avoid the cloudy topic and stick with the harder, colder facts about strokers.
Destroking
The last topic for the day is destroking. While it's rarely done in the 5-liter Mustang universe, destroking is common practice in many Competition Eliminator and Super Stock Modified classes. There are many reasons for destroking, but it's mostly done when building a small displacement engine using a big bore and very short stroke.
By using a short stroke crank, such as a 2.70-2.90-inch stroke, the engine can rev quicker, it will reduce piston speed and there will be less frictional losses found in the ring package. Using a big bore also allows enough room for lots of valve area, sometimes upwards of 2.20-inch on the intake side.
With less cubic inches, engine builders can tailor the cylinder heads, without always worrying about maximum flow. That's because it's easier to fill a 300-cube engine than it is a 400-cube engine.