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Ford Stroker Motor - Stroker Theory
Is Bigger Better? We Think So.
Though you may not realize it, Ford Motor Company has been in the stroker business since the early '60s. In 1962, the Blue Oval boys introduced the 221 V8 that featured a 3.50-inch bore, a 2.87-inch stroke and was installed in lightweight Fairlanes (and later Falcons). By the late '60s, Ford cars grew in size (and weight) and the small-block followed suit. It was first enlarged to 260 cubic inches and then to 289 inches. In 1968 the little V8 grew one final time to 302 cubic inches, still using the same basic block design.
At the time, the 302-inch small-block was factory rated at 230 horsepower (with a 4-barrel carb), but it was not considered a performance engine-at least not considering the power of the hot 390 and 428 engines in the Mustang's lineup. With a 4-inch bore and a 3-inch stroke, the 302 had the potential to make great power, but it was all at high rpm. When Ford found the need for a more powerful small-block with lots of low-end torque, it went with the 351 Windsor, a derivative of the 302, albeit with a raised deck to accommodate a longer stroke. Additionally, the 351 used a different crankshaft main journal size (3.00-inches compared to 2.248-inches), but the basic block design is the same as the 289/302 and cylinder heads are interchangeable.
In the time since the late '60s, engine builders have gone past the standard bore and stroke specifications of the 302 (and 351 and big-block engines, too) using modified and aftermarket cranks, pistons and rods to gain cubic inch displacement. With the current list of available aftermarket parts you can now take a 302 or 289 engine and make it as large as 355 cubic inches. And when using the Ford Racing (M-6010-S351 or M-6010-W351) 351-based 9.5-inch blocks, you can achieve a maximum displacement of 454 cubic inches, all with a great amount of user friendliness.
The reason for increasing engine size is simple-cubic inches equal horsepower. It doesn't matter if you're racing or if you're into street performance, a larger engine will usually outperform a small one. And by stroking your small-block, rather than switching to a big-block, you get the benefit of more engine size while maintaining lighter and smaller packaging. When you turn your 302 into a 331 or 347 you can use the stock hood, stock engine accessories, intake, fuel system and headers. However, in order to reap all the benefits, your stroker engine must be properly designed, built well and run efficiently.
One question you may ask is how big should I make my stroker? The answer is easy, as long as you have a plan of attack. If you're going heads-up racing, you'll be restricted to class limitations that will dictate maximum or minimum engine size. If you race with the NMRA or the FFW it's likely you'll be limited to 310, 360 or 400 cubic inches, depending on which class you run. In the case of NHRA, engine size is dictated by the bore and stroke rather than actual displacement (like in Stock or Super Stock), or by a cubic inch to weight factor, such as in Competition Eliminator.
Either way, remember there has to be a balance between the parts. Building a 347 stroker will do you no good if you're going to use stock heads on top of it and a stock cam in it. Obviously, combination is extremely important.
Thankfully, we can build virtually any variety of cubic-inch size you wish. And you can top your stroker with a myriad of heads and intake combinations to get your small-block screaming.
Over time, much has been written about the Otto 4-cycle (or 4-stroke) internal combustion engine. Performance magazines and most books refer to engines as air pumps because they do pump air, but making copious amounts of reliable horsepower takes a little more understanding and a little knowledge about engine combinations and engine tuning.
The combustion process begins with the engine at top dead center (TDC) and with the intake valve slightly open. As the engine turns, the intake continues to open and air enters the engine and is mixed with the fuel in the carburetor (or in the intake ports, as would be the case with fuel injection). The mixture is then drawn into the individual cylinders as the piston swings down on the intake stroke. It's important to know that a pressure difference exists between the cylinder and the intake manifold (and ports in the heads) which causes the mixture to rush in when the valve opens. Naturally, we can pack in more air and gas with turbochargers, blowers and nitrous kits, but this basic example refers to naturally-aspirated engines.
When the piston crosses a point just past bottom dead center (BDC) the intake valve closes, the piston swings up the bore and the mixture is compressed into the combustion chamber. When the piston reaches a point just before TDC the ignition fires and the compressed mixture ignites and burns rapidly. As the mixture burns it expands, and the force created pushes the piston back down the bore with great force. This force drives the crankshaft, which produces torque. And when the air/fuel mixture is just right, big horsepower results. Finally, the piston reaches BDC, the exhaust valve starts to open and on the final upstroke the burned gasses are expelled and the 4-stroke cycle begins again.
Despite the pumping capabilities (based on large or small displacement), you must remember that it's the chemical reaction of the fuel and air that releases the energy to make horsepower. The pump (our engines) simply contains and harnesses the release of heat and energy. So yes, a larger pump is one way to increase power, provided it can move more air, more fuel, and provided the mixture is burned efficiently. It's also important to remember that by increasing the bore and stroke without increasing the size of the combustion chamber will yield a higher compression ratio and that always has to be accounted for.
Bore & Stroke Basics
There are two important dimensions that determine the displacement (cubic inch size) of any engine and they are bore and stroke. Bore refers to the diameter of the cylinder bore, while stroke refers to the distance the piston travels in the bore. If you use this equation: bore x bore x stroke x .7854 x number of cylinders, you can figure out the displacement of any piston-type engine.
You should know that there are benefits to increasing both the bore and the stroke in an engine, but big jumps in engine size will come after the stroke is increased.
Increasing the bore is beneficial because it opens up the space between the intake valve and the edge of the cylinder bore. This unshrouds the valve and helps flow. By increasing the stroke you increase the swept area that the pistons travel and this yields a much larger increase in engine size when compared to simply boring a block. For instance, overboring a 302 block by .030-inch adds four cubic inches. Yet, stroking the engine .400-inch (less than half an inch) adds 39 cubic inches.
Due to the dimensions of the Ford small-block, we are limited to the amount of boring and stroking that can be done. Blocks are cast with only so much wall thickness, which limits bore size. As for stroke, that's limited by the space available in the crankcase. When stroke is increased, the connecting rod's big end is essentially moved further away from the crank centerline and clearance between the connecting rod big end and the oil pan rail is reduced. In other words, the journals spin in a larger diameter circle.
So, in most cases, it is necessary to grind portions of the block and the pan rail to gain the necessary clearance. This is not a big deal and can be done with a small grinding tool like a Dremel.
Does Size Matter?
In a word, yes, size does matter. And with the variety of aftermarket internals on the market you can choose between an endless variety of cubic inch displacements. In most cases bigger is better, as long as the all the parts support the combination.
However, the intended use of the engine plays a huge role and can effect the end user's performance, reliability and longevity. Some factors to consider include rpm range, type of use (drag racing, towing, street performance, etc.) and the expected life of the engine. When dealing with the standard 8.2-inch block, the most popular stroker kits are the 317, 331 and the 347. All of them feature different strokes using a .030-inch overbore. Popular choices for your 351 are 377, 392 and 408.
Since you can only bore the Ford small-block so far (usually .060-inch max), the major increase in cubic inch must come from extended crank stroke. But upstroking a crankshaft is not a simple thing. If you added stroke and changed nothing else, the pistons would stick out of the top of the block and crash violently into the cylinder heads. On the down stroke the counterweights would slam into the skirts of the pistons. So when upstroking an engine it is necessary to change the connecting rods and pistons, too.
"Ford Motor Company knows how to build engines with lots of power, but they don't build short-deck big cube engines. That's because there is always a compromise, like when you stuff a long stroke crank in a short deck block. It's usually the ring seal that is compromised because the piston is pushed upward with the connecting rod being on such an extreme angle. This can cock the piston, and when that happens you can lose ring seal. And when you trade ring seal for cubic inches the longevity of the engine suffers and the manufacturers can have that," explained Tom Naegele of DSS. "That's why Ford created the taller 9.48- and 9.50-inch deck height blocks when they wanted to go to 351 cubic inches," he added.
In contrast, many builders believe the 347 (with a 3.4-inch stroke) is fine in a 8.2 block. That is, as long as the engine isn't revved past 7000 rpm on a consistent basis. We've seen many 347s make great power both on the street and on the track.
You should also know that increasing stroke also increases piston speed (which is different from engine rpm). And this is something you must consider if you plan on turning your stroker past 7000 rpm. When the stroke is increased, the piston must cover the distance between TDC and BDC in a shorter amount of time (as compared to a stock stroke). Since the pistons have to start and stop quicker, the load on the internal components is increased. This includes the main webbing in the block, the main caps, wrist pins pistons and connecting rods.
Knowing this, I recommend using the strongest and lightest pistons and rods when building a high-rpm stroker. One example is the Probe forged pistons found in the Coast High Performance 347 engine kit. These pistons weigh about 410 grams as compared to a stock 5.0 piston that weighs about 700 grams.
The foundation for any stroker engine buildup is the engine block. When it comes to Ford small-block engines there is no black magic. There are only so many choices, some stock and some not. It's likely that the block you select will be based on cost, but we recommend using the best block you can afford (sometimes).
The stock 5.0 roller blocks are the most common and are great for applications up to (about) 500 horsepower. Naturally, you've seen them make more, way more in some cases, but they are not designed for 600-700 horsepower use and if you make that much you'll be taking your chances.
A better choice for an all-out race engine is a Boss block or the Ford Racing A4 or R-302 blocks. The Boss and the Ford Racing blocks feature 4-bolt main caps, they are beefier and are designed to make well over 850 horsepower. Note: Avoid using a 289 block for any serious stroker because they have shorter cylinder bores (compared to the 302) which will allow the piston to stick out of the block at BDC, creating a stability problem.
If you're using a stock block, we recommend sticking with a .030-inch over bore. The stock block is certainly an economical choice, but you can expect to swap blocks after a season or two if you make over 500 horsepower.
If you are building a 351 stroker, you can select between the many choices in the Ford Racing catalog. You'll find both 9.2 and 9.5 deck height casts that can be used for all types of racing. Whether you use a stock block or a race block we recommend that you have the block Magnufluxed (checked for cracks) and sonic tested if you plan to increase the bore above .040-inch.
No matter which stroke length you select, it will be necessary to notch the bottom of the cylinder bores to add clearance for the rod bolts.
The crankshaft is the heart of any stroker kit. It takes the energy from the power stroke and converts it to torque that's used to drive the car. Each Ford small-block crankshaft has five main journals on the crank centerline. Those journals secure the crank to the engine block, but also allow it to spin freely. The crank also has offset journals (four of them in a V8) for the connecting rods, and counter weights opposite the connecting rod journals that offset the weight of the pistons, wrist pins and the rods.
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.
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.