Jim Smart
October 5, 2011

We're always looking for stealthy modifications that improve classic Mustang performance without being visible. Here's another one-affordable stroker kits from Eagle Specialty Products and Coast High Performance.

"Stroker" is bench-racing slang for engines where displacement has been increased through additional crankshaft stroke. This is done by increasing the distance the pistons travel from bottom dead center to top dead center, which adds displacement for a greater air/fuel charge, not to mention providing a better mechanical advantage (leverage). We get stroke by either offset grinding rod journals of an existing crankshaft or going with a new crankshaft with longer rod journal throws.

To get additional stroke from a small-block Ford, we take the 302's factory 3.000-inch stroke and increase it to 3.300 inches to get 331ci or 3.400 inches to achieve 347ci (with an 0.030-inch overbore). Nearly one-half inch of stroke may not seem like much, but it's considerable because it yields more horsepower and torque from the same 4.000- or 4.030-inch bore block for not much more than a rebuild using original components.

Adding nearly one-half inch of stroke makes your 302 behave more like a 351. And if you're stroking a 351W or 351C to 408 or 427ci, you're getting big-block power from a small-block V-8. If that isn't enough to get your endorphins flowing, consider this. You can stroke your 390ci FE big-block to 428 ci and higher (431ci with an 0.030-inch overbore) for brute Cobra Jet power or a 429ci fat-block to 500ci for awe-inspiring amounts of power.

Engine builders have been increasing stroke since the beginning of internal combustion more than a century ago. What makes stroking more achievable these days are kits that make it easier to get into tire-smoking displacement and power. Your challenge as an engine architect is incorporating the right combination of parts for optimum performance and reliability. As two examples, Eagle Specialty Products and Coast High Performance make it easy to assemble a package because it comes in kit form, ready to install in a freshly machined block. In other words, these folks have already done the brainwork for you, which makes it easy to select the right package. The main thing to remember is not to push the limits of your block with too much stroke, otherwise you could scatter your engine all over the pavement when nailing the accelerator. You want just the right combination of crank, rods, and pistons to get the job done reliably, and this is where Eagle and Coast are helpful with race and street proven kits.

Stroke + Displacement = Power

A 302ci small-block huffs 302 cubic-inches of air and fuel through eight cylinders in two complete revolutions of its crankshaft. Each piston travels 3 inches from bottom dead center to top dead center. In one complete power cycle consisting of intake, compression/ignition, power stroke, and exhaust, each piston travels a total of 12 inches-and that's in just two crankshaft revolutions. Imagine how much air and fuel your engine ingests and burns at 6,000 rpm in a minute's time from 302 cubic-inches. Then imagine 347ci from an engine that looks like a 289/302 externally. No one knows that displacement is there but you.

Alan Davis of Eagle Specialty Products tells us that if we take the 302's 3.000-inch stroke and increase it to 3.400-inches for 347 ci (4.030-inch bore), that's 45 more cubic-inches of air and fuel, or a 14.9- percent increase in displacement. In theory, Alan explains, it means roughly 14.9-percent more power. This means a 350-horse 302 can produce more than 400 horsepower under optimum conditions. Alan stresses that this is strictly theory because engines are the sum total of the parts and technique we use. You can have a lot of stroke but not have a sufficient support system to get it all marching in lock-step order. You're going to need compatible cylinder heads, valve and port size, camshaft profile, valvespring pressures, suitable carburetion or fuel injection, header tube size and length, transmission and rearend capacity, and more. What works well in a 302 won't serve you well with a 331- or 347ci stroker.

When you increase displacement, you can no longer think of your 331 or 347 as a 302. And when you stroke a 351W or 351C to 408 or 427ci, you have to think of your small-block as a big-block. Just about everything you had on your 302 or 351 is no longer compatible due to the increased displacement.

Another important consideration is fitment. What will it take for your block to accommodate a stroker package? The more displacement you cram in there, the greater your concern for fit. A 331ci stroker doesn't cause rod to block clearancing issues like a 347 kit does. The same can be said for a 377 or 392 versus 408 or 427. If you're going to go for the max, be prepared to grind some iron off those block skirts.

How an engine behaves boils down to bore and stroke, along with how we use it. You might look at the FE series 427 and 428 and wonder why Ford produced two big-block engines so close in displacement. Although close in displacement, the 427 and 428 have completely different personalities. The 427 is a large bore, short stroke, high-rpm FE designed for high-performance driving even though it employs the same stroke as a 390. The 427 makes its peak horsepower and torque around 6,000 rpm thanks to that big-bore/short-stroke design. In race trim, the 427's peak horsepower and torque happen around 7,500 rpm.

Ford's 428, on the other hand, has a smaller bore but a longer stroke than the 390 and 427, making it a real powerhouse down low for good traffic light-to-traffic light power. The 428's secret to street and dragstrip power is brute hole-shot performance thanks to its long stroke and obvious mechanical advantage. If you drop a 428 crank into a large-bore 427 block, you wind up with 454 cubic-inches, once again demonstrating the advantages of increased stroke. A 427 stroked to 454 ci becomes a tremendous producer of torque. When stroking the 289 or 302 to 347 ci, or the 351 to 408/427 ci, we're infusing both displacement and raw grunt. It won't feel like a 302 or 351 anymore.

Crankshaft Design & Function

In reciprocating four-cycle engines, we're taking cylinder pressure created by the heat and expansion of combustion and turning it into rotary motion via the crankshaft. You may not believe this, but your engine's crankshaft is quite flexible. It has to be to prevent breakage, but there's more. The harmonic balancer serves as the crankshaft's shock absorber, dampening twist and minimizing oscillation. With each compression stroke and combustion pulse acting on the crank, there's a certain amount of twist and flex (oscillation) around the crankshaft's centerline, causing rod journals to dance wildly around main journals. This affects piston and valve timing more at high rpm under hard acceleration though it's not something you'd notice with the pedal to the metal. It's never the same amount of flex and movement each time around the flagpole.

Suffice it to say that torque originates at pistons and rods, ultimately having an effect on radial oscillations around the crank's centerline. Not only does heat energy acting on the piston and rod make torque, so does the crankshaft, multiplying the torque of combustion and rapid expansion through stroke. When increasing stroke, there's greater mechanical advantage-leverage-acting on the crankshaft's centerline. There's also more stress. The beauty of an iron crank, I-beam rods, and hypereutectic pistons is affordability while providing greater displacement and power.

Eagle tells us that its cast-steel crankshafts will take more punishment than a stock cast-iron crankshaft while offering more power-up to 500 horsepower in small-blocks and a whopping 700 horsepower in big-blocks. All that power comes from stroke and displacement. This means you get a lot of crankshaft durability and stroke without having to sell off the farm. This makes a cast-steel crankshaft suitable for daily driving, weekend racing, towing, and more. The power advantage comes from stroke without having to spend a lot of money. And when you consider machining costs on your factory crank and rods, the cast-steel stroker kit with I-beam rods is an excellent bargain for not much more than all that machine work.

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When you're shopping for a stroker kit, the first question you should ask yourself is how you intend to use the engine. Most driver Mustangs, even with occasional weekend drag racing, can get by with a cast-iron, cast-steel, or nodular iron crankshaft; forged steel I-beam connecting rods; and hypereutectic pistons with ductile iron rings. You really won't need any more than that though most kits already have forged pistons. Weekend drag racers can benefit from forged pistons instead of cast or hypereutectic.

Cast-iron, cast-steel, and nodular iron crankshafts are all basically the same idea-a flexible, durable iron shaft for street and occasional weekend drag racing. Unless you intend to go racing full-time or run nitrous, supercharging, or turbocharging, an iron shaft will live a long time in your engine. When taking an engine to extremes with full-time racing, nitrous, or supercharging, forged pistons become mandatory for durability or you risk breaking a forged steel crank and H-beam rods.

Not only is crankshaft material an important consideration, but so is design. Look closely at journals and how they're machined. You want journals with good load distribution across the face and fillet radii. It's also a good idea to look at oil galleys and journal chamfering while you're shopping. These are points you will need to discuss with a tech rep when ordering your stroker kit.

Another important consideration is stroker crankshaft fitment in your 302, 351, 390, or 460 engine because rotating and reciprocating internals must clear the block and other important components like camshaft and oil pump.

Another less important issue for street cars is crankshaft weight and aerodynamics. For a street Mustang, you're not going to be that concerned with crankshaft weight or aerodynamics. However, weekend and full-time drag racing is another story. You're going to want the least amount of crankshaft weight possible along with counterweights that are knife-edged to slice through air with the greatest of ease. Crankshaft weight and aerodynamics, not to mention dynamic balance, affect how quickly an engine accelerates.

Connecting Rods

Your Mustang was factory fitted with forged steel connecting rods, which work quite well shotpeened and upgraded with ARP bolts when building a stocker. When you make the decision to build a stroker, you're starting out with a clean sheet of paper. Street and occasional weekend racer engines need only I-beam forged steel connecting rods, which are more durable than stock rods. What makes an aftermarket I-beam rod stronger is the material it comes from. Eagle, for example, makes its most basic I-beam rods from SAE 5140 forged steel and fits them with ARP 8740 3?8-inch bolts for unequalled strength. Eagle I-beam rods are fitted with alignment sleeves for optimum cap and rod fit. Machining is done with CNC technology, which keeps your detail work to a minimum, although we suggest checking everything during a mock-up.

Forged H-beam connecting rods are race-only technology and expense. They aren't pieces you'd run with a cast-steel crankshaft, but instead a 4340 steel or steel billet crank. H-beam rods are forged from SAE 4340 steel and are designed for all-out racing activity. This makes the H-beam rod unnecessary for your daily driver and weekend cruiser. Eagle's 4340 H-beam rods sport 3?8-inch ARP 2000 bolts (small-block) and alignment sleeves for rock-solid durability. What takes Eagle H-beams over the top is perfect sizing on the Sunnen Krossgrinding machine. These guys can take up to 900 horsepower if you're so inclined.

Pistons

There are three basic types of pistons available in the marketplace-cast, hypereutectic, and forged. Cast pistons are original to most vintage Ford engines because they're cheap, quiet, and offer good expansion properties. Hypereutectic pistons are cast with high silicon content for strength without the cost of forging. They offer similar expansion characteristics as cast with added strength. You can build a daily driver and weekend drag racer using hypereutectic pistons and save a few bucks.

Forged pistons offer high strength for high-performance applications, yet undesirable expansion properties because they expand more than the cast variety. This means looser tolerances cold, which makes forged pistons noisier on cold start up. Sometimes, forged pistons never get quiet with heat. Just ask anyone with a 4.6L SOHC or DOHC Modular V-8 about piston rattle during cold start-up and they will explain the disadvantages of forged pistons.

When you're amassing a stroker package, most of the brainwork has been done for you, including piston selection, with Eagle and with Coast High Performance. Where it gets tricky is compression ratio, rod length, and crankshaft counterweight sizing. Eagle's Alan Davis tells us the biggest hurdle of building a stroker is understanding the piston, rod, crank combination and how it works together. "Imagine a single cylinder's worth of crank, rod, and piston all assembled at bottom dead center," Alan tells us. "If you increase stroke, it will pull the piston down into the counterweight. Not good. That's why we increase rod length in stroker kits. But if you rotate the crank, everything sticks out of the top of the block, which is why we change wristpin location to get the piston flush with the deck."

Alan adds that wristpin location is known as compression height, yet he tells us that he doesn't agree with that terminology. "Wristpin location isn't the only thing that affects compression height. While changing the compression height by itself would alter the compression, it is not the 'ideal' way to change the compression if that is what you want to do. The location of the pin in the piston is really dictated by the stroke and rod combination you are attempting to build instead of the compression ratio." Alan says he'd prefer to call this dimension pin height rather than compression height.

With all this in mind, how do you cram all this hardware into a 302 block? "This is where block deck height comes into play," Alan says. "Ford's 302 has an 8.206-inch deck height, which is the distance between main centers and block deck. The 351W has a 9.500-inch deck height. You've heard some folks talk about zero deck engines, where pistons come to the top with no clearance whatsoever, flush with the deck."

According to Alan, there's roughly 0.020-inch of deck clearance with most factory engines. This is why you should do a mock-up before final assembly. A raw mock-up without piston rings, but with bearings, allows you to check all dimensions and make any needed changes. If there's too much clearance at the deck, you have too much deck height. If the piston sticks out of the hole, you have no deck height. In either case, you may have to make a piston change to get it right. Situations like this are decidedly rare.

Stroker Science

When adding stroke to an existing block, we make significant changes to the engine's physics. Compression ratio is cylinder and chamber volume with the piston at bottom dead center versus cylinder and chamber volume with piston at top dead center. In other words, with 10.0:1 compression, you have 10 parts volume (bottom dead center) to one part volume (top dead center). This is a dynamic you must be aware of in the planning stages, then confirm physically during mock-up. This dynamic is known as swept volume.

When compression is too high, you can opt for a dished piston or a deeper dish to lower it. The problem there is that a dish piston gets closer to the wristpin. You can also opt for a cylinder head with larger chambers. Those are two solutions, and there are more.

Another element to consider with stroker kits is rod ratio, which we mentioned earlier. Rod ratio is rod length divided by stroke. In other words, rod length versus stroke. Rod ratio is important to power. It's also important to fit and function. You don't want a connecting rod that's too long.

"Rod ratio has several effects," Alan comments. "The most important one as it relates to strokers is side loading." When rod ratio increases, side loading decreases. By the same token, when it decreases, side loading increases. Side loading is the side load placed on the piston as the crank throw comes around and drives the piston skyward, yet right into the cylinder wall. When side load increases, so does piston and cylinder wall wear, friction, and heat.

Alan suggests a maximum rod ratio of 1.70:1 on 302- and 351ci Fords and 1.72:1 on the big-blocks. The generally accepted range is 1.50:1 to 1.70:1. A longer rod yields more dwell time at each end of the cylinder bore, which means more time for air and fuel to amass. That gives us better throttle response thanks to longer dwell time. However, there's also a price to be paid when connecting rods become too long in terms of excessive wear and borderline geometry.

Ford's 428, on the other hand, has a smaller bore but a longer stroke than the 390 and 427, making it a real powerhouse down low for good traffic light-to-traffic light power.

Quick Engine Math

How do you determine an engine's displacement? All you need to know is bore and stroke, then follow this simple formula.

Engine Displacement = bore x bore x stroke x 6.2838

Let's take a typical 302 that has been bored .030-inch oversize with a 4.030-inch bore. You can calculate the new displacement by multiplying the 4.030-inch bore times itself (4.030 inches) times a 3.000-inch stroke times 6.2838 to arrive at 306.1637 cubic-inches. Taking the 302's 3.000-inch stroke and increasing it to 3.400 inches yields 346.985 cubic-inches rounded off to 347ci. Increase bore size another 0.010-inch and you get 348.709 cubic-inches or 349ci. Boring a 302 block beyond 4.040-inches is not recommended, though some go to 4.060-inches, which is cutting cylinder wall thickness way too close. Going beyond 349ci in a 302 block isn't recommended.

Forged pistons offer high strength for high-performance applications, yet undesirable expansion properties because they expand more than the cast variety.

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