Mustang MonthlyHow To Engine
How To Unleash Hidden Horsepower - Get Stroker Power
You can unleash hidden horsepower and torque from your Ford engine build with stealthy stroker kits
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 = PowerA 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 & FunctionIn 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.