Aside from bumping compression higher, increasing an engine’s stroke has always been the fastest way to power. Automakers have always known this. Nearly every Ford engine family has taken on displacement first with increased stroke, then, a fresh block casting with larger bores. In the old days, builders and racers had to offset grind existing crankshafts, cut down journals and counterweights, and find off-the-shelf rods to achieve additional stroke. As one example, 400M cranks were always popular for getting 351W and 351C engines over 400 ci before the advent of affordable stroker kits. All it took was a savvy machinist, tenacity, and the right equipment, but it was never easy to get there for the average enthusiast void of resources and cash.
Thanks to the proliferation of stroker kits with new or reground cranks, custom connecting rods and pistons from companies like Scat Enterprises, Coast High Performance, Eagle Specialty Products, and a host of others, we can infuse displacement with a credit card. And because so much research and development has been performed to get displacement, most of the groundwork has already been done for you.
1 Increased stroke begins here at the crankshaft rod journal. When we extend the throw fu
So why bother with the time and expense of a stroker kit and what are the benefits for your performance improvement dollar? When we add stroke, we’re getting more power from two elements—air/fuel volume and raw mechanical advantage (leverage). The deeper we can haul a piston into a cylinder bore without conflict top or bottom, and the longer it sits at each end, the more power we’re going to make. To get stroke, we’ve got to add reach to the crankshaft rod journals. Yet, at the same time, we’ve got to get our slug flush with the top of the deck and be able to clear crankshaft counterweights when it arrives at the bottom. A crankshaft also has to clear both block pan rails and camshaft while it’s whirling around.
Getting it all to fit involves adjustments to connecting rod length and piston pin location, and oftentimes modifications to the block. And when it’s all buttoned up, no one knows it’s there but you, which can make things darned interesting in a Saturday night traffic light to traffic light magic carpet ride.
It’s easy to get into increased displacement because there are plenty of stroker kits available for Ford V-8s from the aftermarket. Scat Enterprises, for example, offers you not only a broad spectrum of Ford stroker kits, but also abundant information on its website where you can make an educated decision. We spoke with Tom Lieb, owner and founder of Scat Enterprises, about how to choose a good stroker kit. Because Tom has been in business since the 1960s, when he started grinding crankshafts in his garage for people like Carroll Shelby, we thought he’d be one of the best people to consult on this subject. Tom’s time-proven Southern California operation sports 25 CNC machines, 15 crankshaft grinders, and a huge connecting rod production line. That, coupled with a competent team of seasoned engineers makes Scat Enterprises unequaled in the industry.
2 When we add the piston and connecting rod in, we’re including all primary elements of s
Tom tells us stroker kit selection boils down to how you intend to use your engine. If you’re building a street engine for both cruising and occasional weekend dragstrip action, you don’t need a high-end steel crankshaft, H-beam connecting rods, and forged pistons. You can build one heck of a street performance engine with a cast steel crankshaft, high-performance I-beam rods, and hypereutectic or forged pistons because you’re adding stroke and displacement. Today, as always, there’s no replacement for displacement. What does this mean for you?
- 347-355 ci from a 302 block (more from an aftermarket block)
- 427 ci from a 351W or 351C block
- 431 ci from a 360 or 390 FE block
- More than 500 ci (557 ci) from a 460 block
All this displacement and power for not much more than the cost of machine work on your existing rotating/reciprocating stock and a new set of pistons.
Stroke & Compression
When we build stroke into an engine, we have that huge gauntlet to rush through involving dimensions and clearances. Everything has to clear. Next in our mock-up work is calculating compression ratio. Compression ratio is the amount of volume above the piston at bottom dead center versus volume at top dead center. If we have a compression ratio of 10.0:1, that means we have 10 parts volume at bottom dead center versus one part volume at top dead center. In other words, our squeeze is 10 units to 1 unit.
When we take the same combustion chamber and cylinder bore and haul the piston deeper, we have more cylinder volume (known as swept volume) above the piston at bottom dead center. This means we’re going to squeeze more air/fuel from bottom dead center to top dead center, which raises compression ratio. When this happens, we have to somehow increase the space above the piston at top dead center with a larger combustion chamber, head gasket thickness, or a dished piston.
3 Stroker power comes from hauling the piston down as deep in the bore as possible withou
Stroke & Rod Ratio
Another element we don’t address enough during planning is rod ratio (rod length versus stroke). Rod ratio isn’t only about piston dwell time at each end of the bore, but also raw mechanical advantage (leverage) because we’re taking the power generated by combustion pulses and giving it multiplication. And although there’s a lot of discussion about the horsepower that comes from a higher rod ratio, what you really want most from a street engine is torque. Torque is the grunt that gets you off the line quickly in drag racing leaving the other guy in the dust.
Dwell time at each end of the bore is important because it allows the aggressive accumulation of air, fuel, and hot energy-yielding gasses before valves close. Simply put, with longer piston dwell time, that fresh air/fuel charge has time to build above the piston before the intake valve slams shut. And, when it’s time for exhaust stroke, exhaust gasses have more time to scavenge on the way out. And with the right amount of valve overlap between exhaust and intake stroke, dwell time can be to your advantage because we get this thing going called momentum. Hot gasses leave aggressively; causing negative pressure (suction) that draws the fresh air/fuel mixture in with the same kind of fury.
So how do we achieve productive rod ratio numbers? It boils down to rod length, center to center, along with stroke. Let’s say we have a stock 302 with a 3.000-inch stroke and 5.400-inch connecting rod center to center. That gives us a 1.8:1 rod ratio, which is pretty good. We arrive at this number by dividing 5.400-inches (rod length) by 3.000-inches (stroke) to get a 1.8:1 ratio. Truth is, engine builders generally like a minimum rod ratio of 1.5:1 and a maximum of 1.8:1. Anything beyond 1.8:1 isn’t realistic.
When we increase rod ratio, we are reducing rod angle. By the same token, if we decrease rod ratio, we are increasing rod angle. The greater the rod angle, the greater the mechanical disadvantage and cylinder wall wear from side thrust. With too much rod angle, you wind up wasting heat energy and power. With reduced rod angle, there’s less frictional loss. Ideally, you will achieve a nice compromise between not enough angle and too much angle. Another way to look at this is rod length and how it affects piston dimension. You can push the wristpin deep into the ring lands and compromise durability because it gets too close to the fire, causing oil breakdown from extreme heat. Ring support also becomes compromised.
Another way to look at rod ratio is what it does to piston speed. A short rod yields lower piston speed at bottom dead center and more piston speed at top dead center. A longer rod does exactly the opposite—faster at the bottom and slower at top dead center.
Choosing A Piston
- Cast: Good for mild street applications and even some weekend drag racing. Good expansion properties. A quieter, cheaper piston.
- Hypereutectic: Fancy word for high silicon content cast piston. Great for street driving and weekend drag racing because it’s stronger than cast. Can take the heat better. Good expansion properties from cold to hot—a quieter, more forgiving high-performance piston.
- Forged: Conceived for racing applications because it can take greater heat, pressure, and rpm. If you’re going to run nitrous, supercharging, or turbocharging, this is your only choice. Bad news is noise mostly on cold start because forged pistons expand more with heat, which means sloppy tolerances cold.
What’s My Displacement?
Not sure about displacement? All you need to know is bore and stroke, then, try this:
Bore to the Second Power x Stroke x 0.7854 x Number of Cylinders = Displacement
When you add stroke, you have to think of your engine as larger. A 302 stroked to 347 ci has to be thought of as a 350. A 351 stroked to 427 ci has to be thought of as a big-block. This means your stroker’s induction and exhaust has to increase accordingly. Header tube and collector size, exhaust pipe size, carburetor size, intake manifold port size, cylinder head flow, and cooling system capacity should all be chosen to support the larger displacement.
Typical Ford V-8 Stroker Configurations
|Displacement Standard & Maximum||289ci||302ci||351W||351C|
|Bore Standard & Maximum||4.000"||4.000"||4.000"||4.000"|
|Stroke Standard & Maximum||2.870"||3.000"||3.500"||3.500"|
|Rod Length & Ratio||5.155"||5.090"||5.956"||5.778"|
|Bore Standard & Maximum||4.050"||4.130"||4.050"||4.230"|
|Stroke Standard & Maximum||3.780"||3.780"||3.980"||3.780"|
|Rod Length & Ratio||6.489"||6.489"||6.489"||6.489"|
|Bore Standard & Maximum||4.130"||4.362"||4.362"|
|Stroke Standard & Maximum||3.980"||3.590"||3.850"|
|Rod Length & Ratio||6.489"||6.605"||6.605"|
4 This is what we mean about conflict. At the bottom of the bore, look how close the pist
5 Another potential area of conflict is your camshaft, which is at risk for connecting ro
6 Stroker kits must clear the block, oil pump, and main stud girdles by at least 0.060-in
7 So what can you get for your engine-building dollar? Plenty when you consider this Scat
7a For street use, all you need is a cast steel crank, I-beam rods, and either hypereutec
8 Street and weekend race engines don’t need expensive H-beam rods. This sportsman-calibe
9a If you’re going racing or intend to throw nitrous, supercharging, or turbocharging at
9 This 351C forged-steel stroker crank from Scat likes high revs thanks to knife-edged co
10 Piston dynamics and selection are very important to amassing a stroker kit. Make sure
11 Here’s an Eagle Specialties cast steel and I-beam rod stroker package for the 351C eng
11a A close up look reveals MCE’s close attention to detail with smoothly chamfered oil p
12 Trans Am Racing in Gardena, California, has packaged this 351W to 427ci forged steel c
13 The difference between a stock piston and a stroker piston is apparent here. On the ri
14 When we put these guys together, differences in dimension become immediately apparent.
15 Here are two Scat forged steel I-beam connecting rods. On the left is 5.400 inches cen
15a Rod length, as it relates to stroke, is known as rod ratio. Simply divide rod length
16 What type of rod you use depends on how you intend to use your engine. Budget street e
16a Racing, nitrous, supercharging, and turbocharging call for super strong H-beam techno
17 Tom Lieb of Scat Enterprises brought these pieces out for us to see. On the left is wh
18 Marvin McAfee of MCE Engines shows us part of the process of calculating compression r
19 Head gasket thickness, Marvin stresses, also affects compression ratio because it crea
19 The thicker the gasket, the more volume (space) it creates.
20 When compression can’t be lowered via a larger combustion chamber, head gasket thickne
21 Combustion chamber size is measured in cc’s (cubic centimeters). The larger the chambe
22 Stroker kit packages are typically already dynamic-balanced when received. However, we
23 Growing from 302ci to 347ci or 351ci to 427ci? Carburetor size has to go up accordingl
24 Header tube and collector size, along with exhaust pipe size, must increase with strok