Ford Engine Stroker Kits - What About A Stroker Kit?
Increased Displacement (Stroke!) Is The Easiest And Most Affordable Way To Build In Power
From the January, 2012 issue of Modified Mustangs & Fords
By Jim Smart
Photography by Jim Smart
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...
1 Increased stroke begins here at the crankshaft rod journal. When we extend the throw further out from the crankshaft’s centerline, we get both increased stroke (the movement of the piston and connecting rod up and down in the cylinder) and displacement—hence the term “stroker.” A 302 crank sports a 3.000-inch stroke when you count both directions of piston travel or 1.500 inches from crank centerline to rod journal centerline.
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...
2 When we add the piston and connecting rod in, we’re including all primary elements of stroke. To get optimum displacement, both rod and piston dimensions must follow accordingly to get the piston deck height just right and clear crankshaft counterweights. Wristpin location and connecting rod length determine fitment as it relates to crankshaft rod journal throw. It doesn’t work if there’s conflict at bottom and top dead center. Rod length must be such where we get maximum dwell time at each end without contacting the crankshaft counterweights and cylinder head. This is known as rod ratio.
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.
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...
3 Stroker power comes from hauling the piston down as deep in the bore as possible without conflict and extremes of geometry (pushing the limits of your block). Look at the sheer cylinder volume you get from stroking a 351C to nearly 400 ci. This is a whole lot of huff from a cast steel crank, I-beam rods, and forged pistons for well under two grand.
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...
4 This is what we mean about conflict. At the bottom of the bore, look how close the piston comes to contacting the crankshaft counterweight. Stroker pistons must be designed to both clear the crankshaft counterweight at bottom dead center and be flush with the deck at top dead center. This poses challenges for aftermarket companies because engine block architecture can vary considerably. Piston stability at bottom dead center can get tricky from block to block. If given a choice between a 289 or 302 block, choose the 302 because the cylinder skirts are longer.
5 Another potential area...
5 Another potential area of conflict is your camshaft, which is at risk for connecting rod clearance issues if you go too far. A mock-up will prove out clearance issues.
6 Stroker kits must clear...
6 Stroker kits must clear the block, oil pump, and main stud girdles by at least 0.060-inch, though you really want more. This clearance is based on temperature (expansion) and centrifugal force at high rpm.
7 So what can you get for...
7 So what can you get for your engine-building dollar? Plenty when you consider this Scat forged-steel stroker kit can take a 390 FE big-block to in excess of 431 ci making it a Cobra-Jet-spanking powerhouse.
7a For street use, all you...
7a For street use, all you need is a cast steel crank, I-beam rods, and either hypereutectic or forged pistons.
8 Street and weekend race...
8 Street and weekend race engines don’t need expensive H-beam rods. This sportsman-caliber, forged I-beam rod from Scat with cap screws will take a beating without consequence. Use this with a cast steel or nodular iron crankshaft and either hypereutectic or forged pistons and you can count on plenty of street power.
9a If you’re going racing...
9a If you’re going racing or intend to throw nitrous, supercharging, or turbocharging at your Ford, you need a forged steel crankshaft and H-beam rods for durability.
9 This 351C forged-steel...
9 This 351C forged-steel stroker crank from Scat likes high revs thanks to knife-edged counterweights that streamline airflow and reduce turbulence.
10 Piston dynamics and selection...
10 Piston dynamics and selection are very important to amassing a stroker kit. Make sure your stroker kit makes sense. A cast-steel crank makes sense when you fit it with I-beam rods and either cast or hypereutectic pistons. Forged pistons offer durability, but are not always a quiet operation due to expansion properties. This Probe Industries forged piston with a low-friction coating from MCE Engines in Los Angeles can be fitted to a budget or high-end engine.
11 Here’s an Eagle Specialties...
11 Here’s an Eagle Specialties cast steel and I-beam rod stroker package for the 351C engine, which will yield 392 ci. This is real power for not much more money than machine work on existing components, along with a fresh set of pistons.
11a A close up look reveals...
11a A close up look reveals MCE’s close attention to detail with smoothly chamfered oil passages.
12 Trans Am Racing in Gardena,...
12 Trans Am Racing in Gardena, California, has packaged this 351W to 427ci forged steel crank with H-beam rods and dished forged pistons. Pistons are dished to keep compression conservative. Remember, when you increase stroke, you also increase compression ratio depending on deck height, compression height, and chamber volume.
13 The difference between...
13 The difference between a stock piston and a stroker piston is apparent here. On the right is a Scat forged aluminum piston for a 302. On the left is a Scat 347 stroker piston with revised skirt (and overall height) and wristpin location that compensates for the increased stroke.
14 When we put these guys...
14 When we put these guys together, differences in dimension become immediately apparent. On the left is a 302 piston. On the right is a 347. This is how we get more stroke in a cylinder bore along with more rod.
15 Here are two Scat forged...
15 Here are two Scat forged steel I-beam connecting rods. On the left is 5.400 inches center to center. On the right is 5.090 inches.
15a Rod length, as it relates...
15a Rod length, as it relates to stroke, is known as rod ratio. Simply divide rod length by stroke and you get rod ratio.
16 What type of rod you use...
16 What type of rod you use depends on how you intend to use your engine. Budget street engines with occasional weekend drag racing can get by with the forged I-beam from Scat on the left.
16a Racing, nitrous, supercharging,...
16a Racing, nitrous, supercharging, and turbocharging call for super strong H-beam technology on the right.
17 Tom Lieb of Scat Enterprises...
17 Tom Lieb of Scat Enterprises brought these pieces out for us to see. On the left is what an H-beam rod forging looks like prior to machine work. And we wonder why H-beam rods are so expensive? Look what they start with in a raw forging.
18 Marvin McAfee of MCE Engines...
18 Marvin McAfee of MCE Engines shows us part of the process of calculating compression ratio. All space above the piston at top dead center must be figured into compression ratio. Marvin cc’s the valve reliefs to determine valve relief volume, which affects compression ratio.
19 Head gasket thickness,...
19 Head gasket thickness, Marvin stresses, also affects compression ratio because it creates volume (space) between the head and block.
19 The thicker the gasket,...
19 The thicker the gasket, the more volume (space) it creates.
20 When compression can’t...
20 When compression can’t be lowered via a larger combustion chamber, head gasket thickness, or changes to deck/compression height, a dished piston is the last option. The larger the dish, the lower your compression ratio. To raise compression, we sometimes have to dome the piston, the exact opposite of dishing. The more dome we have, the higher the compression ratio, because there is less volume above the piston at top dead center. You have to cram the same amout of air/fuel mixture into a smaller space.
21 Combustion chamber size...
21 Combustion chamber size is measured in cc’s (cubic centimeters). The larger the chamber, the lower your compression ratio, and vice versa.
22 Stroker kit packages are...
22 Stroker kit packages are typically already dynamic-balanced when received. However, we suggest having your stroker package dynamic-balanced by a trusted machine shop after mock-up is performed.
23 Growing from 302ci to...
23 Growing from 302ci to 347ci or 351ci to 427ci? Carburetor size has to go up accordingly or forget making power.
24 Header tube and collector...
24 Header tube and collector size, along with exhaust pipe size, must increase with stroke and displacement, as you will now be moving a much greater amount of air and exhaust through the engine.