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The Basics of Stroked Engines and How to Build One
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Just what is a stroker anyway? A stroker is an engine with increased or decreased stroke. By
increasing an stroke (the distance the piston travels in the cylinder bore), we gain displacement. By
the same token, when we decrease an engine’s stroke, we reduce the distance the piston travels in
the bore. This changes when and how the engine makes power. Short-stroke engines like high
RPM, where they make the most torque. Our focus here is more about increasing stroke in order to
achieve greater amounts of torque and horsepower.
Stroking an engine does more than just increase displacement. It increases torque by giving the
engine more of an internal mechanical advantage. When we increase stroke, we increase the
engine’s crankshaft arm or lever, which makes the most of a combustion cycle. The longer the
stroke, the greater the torque or twist.
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Stroke comes from the length of the crankshaft’s rod journal arm. Then, we double that length to
come up with the engine’s stroke. We double the length of the crankshaft’s arm because we get
that arm in two directions- top dead center (TDC), then bottom dead center (BDC). This is a simple
2:1 ratio. Take the crankshaft arm, measured from the crankshaft centerline, and double the
measurement. If the arm is 1.5-inches, you have a 3-inch stroke.
Factory stock small and middle-block (Cleveland/Midland) Fords have four basic strokes. The 221,
260, and 289ci engines have a 2.87-inch stroke. The 302 and Boss 302 have a 3.00-inch stroke.
The raised deck 351W engine has a 3.50-inch stroke, as do the 351C and 351M middle-blocks.
The 400M has a 4.00-inch stroke.
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Stroker engines generate more power in two ways. First, the greater mechanical advantage of a
longer crankshaft arm generates more torque. Secondly, we are also filling the cylinder with a
greater volume of air and fuel, which gives us more power all by itself. We get torque from
increased stroke and cylinder swept volume. Torque is the truest measure of an engine’s power
output.
When we consider the crankshaft’s arm – the distance from the crankshaft centerline to the center
of the rod journal – this is where torque is born. Torque is an engine’s grunt factor. Grunt is that
physical pressure at your backside when the accelerator is pressed. So what is torque exactly?
Think of the crankshaft’s arm as a lever, like you were taught in high school physics class. Torque
equals the downward force of the stroke times the length of the lever or arm. If we look a 5.0L
engine’s 240 ft-lbs of peak torque, this means each cylinder bore is producing 480 pounds of
pressure on each power stroke. We increase torque when we increase the length of the arm, which
we accomplish by increasing the stroke.
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The greater the distance between the
crankshaft and rod journal centerlines,
the greater the stroke. To increase
stroke, we increase this distance. This
is done using a crankshaft with a
greater throw, or welding and offset
grinding the rod journal on an existing
crankshaft. You can achieve significant
stroke increases by offset grinding an
existing crankshaft.
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Strokers are engines with greater
amounts of stroke than we find from the
factory. For example, this is a 302ci
engine with 3.00-inches of stroke.
When we offset grind the crankshaft,
creating a greater distance between the
crankshaft and rod journal centerlines,
we can increase the stroke to
3.25-inches, which gives us 320+ cubic
inches, depending on bore size.
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We know a stock 5.0L engine’s arm is 1.5-inches. This means the 5.0L engine has a 3.00-inch
stroke. If we add 1/4-inch to the arm, this increases the arm to 1.750-inches. Double the 1.750-
inches and you have 3.50-inches, which combines with the standard 4.00-inch bore to make 351ci.
This gives us 40 additional foot-pounds of torque. Overbore the cylinders .030-inch and you have
355ci. Push the bore to 4.060-inches and you’re courting 360ci. You are also making more torque.
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If we opt for a custom
crankshaft with even
greater stroke, we can
achieve 3.50-inches of
stroke, which balloons our
302 into 355ci.
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Despite the advantages of a stroker, there are disadvantages as well, especially if you’re bent on
pumping the most displacement possible into a 302 or 351ci engine. When we stroke a 302 or 351
to its limits, we lose piston skirt, which hurts stability. We also push the piston pin into the piston ring
land area, which weakens piston design. It also puts the pin close to the piston dome, which exerts
too much heat on the pin and boss. These are disadvantages that shorten engine life.
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Stroker kits are packages that enable
us to increase stroke without a lot of
complex research. Someone else has
done the thinking for us.
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This is the Eagle 331ci stroker kit from
JMC Motorsports in San Diego,
California. It is very high end, with a
forged steel crankshaft, H-beam rods,
and forged pistons. This kit employs a
custom crank, rods, and pistons that
are ready to go.
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Another factor with stroking is rod length. When we haul that piston deep into the cylinder bore, we
are also bringing it closer to the crankshaft counterweights, which creates conflict. This means we
need a longer connecting rod to get the piston down there without interference with the
counterweights. Sometimes, we can find off-the-shelf connecting rods to complete our stroker. And
other times, we are forced to custom make connecting rods that will work. More expensive stroker
kits have custom parts, like rods and pistons. More affordable kits have off-the-shelf parts that have
made the kit possible without expensive tooling costs.
Whenever we have to custom make connecting rods, this drives the cost of a stroker kit up. The
same is true for custom pistons. Stroker kits often mandate custom pistons to keep things friendly
at the top of the bore. A 347 or 355ci stroker, for example, has custom pistons with pin bosses
pushed way up into the ring lands. This also drives the cost up and shortens engine life for reasons
explained earlier.
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Understanding Bore, Stroke, & Compression Ratio
Whenever we increase the stroke length, we are squeezing more volume into the same combustion
chamber. This means we need to concern ourselves with an increase in compression. With that
increase in stroke and compression comes an increase in cylinder volume. In that volume comes an
increase in air and fuel, which gives us more power.
Compression ratio is a subject that is often misunderstood. One popular misconception is that
pistons, alone, determine compression ratio. But this isn’t true. Compression ratio comes from not
only piston dome features, but also from stroke, bore, and combustion chamber size.
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Compression comes from piston travel from bottom dead center to top dead center with both valves
closed. We are simply squeezing the cylinder volume (displacement) into the combustion chamber.
This is called compression ratio. Compression ratio is cylinder volume at top dead center versus
cylinder volume with the piston at bottom dead center. If cylinder volume with the piston at bottom
dead center is 10 times more than it is with the piston at top dead center, then we have a
compression ratio of 10.0:1, or simply 10 to 1. It is the proverbial ten pounds of fertilizer in a
one-pound bag, not that we would be inclined to show you an example.
Five basic facts affect compression ratio. Cylinder swept volume, piston dome shape, head gasket
thickness, clearance volume, and combustion chamber size. Swept volume is how much air or
volume the piston displaces during its journey to the top of the bore. If we enlarge the swept volume
by boring the cylinder oversize or increasing stroke, we increase compression ratio.
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We may also increase or decrease compression ratio by changing the piston dome. If we dish the
top of the piston, we lose compression. This is common with stock pistons that are often dished to
reduce compression. A good example is a the 289-2V and 4V engines from 1965-67. The 289-2V
cast piston is dished to keep the compression around 9.0:1. The 289-4V engine has a flat-top
piston to increase compression to 10.0:1. The same is true for the 289 High Performance V-8.
Late-model 5.0L High Output engines sport compression ratios of 10.0:1 with flat-top pistons and
valve reliefs.
To raise compression ratio, we can dome the piston, with a surface shaped like the combustion
chamber. This reduces clearance volume at the top of the bore. When we reduce clearance
volume, we increase compression ratio.
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Compression ratio may also be increased by reducing combustion chamber size. For example,
older 289/302/351W heads have smaller chambers, which means plenty of compression with a
stroker kit. While this has its power benefits, it also can cause engine damage and destruction. A
347ci stroker kit with a 289’s 57cc chambers can yield too much compression, and with catastrophic
results. For one thing, a 289 head wouldn’t be an intelligent choice for a 347ci stroker. Port size
would greatly limit the engine’s potential. This leads us to a better aftermarket cylinder head for the
347. And don’t discount the 351W head for your 347ci stroker either. It makes for a stealthy factory
cylinder head for the 347.
Whenever you step up to an aftermarket head, keep combustion chamber size in mind. Most
aftermarket heads have chamber sizes around 64cc. If you desire greater compression, you can
make adjustments with proper piston selection.
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Previous | Next
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This has been a sample page from
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How To Build Big-Inch Ford Small Blocks
by George Reid
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Have you been dreaming about a little extra displacement for
your Ford? By increasing the bore and stroke of your current
engine, you can add those cubic inches without the hassle of
switching to a big block. George Reid thoroughly explains the
concept of building a stroker, paying special attention to the
effect that increasing the bore and stroke have on the engine
as a whole. With this information, you’ll be better able to tailor
your heads, cam, intake manifold, carburetor, and exhaust
system to get the most out of the extra cubes. Also included
is a complete guide to factory head and block castings, as
well as aftermarket block and head guides, so you can
choose exactly the right parts for your project. This book is
the definitive guide for building a big-inch Ford small block,
complete with four engine buildups ranging from 331 to 408
cubic inches. Read the sample pages to learn more!
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Click below to view sample
pages from each chapter
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Chap. 1 - Ford Small Block
Chap. 2 - Stroked Engines
Chap. 3 - 289 and 302 Stroker
Chap. 4 - 351W Stroker Kits
Chap. 5 - 351C Stroker Kits
Chap. 6 - Stroker Vehicles
Chap. 7 - Engine Math
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Sftbd.
8-1/2 x 11
128 pages
300 black & white photos
Item #SA85
Price: $18.95
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Click here to buy now!
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Other items you might be interested in
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How to Rebuild the Small-Block Ford
In this 144 page book you're walked step by step through a rebuild,
including: Planning your Rebuild; Disassembly and Inspection;
Choosing the Right Parts; Machine Work; Assembling your Engine;
and First Firing and Break-In. Includes 289, 302, 351W, 351C, 351M
and 400.
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Price:
$18.95
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The Step by Step Guide to Engine Blueprinting
Practical methods for racing and rebuilding, selecting and preparing
parts, and how to buy machine shop work. This completely revised and
updated version containing an additional 32 pages is simply the best
book you can buy on engine preparation for street or racing! Engine
Blueprinting shows the reader how to use precision measuring tools,
calculate compression ratios, degree a camshaft, and much more!
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Price:
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How to Build Max Performance Ford V-8 on a Budget
This book addresses high-performance V-8 engines such as the 289,
302, 351ci small-blocks found in Mustangs, as well as the FE series of
big-blocks. Includes realistic, low-cost formulas for building serious
horsepower in Ford V-8 engines.
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Price:
$18.95
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MRE
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