Text and Illustrations by Jon
Mikelonis
Introduction
More displacement and greater leverage means more torque. This
concept is obvious when you compare the torque ratings between
factory small block motors and factory big block motors. However,
nowadays it is not necessary to suffer the time and switching
costs of leaping to a larger block if you are only after more
displacement. Displacement is just a factor of bore and stroke,
by increasing the stroke of your current motor you can enjoy the satisfaction of
more torque disguised in the same package.
| Formula A. Displacement.
Simply a factor of bore and stroke. Increase the stroke
of your current motor and reap the benefits of more torque. |
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Widespread awareness of the facts above and an abundance of
aftermarket stroker kits have made the stroker option extremely
popular. If you are out for performance, a stroker is a wise
alternative to building a motor that only meets the factory
displacement. Whether you have already built a stroker motor
or are simply researching them, take a little time to learn
the basics and understand the benefits and possible compromises
of the now popular engine building practice.
Stroker Motor (def.)
A motor that has greater than stock displacement due to an
increase in the factory crank throw. An increase in crank throw
increases stroke (the difference between the piston's top dead
center and bottom dead center position).
The illustrations below show the difference between a stock
and a stroked rotating assembly. Study the differences and you
can see what makes up a typical stroker motor. Though a bit
exaggerated for effect, the stroked cross section in Figure
2 incorporates:
 Increased
Crank Throw (distance between C and D)
Increased
Rod Length (distance between B and C)
Decreased
Piston Compression Height (distance between A and B)
Keep in mind that rod length
does not affect the displacement of the engine, it is common
to have a stroker motor that uses an increased crank throw,
decreased piston compression height, and stock rod length to
achieve additional stroke. We'll discuss why longer rods are
often used in stroker motors later in the article.
Figure
1. Stock Cylinder
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Figure
2. Stroked Cylinder |
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The animation below helps visualize the effect of increased
stroke and rod length on piston travel and speed.
Figure
1. Stock Cylinder |
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Figure
2. Stroked Cylinder |
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Stroker Evolution
Stroker engines are nothing new, and in fact they are not
even an aftermarket invention. If you look
closely at factory engine offerings, you'll see that changes
in displacement are often nothing more than a change in stroke.
This was a cost effective way for the factory to increase power
for larger vehicles, or future models, while reusing the same
block and accessory components.
| Figure 3. Offset Grinding. The rod journal is offset ground to move the centerline of the rod journal further from the centerline of the main journal. Result is increased stroke. |
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Performance enthusiasts then caught on and they found that creative
machining and parts matching could yield more cubes while hidden
in the stock block to fool fellow racers.
One of the methods used to increase stroke with a stock crank,
is called offset grinding. By offset grinding the rod journal
you move the centerline of the rod journal away from or toward
the centerline of the main journal. This will result in increased
or decreased stroke. Figure 3 above illustrates the case we
are interested in, the rod journal is ground in a manner to
increase stroke. Keep in mind that when the rod journal is offset
ground it now has a smaller diameter. The motor will require
special connecting rods with correctly sized bearing bores.
Additionally, if the rod journal is ground too much it becomes
weak. Unless you add material and regrind, you can only stroke
a motor so far with a stock crank.
Due to a demand for more stroke than offest grinding a stock
crank could achieve, many aftermarket companies developed specialized
cast and forged cranks with relocated rod journals. The specialized
stroker crank has dramatically increased the amount of stroke
you can add to your stock bottom end. Stroker cranks require
a shorter piston to keep the factory sized piston from extending
beyond the deck surface, it is also shortened to accommodate
a longer rod. In the past the only way to complete a stroker
motor was to find the right combination of rod lengths and piston
heights. This often meant researching other factory motors for
the right dimensions. It was not uncommon to have a Small Block
Ford stroker motor consisting of Pinto rods and Chevy pistons.
Longer rods are often required to increase leverage and minimize
the high degree of rod angularity created by the increase in
stroke. The longer rod also prevents the piston from being pulled
out the bottom of the cylinder bore. Rod Ratio and rod angularity
are especially important issues to consider before simply choosing
the stroker kit that yields the largest displacement for your
application. We will discuss these topics in the following section. |
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 (Rod Ratio Explained) |
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In
This Article:
An illustrated review of the geometry behind the increasingly
popular "stroker motor". This article covers
the fundamentals and makes special note of the benefits,
compromises, and limits of stroking your block. |
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Recommended References
“How to Build Big-Inch Small Blocks” by George
Reid, covers the theory behind increasing displacement via bore
and stroke modifications and then dives right into stroker kit selection
and assembly. The book has pictorial build-ups on 302, 351W and
351C based stroker engines. Reid also discusses proper cylinder
head and camshaft considerations for the bigger displacement engines.
Only $12.32
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