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How to Balance an Engine
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Every driver has felt the effects of an out-of-balance tire. The entire car shakes and shimmies, and
the steering wheel almost vibrates out of your hands. Now, just imagine what takes place inside an
unbalanced engine. Instead of turning at a leisurely 600 rpm like a tire, the crank may be spinning
at over 6,000 rpm. At these engine speeds, the pistons, rods, and crankshaft all become incredibly
heavy; a piston whose weight is measured in ounces can exert thousands of pounds of force when
it changes direction at TDC. A minor imbalance on a crankshaft counterweight is magnified many
times by the centrifugal force of the spinning assembly. A single ounce of metal (28 grams) has a
dynamic weight of over 700 pounds when it’s placed on a rapidly turning crankshaft counterweight.
This is why balancing is a vital part of engine blueprinting.
Balancing a tire is a breeze compared to balancing an engine. A tire rotates in one plane; an
engine has a crankshaft that is turning, pistons that are moving up and down, and connecting rods
that are doing a little of both. This helter-skelter motion produces some very strange vibrations.
The number and arrangement of the cylinders also has a tremendous impact on engine balance. A
conventional V-8 engine with cylinder banks spread 90 degrees is a beautiful solution to many
balancing problems. Yet if you lop off two cylinders to make a 90-degree V-6—as several
automakers have recently done—you have an engine that is a disaster from the standpoint of
balancing. To keep such an engine from shaking itself apart, the engineers devise all sorts of
ingenious solutions, including offset crankpins, super-soft engine mounts, and special balancing
techniques.
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There are some engine designs that can never be perfectly balanced, no matter how much time,
money, and equipment you devote to the project This is why some automobile and motorcycle
manufacturers use complex counterbalancing shafts on inline fours when smoothness is an
important consideration. Rearrange those four cylinders into a “V” with the proper angle between
the cylinder banks, however, and the reciprocating assembly can be balanced much more easily.
You don’t have to understand the detailed physics behind these effects to appreciate that some
motors will always have a reputation as “shakers,” while others, with a different number or
configuration of cylinders, can be silky smooth.
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There are no low-buck alternatives
when it is time to balance an engine.
The equipment is expensive, and it
takes a skilled machinist to do the job
right.
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Rotating and Reciprocating Weight
For balancing purposes, the parts of the crankshaft assembly are divided into two categories:
rotating weight and reciprocating weight. The crankshaft spins, so it is obviously part of the rotating
mass. So are the components that spin with it, like the rod bearings. The pistons, rings, and wrist
pins, on the other hand, move up and down, so they are part of the reciprocating weight. The
connecting rods are a mixed case: the small end reciprocates, while the big end rotates. When an
engine is balanced, all the components that are part of the reciprocating weight are matched so
that none of the piston/ rod assemblies is heavier than the others. In a well-balanced engine, the
weight of one piston is always offset by the weight of another piston moving in the opposite
direction.
Before balancing the rotating components, a machinist must first compute the bobweight. The
bobweight is the mass on a single rod journal. It takes into account the weight of the piston, pin,
piston locks, rings, the reciprocating and rotating parts of the rod, and the rod bearings.
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When calculating the bobweight for a conventional V-8 engine, the formula used by most
machinists calls for adding 50 percent of the total reciprocating weight to the bobweight. Other
engines can require a different percentage, however. The formula used to compute the bobweight
for a 90-degree V-6 Chevrolet, for example, calls for adding only 46 percent of the reciprocating
mass.
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Before the crankshaft is balanced,
the bobweight must be computed.
The formula used to calculate the
bobweight varies according to the
engine design. The estimated weight
of the oil on pistons and rods, and
the percentage of overbalance (if
any), must also be included in the
calculations.
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The ringer in all bobweight calculations is the weight of the oil on the components. If you dip a
piston and rod assembly into a can of oil and then weigh it, the oil will add between two and ten
grams to the total weight. How much oil actually clings to the rods and pistons inside a running
engine—and how much it affects rotating and reciprocating weight—are difficult questions.
Some engine builders feel that the oil mist in the crankcase clings together to form a rope-like cloud
that twists around the spinning crank and rotates with the assembly, which will, of course, affect the
balance. Experienced engine balancers usually include an arbitrary amount of weight in the
bobweight calculation to represent the oil film on the moving parts.
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When the crankshaft counterweights
are not heavy enough to balance the
engine assembly, additional weights
are added to the flywheel (or converter
flexplate) and harmonic balancer. This
flexplate and damper are from an
externally balanced 400ci Chevrolet
small-block.
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High-rpm racing engines are sometimes overbalanced. When an engine is overbalanced, the
percentage of the reciprocating weight that makes up the bobweight is increased from the
customary 50 percent to about 51 percent or 52 percent. Although there may be some
theoretical advantages to overbalancing, the benefits are difficult to discern.
Advocates of overbalancing point out that bearing life is increased, while disbelievers maintain
that the practice makes no difference in either power or reliability. Overbalancing is one of those
engine-building techniques that fall into the gray category labeled “Probably Doesn’t Hurt.” For a
street performance engine, using the traditional bobweight formula will provide perfectly
satisfactory results.
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Previous | Next
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This has been a sample page from
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The Step-By-Step Guide to Engine Blueprinting
by Rick Voegelin
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Practical Methods for Racing and Rebuilding
How to buy machine shop work
Selecting and preparing parts
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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! Rick Voegelin's highly
acclaimed combination of savvy writing and wrenching skills puts
this best-seller in a class by itself. All important preparation
techniques are clearly illustrated and explained in this easy-to-
read text. Engine Blueprinting shows the reader how to use
precision measuring tools, calculate compression ratios, degree a
camshaft, and much more! Loaded with helpful advice, this book
should be in every enthusiast's tool box.
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Click below to view sample
pages from each chapter.
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"Rick Voegelin's book, The Step-by-Step Guide to Engine
Blueprinting, is an excellent source of performance-oriented
engine building information for the beginner and the seasoned
veteran alike. This digest should be in every enthusiast's greasy
mitts."-- Steve Magnante, HOT ROD
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Chap. 1 - Engine Blueprinting
Chap. 2 - Cylinder Block
Chap. 3 - Crankshaft
Chap. 4 - Connecting Rods
Chap. 5 - Pistons
Chap. 6 - Cylinder Heads
Chap. 7 - Camshaft
Chap. 8 - Compression Ratio
Chap. 9 - Balancing
Chap. 10 - Assembly Tips
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Softbound
8-3/8 x 10-7/8
160 pages
400 b/w photos
Item #SA21
Price: $18.95
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Click here to buy now!
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How to Rebuild the Small Block Ford
This 144 page book guides you step by step through
a
rebuild, including: planning, disassembly and
inspection,
choosing the right parts, machine work,
assembling your
engine, first firing and break-in. It also
gives you helpful hints
and tips on performance
upgrades, including cams, heads,
ignition, induction,
and more. It also points out problem areas
to watch for,
professional builder tips, jobs that need special
care or
special tools, and more. Includes 495 color photos
and
covers the Ford 289, 302, 351W, 351C, 351M and 400.
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Price:
$
22.95
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Turbochargers
How to select and install the correct turbo for big or
small
horsepower gains. Discusses turbocharger design,
sizing,
matching, controls, carburetion, exhaust,
ignition,
intercooling, marine and high altitude
applications. The most
comprehensive book available.
Turbo suppliers and kit
maker addresses are included.
“Everything you could possibly
need to know about
turbochargers for automotive applications
is in this
book.
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Price:
$
18.95
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How to Build Big-Inch Ford Small Blocks
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 building of a
small block Ford stroker, paying special attention to the effect
that increasing the bore and stroke have on the engine as a
whole. 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.
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Price:
$
18.95
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Payment, Shipping & Sales
Tax: Iowa
residents must pay 7% sales tax. Items usually ship
within one
business day of receipt of payment! Standard shipping is a flat rate of
$4.95 to
anywhere in the United States with USPS Media Mail.
Priority Mail shipping is available for an
additional $3.00, or
$7.95 for shipping. Shipping is combined and discounted for multiple item
purchases as follows: first item regular shipping price, add
$1.95 for each additional item. For
purchases of 3 or more items
shipping is automatically upgraded to Priority for no additional
charge! We offer world wide shipping and ship to Canada and Mexico
with USPS Priority Mail
International for $14.95, and to most
locations in Europe, Australia, Asia, Japan and South
America for
$18.95. Satisfaction is Guaranteed. Our store has a NO HASSLE RETURN
POLICY
within 30 days of purchase.
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