One of the keys to a smooth running, long lasting engine is proper balance of the reciprocating and rotating parts. When a crankshaft is out of balance, the uneven distribution of weight can generate centripetal forces that shake the engine with increasing intensity as engine rpm goes up.
Centripetal force (which many people mistakenly call “centrifugal” force) tries to pull the crankshaft toward the heaviest part of the imbalance as the crankshaft spins around. This makes the crank wobble as it rotates, which produces shaking that can be seen and felt in an engine that’s out of balance.
The greater the imbalance and the further it is from the center of the crankshaft, the more the engine shakes and hammers the main bearings and crank. Over time, this can lead to metal fatigue and cracks that may cause the bearings to fail or the crank to break.
With low-revving diesel truck engines that rarely see this high side of 4,000 rpm, the centripetal forces created by imbalance can be as great or greater than those in a typical passenger car gasoline engine or even a racing engine because the pistons, rods and crankshaft counterweights are all much heavier. An out of balance diesel engine can cause annoying vibrations that wear on a driver’s nerves and shorten the life of the engine.
Balance is just as critical at the other end of the spectrum, too. A high-revving small displacement motorcycle engine that revs to 12,000 rpm has much lighter reciprocating and rotating components. But at such high speeds, even a small amount of imbalance is multiplied many times over and produces significant loads that can shake and vibrate the engine.
When it comes to building performance engines for the street, strip or circle track, near perfect balance is absolutely essential for engine smoothness, durability and maximum horsepower. NASCAR engines typically run as high as 9,800 rpm for much of the race, so more than a few grams of imbalance can create harmonics and destructive forces that reduce power and increase the risk of the engine not finishing the race. On some of these engines, they are even balancing the camshafts to dampen valvetrain harmonics that can rob power at high rpm.
With drag racing, it’s all about a brief burst of power and maximum acceleration. The engine doesn’t run at a constant speed but accelerates between each gear change. This can produce harmonics at various rpm ranges that reduce power. Balancing can tune out some of these harmonics or shift them to an rpm range that has less of an effect on the engine’s power output.
Engine balance within 4 grams (0.14 ounces) has been a traditional benchmark for street engines, and 2 grams (0.07 oz.) or less for performance engines. But in today’s highly competitive professional motorsports, close enough is not good enough. Many performance engine builders are now balancing engines down to tenths of a gram! It all depends on the engine and the application.
How much is 1 gram? Not very much. A dollar bill weighs about one gram. A penny, by comparison, weighs about 2-1/2 grams. An ordinary sheet of office paper tips the scale at a whopping 5 grams, which is more than the amount of imbalance that’s generally desired in a street engine.
Realistically, 3 grams is probably close enough for a big block Chevy performance engine that may never see the high side of 5,500 rpm, but 2 grams or even 1 gram may not be close enough for a high revving Chevy small block in a circle track race car. Because of this, many performance engine builders will strive to achieve 1/2 gram or less of imbalance to keep their customers happy.
The final tolerances will depend on the weight of the reciprocating and rotating parts, how far from the axis of rotation any residual imbalance is located, and the rpm range of the engine. A small amount of imbalance at the outer edge of a counterweight or a flywheel can produce just as much force as a much larger imbalance located in close to the center of the crankshaft or flywheel. So it’s important to know not only how much imbalance there is, but where the imbalance is located.
Why Balancing Has Become Absolutely Critical Today
Most shops that are doing custom engine building today are assembling new parts that have never been in an engine before. The only parts that are reused in many performance engines are the block and maybe the cylinder heads and often even these parts are replaced with aftermarket castings.
Most engines are put together with a new crank (usually a stroker), new connecting rods and new pistons. Depending on where the parts are sourced, the weights of the rods and the weights of the pistons may be fairly even. Even so, it’s always a good idea to check the weights, and to equalize the weights as needed. Some engine builders aim to equalize connecting rod and piston weights to a tenth of a gram or less.
The weights of the reciprocating parts then have to be balanced to the counterweights on the crankshaft. Aftermarket performance parts (rods and pistons, that is) are almost always lighter than the stock parts they replace. So if the original crank is being reused, the counterweights will have to be drilled to compensate for the reduced mass of the reciprocating parts.
If the engine is being built with a stroker crank, balancing is an absolute must. Some suppliers of stroker cranks publish a “target bobweight” for their cranks so engine builders can more easily estimate how much work it will take to balance the crank with a given combination of parts. Others just give you the crank and you’re on your own to figure it out.
It’s not unusual to see brand new stroker cranks that are out of balance by as much as 200 to 300 grams! That’s a lot of extra mass on the counterweights that will have to be removed to balance the crank. One reason why many stroker cranks are heavy is because they are forged with extra metal in the counterweights so the engine builder doesn’t have to add heavy metal (tungsten plugs) to achieve proper balance. Drilling holes is cheaper and easier than installing heavy metal.
A one inch hole drilled one inch deep removes about 100 grams of metal. So to balance a stroker crank that is 300 grams too heavy, you may have to drill 3 or 4 holes in the counterweights to bring it into balance.
If an engine is being built with a lightweight racing crank, on the other hand, there’s less metal to work with when it comes to balancing the crank. On some of these cranks, you may have to use heavy metal to bring balance down to where you want it.
Heavy metal may also be required if an externally balanced engine such as a big block Chevy is being converted to an internally balanced engine. On externally balanced engines, the crankshaft is balanced with the harmonic balancer and flywheel attached. This provides a lot of metal area to work with if you have to drill holes to lighten the assembly. On internally balanced engines, the balancer and flywheel are balanced separately. Consequently, the crank often turns out to be too light and requires heavy metal to bring it into balance.
Customers should be told what type of engine balance they have (internal or external), and warned about marking the position of the flywheel if the engine is externally balanced should the flywheel have to be removed. Marking the index position of the flywheel is necessary so it can be remounted in the same position as before to maintain balance. They also need to be aware of the fact that replacing the flywheel or harmonic balancer with different parts can upset balance on an externally balanced engine.
Balancing requires the use of “bobweights” when spinning the crankshaft on certain kinds of engines to simulate the effects of the rotating and reciprocating parts inside the engine. The bobweight should usually equal half of the reciprocating weight plus the rotating weight.
The rotating weight is the big end of the connecting rod, the rod bolts and rod bearings, plus about 4 grams for the oil that is between the bearings and crank journal when the engine is running.
The reciprocating weight is the small end of the connecting rod, the piston, wrist pin, retainers (if used) and rings, plus 4 to 5 grams of oil for the oil that clings to these parts when the engine is running.
Bobweights are necessary on each crank journal when balancing V6, V8 and V10 engines, and also most one, two, three and five cylinder engines. Bobweights are not needed on inline four and six cylinder engines, or horizontally opposed flat fours or sixes (Porsche and Subaru) either. On these applications, the motion of the pistons is opposite each other so the forces cancel out provided the weight of the pistons and rods are evenly matched and equalized to the lightest weight.
Most balancer packages come with bobweights for standard engine applications, but additional bobweights may be needed to handle special applications. Once the rotating and reciprocating weights have been measured, the bobweights can be assembled by referring to bobweight tables or the software on the balancer. On some machines, the scale inputs the weight of the individual parts directly into the software to simplify the math and reduce the risk of entering the wrong information.
On some oddball engines like a Volkswagen VR6 engine, the bobweight percentages will be different than usual because of the narrow angle between the cylinder banks. On these engines, you only use 20 percent of the reciprocating weight, not the usual 50 percent.
Some racers have found that slightly underbalancing or overbalancing an engine by using 1 to 4 percent more or less weight on the bobweights actually produces more power and less vibration at certain rpm ranges. Underbalancing and overbalancing requires a lot of trial-and-error experimentation to find the weight that works best for a given engine, so some say it is more of a black art than a science as conventional balancing theory doesn’t fully explain it.
Balancing The Crank
Once the bobweights have been made up, they are installed on the crank so the crank can be spun on the balancer. This often takes longer than spinning and correcting the crank itself.
On a typical street engine, it often takes less than an hour to set up and balance a crankshaft. On a performance engine, the job may take up to several hours depending on how close you want to get the crank to zero, and whether or not you have to install heavy metal to bring it into balance. The more drilling and heavy metal it takes to achieve balance, the longer it will take to balance the crank. That’s why you have to price the job accordingly. If you quote a flat rate to balance the crank, and end up putting a lot more time into it than you originally thought it would take, you are selling yourself short.
Most of today’s balancers are PC-based with Windows software and graphical displays that make balancing much easier than older balancers. The software eliminates guesswork and reduces the time and effort it takes to make corrections. It also makes balancing less intimidating for a novice because the software does all the calculations.
After the first spin, the software calculates the imbalance, shows you where it is, and tells you how much metal has to be added or removed, and where. If you don’t like the location of the recommended correction(s), you can tell the software to recompute the data so the correction can be made elsewhere on the crank.
It may take several spins to fine tune all of the corrections and to verify the crank is within the desired range of balance. Most cranks can be brought down to a few grams with two to three spins, but if you are aiming for a couple tenths of a gram or zero balance, it make take 7 to 10 spins to nail it down.
Balancing As A Profit Center
Compared to some other pieces of equipment in your machine shop, an engine balancer can produce an excellent return on your investment. If you charge $200 to balance an engine, and it takes you an hour, you’ve made $200 per hour. On the other hand, if you change $200 to balance a stroker crank that takes you three hours to complete, you’ve earned the equivalent of $66 per hour which isn’t bad but probably isn’t enough to fairly compensate you for your time and effort. That’s why you need to change for your actual time rather than quote a flat rate.
With performance cranks, you never know how much time it will take to balance the crank until you get the crank on your balancer and spin it up. Once you’ve gained some experience with a particular brand and stroke of crankshaft, you’ll have a pretty good idea of how long it will take to balance the crank on the next job. But until you’ve gained that experience, each job you do will probably be a whole new learning experience.
The most profitable applications for balancing include small high revving engines such as those in motorcycles, boats and go-karts. Some shops get $100 or more to balance a single cylinder Briggs & Stratton engine, which requires a lot less time and effort than a V8.
Buying A Balancer
Most shops that are doing performance engine building today already have a balancer. But is their balancer up to date with the latest software? Is the software fast, accurate and easy to use? And does the balancer have the flexibility to handle other kinds of work that may come into your shop?
Some balancers that mount on a milling machine start as low as $7,000. Stand alone balancers typically start in the $15,000 range, and can go up to $30,000 or more depending on what comes with it (such as a drill stand). Some balancer manufacturers say that if you do only one or two jobs a month, you can cover the lease payments on a new balancer. Anything beyond that is pure gravy.
Though most shops that buy a balancer do so specifically for performance engine work, there’s no reason why a balancer can’t be used for other types of jobs, too. This includes balancing boat props, prop shafts, driveshafts, rotors, drums, even CV joints. Driveshafts can be tricky because they require special end clamps and supports, and the machine has to be long enough to accommodate the length of the shaft.
Any rotating industrial component that can be physically mounted on a balancing machine can also be balanced which opens up a lot of new revenue possibilities if the automotive market is slow in your area.
Special balancers are also available for balancing turbocharger impeller and turbine wheels. These parts spin at extremely high rpms, so accurate balance is critical to their longevity.
BALANCING EQUIPMENT BUYERS GUIDE
Contact: Randy Neal
G & H Balancer Service
Contact: Gary Hildreth
Hines Industries Inc.
Ann Arbor MI
Contact: John Witt
Fort Mill SC
Contact: Rick Irland
Schenck Trebel Corp.
Deer Park NY
Contact: Rich Idtensohn
T & S Machines and Tools Inc.
Contact: Tim Whitley Sr.
Contact: Warren Turner
Winona Van Norman
Contact: Britton Harper
These listings are taken from the 2008 Engine Builder Buyers Guide.