42-47 CoverFeatBalance 2/18/14 9:15 AM Page 42
Weighing In on
Feature Balancing Work Reducing shaking and stress on an engine’s
bottom end BY LARRY CARLEY, TECHNICAL EDITOR
hether you're building a that can be felt throughout the Minimizing Balance stock engine, a high-revving drivetrain and chassis. And even Problems Wperformance engine or a though the engine may never be The basic idea behind engine balancing slow-turning diesel engine, you can't pushed that hard, the forces generated is to equalize reciprocating forces as overlook the importance of balance. by imbalance will, over time, shorten much as possible, and to make sure all Every time the pistons in a the life of the engine and other rotating mass (the crankshaft and the reciprocating internal combustion drivetrain components (including the lower half of the connecting rods) are engine change direction, they generate motor mounts). in balance so the crank spins smoothly a force. That force is created by the With slow turning diesel engines, about its center axis. reciprocating and rotating mass of the balance may not seem that important. When a rotating object such as a piston and rod assembly. If this force is But most diesels are hard working crankshaft, flywheel or tire is out of not balanced by an equal and opposite engines, so even a small imbalance balance, it wobbles because its center of force, it will create a vibration. over a long period of time can shorten mass does not align with its axis of In a low revving engine, a little the life of the crankshaft and bearings. rotation. The heavy spot wants to pull vibration may seem hardly noticeable. Over-the-road truck drivers also the object off-center as it rotates. But over many miles and millions of appreciate a smooth running diesel The centripetal force generated by cycles, even a small vibration can take engine with minimal NVH. the imbalance multiplies exponentially a toll on the crankshaft and bearings. Balance not only reduces NVH for a with the speed of rotation (doubling In a high-revving engine, any smoother running engine, it also the speed quadruples the force), imbalance multiplies exponentially as improves engine reliability and creating a shake or vibration. the RPMs go up. A small imbalance at durability by minimizing forces inside Balancing a rotating object requires 1000 RPM becomes a huge imbalance the engine that could do it harm. placing it in a spin balancer so the at 8,000 RPM. An imbalance of only a heavy spot can be pinpointed. In the few ounces can generate a force of over 200 lbs. at 8,000 RPM! That's a lot Some well-known engine builders have of shaking and stress on the engine's expressed that proper balancing can add bottom end. 20 horsepower to an engine. Most race car drivers don't care that much about NVH (Noise Vibration Harshness) but they do like to finish races. An engine with a severe imbalance can literally shake itself to death. The forces generated by imbalance can cause fatigue cracks in crankshafts and fretting in bearings. So if something breaks as a result of imbalanced forces inside the engine, the race is over for our unfortunate competitor. Passenger car and light truck owners do care about NVH, and they also want their engine to last as many miles as possible. An engine that isn't properly balanced can produce annoying vibrations and harmonics
42 February 2014 | EngineBuilder 42-47 CoverFeatBalance 2/18/14 9:15 AM Page 44
Balancing Feature case of a tire balancer, a weight equal to Balancing Various positioned counterweights on the the imbalance is then attached to the Engine Types crankshaft. wheel rim opposite the heavy spot to With a horizontally opposed four- In a 90-degree V6 or V8 engine, the equalize the forces. cylinder engine, two pistons are mass of the counterweights should With a crankshaft or flywheel, it's always moving in when two pistons equal 100% of the rotating mass (lower much easier to lighten the heavy spot are moving out. half of the connecting rods and rod than it is to add weight, so the balancer Consequently, the forces, equal and bearings), plus 50% of the reciprocating pinpoints the heavy spot so a hole or opposite, are essentially balanced mass (the upper half of the rod, piston, holes can be drilled into the component provided the weights of each piston wrist pin and rings) times two (because to reduce the imbalance. It usually and rod assembly is equal. Balancing each rod journal has two connecting takes several spins and corrections these types of engines is fairly simple rods and pistons). before balance is achieved. because all you have to do is equalize A few extra grams also needs to be The really tricky part is balancing the weights of the piston and rod added to compensate for oil in the the reciprocating forces inside a V6 or assemblies. bearings and clinging to the parts. The V8 engine. Unlike rotating forces that With an inline four-cylinder total mass you end up with is what the spin around an axis of rotation, engine, two pistons are moving up counterweights have to equal to reciprocating forces are moving back while two pistons are moving down. balance the engine. and forth. The motions of the pistons offset each For example, if each piston weighs In a single-cylinder engine, there is other, but because they are not 680 grams, the wrist pins weigh 190 nothing to counter these forces except horizontally opposed the crankshaft grams, the ring set weighs 60 grams, the counterweight on the crankshaft. needs counterweights to offset the and the small end of the rod weighs If the mass of the counterweight reciprocating forces. 290 grams, the total reciprocating equals the weight of the upper half of Things get much more complicated weight for each piston and upper rod the rod, piston, wrist pin and rings, the when we start talking about V6 and V8 assembly is 1220 grams. Take half of forces will be balanced and the engine engines, because one bank of pistons is that amount (610 grams), then multiply will run smoothly. moving at an angle to the other bank of by two and we end up with 1220 If not, the engine will shake. pistons. grams of reciprocating weight that This creates force interactions that needs to be offset by counterweights need to be offset by carefully on the crankshaft. To that, we have to
Circle 44 for more information 44 February 2014 | EngineBuilder 42-47 CoverFeatBalance 2/18/14 9:15 AM Page 45
Balancing Feature
add the rotating weight of the big end corrections are needed. and/or harmonic balancer to offset any of each connecting rod and bearing With 60-degree V6 engines, the residual imbalance. This is called (650 grams), also times two because angle between the pistons is less so a externally balancing the engine. there are two rods per journal. What different value must be used when Externally balancing an engine we end up with is a combined weight calculating the reciprocating mass for requires mounting the flywheel, of 2520 grams that the counterweights the bobweights. This will vary flexplate and/or harmonic balancer on have to offset to balance the engine. depending on the application, and may the crankshaft when it is spun in the This is referred to as the "bobweight" to range from 35% to 40 or 45%. The balancing machine. which the crankshaft is balanced using value may also vary depending on Engines that are externally balanced bolt-on weights. whether or not the engine has balance from the factory include older small When a 90-degree V6 or V8 shafts. block Fords (302 & 351W) and Chevy crankshaft is balanced, bobweights that The balance shafts have offset 400. Chevy small blocks, most Chevy equal 50% of the reciprocating mass weights that help counter both engine big blocks, Chevy LS and late model and 100% of the rotating mass are dynamic vibrations and vibrations Ford V8s (4.6, 5.0L & 5.4L) are all assembled and attached to each rod caused by uneven firing orders. internally balanced from the factory. journal on the crankshaft to simulate Balance shafts must be correctly timed Internal balance is always best the rods and pistons. The crank is then to the crankshaft, otherwise the forces because it keeps the offsetting weight balanced to the bobweights, usually by won't cancel each other resulting in closer to the reciprocating forces that drilling the counterweights to equalize unwanted vibrations. need to be balanced. Moving the the forces. In some cases, weight may have to weight out to either end of the Counterweights can also be milled be added to a crankshaft to offset crankshaft can sometimes create to remove weight for a cleaner, more heavier pistons/rods. This can be done additional dynamic forces that cause aerodynamic result, but this requires by drilling holes in the counterweights vibrations of their own. Also, changing removing the crank from the balancer and installing heavy metal tungsten the index position of the flywheel or and removing the bobweights so the (Mallory) plugs. harmonic balancer on an externally crank can be milled, then carefully If the counterweights are not heavy balanced engine (or replacing either reinstalling the bobweights in the exact enough to completely balance the component) will upset the balance. same position as before and respinning engine internally, additional weights On most V8 cranks, there are only the crank to see if additional can be added to the flywheel, flexplate six counterweights to save cost and
Circle 45 for more information EngineBuilderMag.com 45 42-47 CoverFeatBalance 2/18/14 9:15 AM Page 46
Balancing Feature
reduce weight. The counterweights are target bobweight of positioned in such a way that they the crank, heavy metal offset not only the piston and rod and/or external assemblies on their own journals but balancing will be also the adjacent journals. Some racing required to achieve cranks are available with eight proper balance. counterweights (two for each pair of This is important pistons), which often helps reduce high information to keep in RPM vibrations even better, especially mind if you are using on long stroke cranks. parts from different Crankshaft counterweights are suppliers, or are typically made to a specific target replacing one set of bobweight. In other words, the size, pistons or rods with a positioning and mass of the lighter set of pistons counterweights is designed to offset a or rods. You may end specific piston and rod weight, (plus or up with a mismatch minus a couple percentage points). that requires a lot of With stock cranks, the bobweight is drilling or milling to Balance between the crankshaft and its related compo- more or less equal to the weight of the achieve proper nents is critical to providing a smooth, trouble-free pow- stock pistons and rods. With balance. erplant. performance cranks, various Speaking of bobweights are available depending on balance, most 90- are offset and equal. But in some high- the rod/piston combination you want degree V6 and V8 engines are "neutral" revving performance applications such to use. If the rods and pistons are balanced, meaning they are balanced as NASCAR where an engine may be lighter than the crankshaft target using 100 percent of the rotating running at 8,000 to 9,500 RPM or bobweight, the counterweights will weight and 50 percent of the higher for most of a race, this may not have to be drilled or milled to balance reciprocating weight of the pistons and be enough. The side effects of the engine. Likewise, if the rods. Neutral balance works best in combustion at such speeds can create pistons/rods are heavier than the most applications because all the forces additional forces that may have to be
Circle 46 for more information 46 February 2014 | EngineBuilder 42-47 CoverFeatBalance 2/18/14 9:15 AM Page 47
Balancing Feature
offset by "overbalancing" the engine. possible to shoot for near zero balance. done on the crank to balance the Some engine builders say they have But the closer you get to near zero, the engine. found additional horsepower and more spins and corrections it takes to Suppliers of performance pistons smoothness at high RPM when they achieve such near perfect balance. and rods are doing a much better job slightly overbalance an engine. Instead Realistically, an engine doesn't have these days of accurately matching of using the standard 50% to be that close to be good enough. weights between parts. Even so, you reciprocating weight when calculating Most stock street engines will run should still weigh all the parts, and their bobweights, they use 51 or 52% or smooth enough if they are balanced to equalize to the lightest in the set when whatever to achieve a certain 1 oz. (28 grams), although many late calculating a target bobweight for the overbalance. model engines have much tighter crankshaft. This will obviously upset normal factory specifications (as little as 4 to 6 engine balance at lower RPMs, but grams). For a high-revving Balancing Tips may provide some performance performance application, aim for the If you are doing your own engine advantages at high RPMs. The only recommended 2 grams or less. balancing, one of the keys to achieving way to know for sure whether or not If you are buying a rotating reliable repeatability is to make sure overbalancing produces any significant assembly (crank, rods and pistons) your equipment is accurately gains is to test the engine on a dyno. from a supplier, buying a pre- calibrated. How close should an engine be balanced assembly will save you time This includes not only the balancer, but balanced? It depends on the and trouble and assure a smooth also the scale you use to weigh application, but the closer you can get running engine. individual parts. You should also to neutral balance the better -- If you are buying a crank from one follow the same procedures especially for high-revving engines. supplier, and pistons and rods from consistently. Several crankshaft manufacturers other suppliers, find out the exact When you're mounting bobweights we interviewed for this article said weights of the parts to determine the on the crank, make sure the weights performance cranks should be target bobweight for the crank are installed in the exact same position balanced to plus or minus 2 grams or BEFORE you buy the crank. every time. Also, make sure the nuts less. This will simplify the balancing that hold the bobweights together are Many electronic crank balancers are process and minimize the amount of tight and don't come loose while the accurate to 0.1 grams (0.004 oz) so it is drilling or milling that needs to be crank is spinning. ■
Circle 47 for more information EngineBuilderMag.com 47