Engine Balancing - Engine Builder Magazine

Engine Balancing

In the dark ages of the automotive industry, engine balancing was considered an art practiced exclusively by performance engine builders. Only those wizards of speed (it was widely believed) could understand the forces that caused drivetrain vibration – and more importantly, those that stopped it. Those few who could balance an engine were looked upon with admiration and awe.

Of course, in today’s enlightened world, it’s clear that balancing is beneficial to ALL rebuilt engines – and is often a necessary part of the process if you and your customers expect the engine to provide trouble-free service.

“In performance, balancing was always important. But with standard automobiles, balancing never used to be a factor,” explains Randy Neal, CWT Industries. “Now, through better manufacturing processes – including balancing — the OEMs have tried to reduce any force that causes wear. As a result, we’re seeing engines that go 100,000 miles instead of 50,000 miles.”

Neal points out that more precision at the OE level means a smoother running, longer lasting car, which we have all come to expect. The tradeoff comes at the rebuilding level, when you are required to not only meet the boring, alignment and surface finish needs of the engine but the balance as well. If standard engine rebuilders, who may not be used to thinking about performance and balance, don’t pay attention to the weight differences between parts, they could be setting an engine up to be a real shaker.

“When we introduce aftermarket components into the engine, in all probability we vary the balance,” says Neal. “There are wide variations in the parts manufacturing process. Even parts that are physically identical can have significant differences in weight. Mixing and matching connecting rods to make a set – even if they are all the same part number – may very well introduce a variation into the engine that will cause an unbalance.

“It’s critical that we as quality aftermarket suppliers build the equivalent of OE quality or better,” Neal says. “To do less is to undermine the industry in which we work.”

A smooth running engine isn’t just a nice thing to have – it translates directly into power. If the crankshaft, connecting rods and pistons aren’t tying to tear each other apart and in the process shake the vehicle to pieces, more power will be transferred to where it’s needed: the wheels.

Failure to balance the components properly can result in many different problems: piston rings failing to seal; main and rod bearing wearing early or irregularly; harmonic balancer deterioration; valve spring failures; timing chains stretching; bolts and fasteners working loose.

Don’t ignore comfort, though: fewer vibrations also mean accessories will last longer, and the vehicle will be more comfortable to drive.

“Think of the mom waiting to pick her kids up from school. Sitting there with a cup of coffee while the van is idling,” explains Tim Meara, Sunnen Products. “It’s not performance, – she may not be turning 6,500 rpms going down the highway, but if the rebuilt engine isn’t smooth and that coffee starts splashing across the dashboard, even she’ll know there’s a balance problem.”

Babcox Technical Editor Larry Carley explains that understanding engine balancing is actually quite simple – well, as simple as high school physics. “As everybody knows, a rotating object generates ‘centripetal force,’ which is an actual force or load generated perpendicular to the direction of rotation. Tie a rope to a brick and twirl it around and you’ll feel the pull of centripetal force generated by the unbalanced weight of the brick. The faster you spin it, the harder it pulls,” he explains.

“Centripetal force should not be confused with ‘centrifugal’ force, which is the tendency of an object to continue in a straight trajectory when released while rotating,” says the professor. “Let go of the rope while you’re twirling the brick and the brick will fly off in a straight line.

“As long as the amount of centripetal force is offset by an equal force in the opposite direction, an object will rotate with no vibration. Tie a brick on each end of a yardstick and you can twirl it like a baton because the weight of one brick balances the other,” says Carley.

A crankshaft is a heavy rotating object. What’s more, the piston and rod assemblies reciprocating back and forth along its axis greatly complicate the problem of keeping everything in balance.

“With inline four and six cylinder engines, and flat horizontally opposed fours and sixes (like Porsche and Subaru), all pistons move back and forth in the same plane and are typically phased 180

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