The curious case of the confusion of counterweights
No story about engine balance is complete without mentioning the harmonic damper attached to the front of the crankshaft. In OEM form, the damper (also called a balancer with external balancing) is simple and inexpensive, but only cancels a narrow range of vibration (aka, frequency). To understand the nuances of externally balanced motors, we contacted fluid amplifier to have a discussion of the what, why, and how to hang extra weight off the end of the crankshaft.
Types of balance
There are three types of motor balance. The first is the primary equilibrium of the rotating assembly. An unbalanced rotating assembly is like losing the weight of a wheel. It will make a noticeable thud at one speed and may be less noticeable (but still very present, as far as your engine is concerned) at other speeds. Vibration can cause high engine wear (or worse), so matching piston and connecting rod weights with crankshaft counterweights is key to minimizing unbalanced vibration.
Another type of vibration is axial, which is the back and forth movement of the crankshaft. Your thrust bearings and main bearing support plates prevent this type of movement. It’s quite simple to control.
Torsional vibrations are more difficult to isolate and control – the crankshaft twists during combustion events. Also, all the existing vibrations get harsher when you increase the power. The harmonic balancer (damper) manages the torsional reaction for better efficiency and durability, and also, as a side benefit, it releases a bit more power. This is the area where companies like Fluidampr excel with their viscous dampers.
Primary engine balance is achieved in two ways, either internally balanced, where the crankshaft counterweights match the total weight of the connecting rods and pistons. With internally balanced engines, the counterweights on the crankshaft are large enough to cancel out the rotating mass of the pistons and connecting rods. With this type of balance, there is less weight on the crank nose.
In some cases, if there is not enough room, a counterweight is moved outside the engine block to the front and rear of the crankshaft. For the purposes of this article, we are looking at the external balance as we have a small-block Ford project (Retro Project 5.0) which requires this type of equilibrium.
Some builders choose to convert externally balanced motors to internally balanced motors, but it depends on the application and if there is enough space inside the block. Some shot combinations that were originally internally balanced (i.e. 350 ci SBC) became externally balanced after adding this long stroke crankshaft (SBC 400). Some call it the “farmer’s kit”. But we digress.
The SBF 302 evokes a special situation for many passionate builders. Although the motors are externally balanced it depends on the model year whether it is an ’82 and later high output (HO) or earlier as there are two different cranks and balancers . But you can’t necessarily tell from the outside as they both have the same bolt pattern and look alike.
Fluidampr’s Nick Orefice says there are limitations with some engines that require external balancing, such as when there isn’t enough material on the crankshaft for the counterweight to balance all the parts. “External balance comes down to the length of the rods and, really, your overall space. If you don’t have space to balance inside the motor, you’ll have to do it with a flywheel or an externally balanced flex plate and then the damper itself, but Orefice warns that it also puts more pressure on the nose of the crankshaft, so there are limitations as well.
“With all of these components, you need this external balance [weight] to smooth it out. Otherwise, you’re just going to have a hot mess; it’s going to vibrate like crazy,” says Orefice.
Several applications are externally balanced from the factory, such as some diesel engines (Powerstroke and Duramax) and most small-block Fords. But there has been some confusion as to which balancer to use with small-block Fords, as there are two types. They are available in 28 or 50 ounce counterweights. The 50 ounce application is for HO models and is actually 34 ounces on the shock and 16 ounces on the flywheel.
OOne of the issues people have, according to Orefice, is that the engines are older and the current owner may not know what was done until they got it. “I think a lot of the confusion is just that so many people have gotten their hands on this over time,” Orefice says. “A previous owner may have mixed up some parts or didn’t know what they were doing. So we often go back and forth with customers who build 302s. Sometimes we find they have a 28-ounce engine over a 50-ounce engine or vice versa. Or, it’s an internally balanced motor, and they have a 28oz on it and nothing on the flywheel side. We see all kinds of crazy scenarios.
Orefice notes that confusion over these engines is usually not the fault of the current owner. “It’s usually soup they pick up on the road. Few people want to fire the engine to get a definitive answer. They can see that someone put the wrong crank in the engine, or they’re sure it’s an old one. before the 80s 302, so they expect a 28 oz balancer. But then they find out that the crank is from the early 90s, and it’s the one with the lightened counterweights that requires a 50-ounce balancer.
Orefice remembers seeing this play out on a new crate engine. “We’ve had a call from guys probably a dozen times. We talked for about an hour each time. He just bought his dream car and was trying to do it right. He said the engine had a few flashes of vibration at 1700 rpm, then 2900 rpm or something – just a few random spots. He just knew something was wrong. So he removed the engine and replaced it with a crate engine. He expected it to be a drop-in-and-go, but it didn’t work out that way.
After the customer took the engine apart to see what was wrong, it turned out that the crankshaft was ultimately the wrong one. “He calls us and says, ‘I have this motor, and it’s a stock shock. It’s stock this and that. To his knowledge, everything was identical to the original. So I did a little digging and found out it was supposed to be a 50oz motor. It was the wrong engine for the year of the car. Its model year denotes the 28 oz engine, and it is the old 302. Its engine should be 50 oz, which is the newer one.
It took this guy to pull the engine, take it apart, and take it to his local engine shop to find out what was wrong. The store knew this from the start. So it’s a common thing. —Nick Orefice, Fluidampr
Fluidampr’s viscous damper is an essential upgrade for anyone making modifications to their engine. The OEM shock is two pieces held together by an elastomeric rubber that wears and cracks over time. These shock absorbers are also not homologated for competition because they can be dismantled. The Fluidampr seals a steel ring with a unique silicone material that can smooth out vibration across a wide rpm range. OEM style shocks are only designed to cancel vibration within a narrow factory specified range. Any changes to the motor and vibration point can move up or down out of range of the stock shock.
Orefice says another term that people confuse (including this writer), is the definition of balancer and damper. A harmonic balancer aids in the actual motor balance seen in externally balanced motors, while the harmonic damper is strictly intended to dampen torsional vibrations. “Whatever you call it, balancer or damper, it plays no role in balancing unless it is an external balance,he says. The more you know.