External Balance: The Curious Case Of Counterweight Confusion

External Balance: The Curious Case Of Counterweight Confusion

No story discussing engine balance is complete without talking about the harmonic damper attached to the front of the crankshaft. In OEM form, the damper (also called a balancer with external balance) is simple and inexpensive, but only cancels out a narrow range of vibration (aka, frequency). To understand the nuances of externally balanced engines, we reached out to Fluidampr to have a discussion on the what, why, and how of hanging extra weight off of the end of the crankshaft. 

Types Of Balance

There are three types of engine balance. The first is primary balance of the rotating assembly. An unbalanced rotating assembly is like losing a wheel weight. It will make a noticeable thump at a certain speed and possibly be less noticeable (but still very much there, as far as your engine is concerned) at other speeds. The vibration can cause high engine wear (or worse), so it is critical to match the weights of the pistons and connecting rods with the crankshaft counterweights to minimize unbalanced vibration.

Another type of vibration is axial, which is the forward and backward movement of the crankshaft. Your thrust bearings and main bearing support plates prevent this type of movement. This is pretty straightforward to control. 

Torsional vibration is more challenging to isolate and control—the crankshaft twists during combustion events. Furthermore, any existing vibrations become harsher when you make more power. The harmonic balancer (damper) manages the twist reaction for better efficiency and durability, and also, as a side benefit, it unleashes a little more power. This is the area where companies like Fluidampr excel with their viscous dampers. 

Small-block Fords are commonly balanced externally from the factory, but there are two different styles from which to choose, and many people get tripped up thinking they have a 28 oz application when they have a 50 oz or vice versa.

Primary engine balance is achieved in one of two ways, either internally balanced, where the crankshaft’s counterweights match the total weight of the rods and pistons. With internally balanced engines, the counterweights on the crankshaft are big enough to cancel out the rotating mass of the pistons and connecting rods. With this type of balance, there is less weight on the nose of the crank. 

In some cases, if there’s not enough room, some counterweight is moved outside of the engine block to the front and rear of the crankshaft. For the sake of this article, we are looking at external balance because we have a small-block Ford project (Project Retro 5.0) that requires this type of balance. 

Some builders choose to convert externally balanced engines to internally balanced ones, but it depends on the application and if there’s enough clearance inside the block. Some stroker combinations that were originally internally balanced  (i.e., 350 ci SBC) became externally balanced after adding that long-throw (SBC 400) crankshaft. Some people call it the “farmer’s kit.” But we digress.

On Fluidampr’s balancer for the ’82 and later H.O. 302 (P/N: 650221) a 34-ounce counterweight on the harmonic balancer is combined with a 16-ounce counterweight on the flywheel, for a total of 50 ounces. Using the 28-oz version for pre-1980s engines will fit, but it won’t cancel out all of the vibrations.

The SBF 302 conjures up a peculiar situation for many enthusiast builders. While the engines are externally balanced, it depends on the model year, whether it’s an ’82 and later high-output (HO) or earlier because there are two different cranks and balancers. But you can’t necessarily tell from the outside because they both have the same bolt pattern and look similar. 

Nick Orefice at Fluidampr says that there are limitations with some engines that require external balance, like when there’s not enough material on the crankshaft for the counterweight to balance all the parts. “External balance comes down to rod lengths and, really, just your overall space. If you don’t have space to balance inside the engine, you’ll need to do it with an externally balanced flywheel or flexplate, and then the damper itself.” But Orefice cautions that it also puts more pressure on the nose of the crankshaft, so there are limitations there as well.

If you plan on going 10.99 or quicker at an NHRA track, you’re going to need this SFI approval on the balancer. With the amount of power wer’re hoping to make by the end of this project, this approval will be required.

“With all those components, you need that external balance [weight] to smooth it out. Otherwise, you’re just going to have a hot mess; it will vibrate like crazy,” Orefice says.

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 over what balancer to use with small-block Fords since there are two types. They come in either 28-ounce or 50-ounce counterweights. The 50-ounce application is for the H.O. models and is actually 34 ounces on the damper and 16 ounces on the flywheel. 

One of the issues people have, according to Orefice, is that the engines are older, and the current owner may not know what was done before they got it. “I think a lot of the confusion stems from just the fact so many people have had their hands in there over time,” says Orefice. “A previous owner may have mixed some parts up or didn’t know what they were doing. So we often go back and forth with customers building 302s. Sometimes we find out they’ve got a 28-ounce on a 50-ounce engine or vice versa. Or, it’s an internal-balance engine, and they have a 28-ounce on there and nothing on the flywheel side. We see all kinds of crazy scenarios.”

Orefice notes that the confusion over these engines is generally not the fault of the current owner. “It’s usually some soup that they picked up down the road. Not many people want to pull the engine to get a definitive answer. They may see that someone put the wrong crank in the engine, or they are sure it’s an old pre-’80s 302, so they’re expecting a 28-ounce balancer. But then they come to find out the crank is from the early ’90s, and it’s the one that has the lightened counterweights that requires a 50-ounce balancer.

The front view of Fluidampr’s SBF harmonic balancer shows the standard four-bolt pattern for all of the Ford units, so it can be difficult to tell from looking at the front of the crankshaft which one you have. And, because these are older engines that have typically had many owners and modifications, it can be even more of a challenge to figure out if you have the right crankshaft and flywheel for the application.

Orefice recalls seeing this play out on a new crate engine. “We had a guy call probably a dozen times. We talked for about an hour each time. He just bought his dream car and was trying to get it right. He said the engine had a couple of flashes of vibration at 1,700 rpm and then 2,900 rpm or something like that — just some random spots. He just knew something was wrong. So he pulled the engine out and replaced it with a crate engine. He was expecting it to be a drop-in-and-go, but it didn’t work that way.”

After the customer tore the engine down to see what was wrong, it turned out that the crankshaft was not the correct one after all. “He calls us and says, ‘I have this engine, and it’s a stock damper. It’s stock this and that.’ To his knowledge, everything was the same as the original. So I dug a bit and discovered it was supposed to be a 50-ounce engine. It was the wrong engine for the year of the car. His model year designates the 28-ounce engine, and it’s the older 302. His engine should have 50 ounces, which is the newer one.”

It took this guy pulling the motor, tearing it down, and taking it to his local engine shop to find out what was wrong. The shop knew it right off the bat. So this is a common thing. — Nick Orefice, Fluidampr

The viscous damper from Fluidampr is an essential upgrade for anyone making modifications to their engine. The OEM damper is two pieces held together by elastomer rubber that wears out and cracks over time. These dampers are also not approved for racing because they can come apart. The Fluidampr seals a steel ring with a unique silicone material that can smooth out vibrations through a wide RPM range. OEM style dampers are only designed to cancel out vibration in a narrow range specified at the factory. Any modifications to the engine and the vibration point may shift up or down, out of the range of the stock damper.

Orefice says that another term people confuse (including this writer), is the definition of balancer and damper. A harmonic balancer aids in the actual balance of the engine seen in externally balanced engines, whereas the harmonic damper is strictly for damping torsional vibration. “Whichever you call it, balancer or damper, it plays no role in balancing unless it is external balance,”  he says. The more you know.

 

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About the author

Brendan Baker

Brendan Baker is an automotive writer who lives in Akron, Ohio with his wife and two dogs. He started racing and building cars at a young age, building his first "racecar" at age 12 (a quarter-midget), which he put on pole position in its first race.
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