To balance or not to balance?
That is a great question but a perfectly balanced engine is not always a good thing. The real question is when to balance and how to balance.
Part of the difficulty with understanding how to balance an engines rotating weight is the fact that not all of it rotates. The crankshaft spins but part of the connecting rods and the pistons don’t, they move up and down so how do you balance it all out?
The answer is actually quite simple. When calculating weight which is attached to the crank, the rod bearings and the big end of the connecting rods are considered “rotating weight” while the upper parts of the connecting rods along with the wrist pins, pin locks, pistons and rings are all considered “reciprocating weight” because it moves up and down. Typically, 100% of the rotating weight is added to 50% of the reciprocating weight to determine bobweight. Depending upon what size engine is being balanced, many engine builders will add 4 to 6 grams to the bobweight to simulate oil which will be clinging to the engine parts.
Once the bobweight is calculated, actual “bobweights” are attached to the crank’s rod journals to simulate the weight of the rod/piston assemblies which is then spun on a balancing machine. In the case of balancing an engine here at Asheville Engine, Inc., we use a state of the art CWT Model 5500 balancing machine.
Today, almost all quality aftermarket components come “weight matched” so other than checking them before assembly, there is nothing to do if you are starting with a balanced crankshaft.
As stated earlier, a perfectly balanced engine is not always a good thing. In some cases it is better to “overbalance” an engine and in other cases, it’s better to “underbalance” an engine. Overbalancing is when the percentage of reciprocating weight is increased by a few percentage points when the bobweight is calculated. Of course, when underbalancing an engine, the weight is removed. I won’t go into great detail because we are building Ford Powerstroke diesel engines and even with over size pistons in it, the bobweight only changes about 4 to 5 grams which is totally irrelevant because the weight of the piston is “Reciprocating weight” and does not spin and Powerstroke engines have such a large crankshaft that an overbalance of 4 to 5 grams is nothing! These engines also have an external harmonic dampener attached to the crankshaft which will easily smooth out a minor imbalance.
Here is an example of where it does matter.
An engine builders who used to work for Asheville Engine at one time worked on a pro drag racing team. They built a perfectly balanced engine (100% rotating/50% recipriocating weight) which was designed to run at just over 8,000 RPM. The driver reported that the vibration was so bad right before he got to the lights that his eyes teared up and it was hard for him to see. In the short amount of time that the car was on the track, the vibration had fatigue-cracked the intake manifold and oil pan.
The engine was disassembled and the crank was overbalanced at 52% (100% rotating/52% reciprocating weight) and the vibration was reduced significantly at the high RPM. The next time that the engine was torn down, the crank was overbalanced 54% which caused a pretty violent vibration in the low RPM range but reduced the high RPM vibrations. It not only reduced the high RPM vibrations but it also decreased the elapsed time on the track which is what the team was really after.
The next time that somebody tells you that a Powerstroke diesel engine, which is designed to be operated at roughly 3,000 RPMs needs to be perfectly balanced, keep in mind that the slight amount of vibration from an extremely low bobweight imbalance will be easily picked up by the harmonic dampener and will not affect performance or longevity of the engine.
That being said, Asheville Engine has always balanced engines for a small additional charge for people who want their engines balanced. For the average guy who’s using his truck to work, haul a camper and get groceries, you don’t really need it but if it’s what you want, it helps us pay for our machine.
Asheville Engine, Inc. We Keep You Strokin’