Engine Balance and the Balance Shafts

Jan 7, 2010

As with any engine, balance is particularly important, especially at high rpm. It’s a common misconception that the balance shaft is used to balance the engine, when it actually creates a vibration to counter act the vibration you would feel at low engine RPMs. The reason behind this is when the crank is spinning in a circle; the weight at any one point needs to be equal so there are no uneven forces that would move the engine in any particular direction during its rotation, thus balancing the spin. A simple example can be found in your vehicles tires. If your wheels are not balanced, and you try to drive, you will feel your vehicle shaking. At high rpm, you will feel strong vibrations that can literally shake your engine apart. In the case of the motor, you add or remove weight from the crank, make sure the rod’s and piston’s weights are equal, and you make sure the balancer and fly wheel are both in balance. The better the balance, the higher the rpm your engine will be able to survive at.

Let’s take a look at the math. An imbalance of one ounce (the weight of a rod bolt), One inch away from the center at 2,000 rpm will be subjecting a force of seven lbs. At 4,000 rpm, that force grows to 23.5 lbs! Double the speed again to 8,000 rpm and the force becomes 114 lbs! Keep in mind that this is only one inch away from the center of rotation. The idea of this example is to demonstrate how balance issues are negligible at low rpm, but become drastically more important at a higher rpm.

The Ecotec engine has four cylinders that move in pairs. Two will go up, while the other two go down. This design of the four cylinder engine creates shaking when idled. The shaking does not necessarily mean that the engine isn’t balanced, it actually is. The shake comes at the moment the cylinders change direction. The cylinders moving up change direction to go down, and the ones going down, go up, creating a slight shock that is felt as engine shake. As the engine speed rises, so does the frequency of the shock, and it isn’t felt as much, as it is being absorbed through the mounts and chassis. A good example of this is a subwoofer vs. a tweeter. Bass is a lower frequency, and can be heard at longer distances. But your tweeters are on a higher frequency and do not travel as well. The principle is essentially the same.

 The balance shafts are actually out of balance rods that create a vibration. This is done on purpose, as it cancels out the vibration inherent to a four cylinder engine. Mainly, they do their work at idle and low engine speeds, where vibrations are felt the most. Being out of balance is bad for performance; however this trade-off is worth it for the average consumer. The real problem comes in when you want to use the engine at high speeds. Recall the math from above and how the forces relate to rpm. A factory balance shaft has a radius just over an inch, making the counter weight a lot more than an ounce, which in turn creates terrible engine shake. For high rpm engine operations, it is essential to eliminate the factory balance shafts, or run neutral shafts (which can spin at any speed without causing vibrations).  It also helps to add on 10/210, and the ATI super dampener mod. We went through a lot of stock crank pulleys until we gave up and finally ran this.