Posts Tagged ‘balance’

Internal Balance

June 27th, 2009 No comments

I talked with Bob again the other day about internal balancing, and I found out a couple things.

In an externally balanced engine, like the stock small block Ford, weights are installed on the flywheel and the harmonic balancer that balance the rotating assembly. Pre-1981, this amounted to 28oz or so. After 1981, 50 oz.

This is fine for engines that get mostly street use, with some high revving now and then. When you start revving high a higher percentage of the time, and/or up your revs higher than about 6k, you start to see the disadvantage. The weights on the end can set up an oscillation in the crank at high RPMs. As you can imagine, the bearing clearances aren’t very high – even a slight oscillation can cause damage to the main bearings.

Moving the weight closer to the radial and axial center reduces this tendency toward oscillation (or at least lowers its amplitude) and the bearings last longer in a high-stress motor. This also reduces the stress on the crank itself and it’s less prone to fatigue.

Categories: Engine and Drivetrain Tags: