Engine Balancing - Tech - Balance of Power

When it is powering a race car--with all the rumbling, vibrations,beating, and banging going on--it can sometimes be easy to forget thatthe engine is a precision piece of equipment. In the hands of the rightengine builder, all the moving pieces are as finely tuned and move withthe precision of the proverbial Swiss watch.

That's a good thing. Because of the rpm being turned by race engines,any unbalance can have severe results. Two decades ago, if one pistonand rod combination was heavier than the rest, it usually wasn't aproblem. The entire mass of the rotating assembly was so much that adifference of 10 or more grams didn't make much difference. Today,however, component manufacturers are pushing the design limits ofhigh-strength/low-weight components; a piston and rod combination thatis just a few grams too heavy or light can mean ruined bearings, shearedflywheel bolts, and worse. That's why properly balancing your raceengine's rotating mass is more important than ever before.

Calculating Weight

Part of the difficulty with understanding how to balance an engine'srotating weight is this: Not all of it rotates. Yes, the crankshaftspins, but don't the pistons and part of the rods move up and down? Howdo you balance it all out?

The answer is actually a very simple calculation. When calculating theweight that is to be attached to the crank, the rod bearings and the bigend of the rod are considered rotating weight. The small end of the rod,piston, wristpin, rings, and pin locks are all considered reciprocatingweight (instead of spinning around, it moves up and down).

Typically, 100 percent of the rotating weight and 50 percent of thereciprocating weight are added together to form the total for the bobweight. Some engine builders include four or five grams in the total tosimulate the weight of oil clinging to the various parts. The bobweights are actual weights that are attached to the crank's rod journalsto simulate the weight of the rod/piston assembly. With the bob weightsin place, the crank is ready to be spun on a balancing machine.

Understanding Overbalance

If you have hung around engine shops, then you may have heard that acrank has been "overbalanced." You may have even heard the phrase "underbalance," but hopefully never in association with a circle track racingengine. Overbalancing is when the percentage of the reciprocating weightis increased by a couple of percentage points when calculating the bobweight. The idea is that this reduces high-rpm vibrations.

"There are always two critical imbalances in any V-8 engine," explainsKeith Dorton, owner of Automotive Specialists. "Normally, there's alow-speed and a high-speed imbalance. Properly balancing the crankshaftreduces it, but it doesn't get rid of it.

"My first experience with overbalancing was early in my career withmotorcycle engines. They were high rpm engines, and we were getting upto 60 to 70 percent overbalance on them to keep the high-rpm vibrationsdown.

"One of the first applications I remember overbalancing a V-8 was wayback in the '60s on a big-block 427 Ford that was drag racing in the AGas class. Back then, it was making somewhere around 570 to 580horsepower.

"The racer that ran the car was pretty much ahead of his time. Backthen, the NASCAR guys were running their engines to 7,000 or at most7,200 rpm, but this guy was turning a big-block close to 8,000 in adragster. We balanced his rotating assembly using common practices forthe time--50 percent reciprocating weight and 100 percent rotatingweight. But he complained that when he really got the thing going inhigh gear, right before he got to the lights, the engine startedvibrating so badly it actually blurred his vision.

"The high-rpm vibration was vibrating things off the car. In the shortamount of time the car was on the track, the vibration wasfatigue-cracking the sheetmetal valve cover and the intake manifold.Plus, this driver had a really good feel for the car, and he could tellthat when the engine got into this vibration, it was also laying down.

"So we brought the engine back in and overbalanced the crank. Weoverbalanced it at 52 percent (each bob weight weighed 100 percent ofthe rotating weight and 52 percent of the reciprocating weight), and itreduced the high-rpm vibration substantially. The next time we had it inthe shop, we brought the overbalance up to 53 or 54 percent and thathelped it even more. The change caused a pretty violent vibration in thelow-rpm range. I think it was between 3,000 and 4,000, but that wasn't aconcern because we weren't racing there. The change didn't just reducethe high-rpm vibrations. It decreased the elapsed time on the track,which is what we were really after."

For circle track racing, Dorton says that most engines are onlyoverbalanced by a couple of percentage points. That's because an ovaltrack car, especially for short tracks, will have to pull through a widerange of rpm. Although the engine may rev beyond 9,000 rpm, it will bein that range only for brief moments at the end of the straights. Moreimportant is how well the engine pulls at much lower rpm when the car isaccelerating out of the turn. The goal is to adjust the components sothat the critical vibrations fall at rpm levels below and above yournormal operating range.