The Stovebolt Site Forums The Shop Area Engine Shop Dynamic engine balance

I'm curious to know exactly what is involved in a "dynamic balance" of an engine, in particular (if it matters) a straight-6 like a 235.

Are you going to race with the engine, Woody?


Straight-six engines are naturally well-balanced (like most of their drivers)

Balancing certain engine components during a rebuild would seem to be prudent. Pistons and rods would be easy to match - is a crankshaft normally balanced when it is ground/machined?

When I'm building a race engine, I weight-match all the moving parts such as pistons, rods, wrist pins, rings, etc. to a tolerance of +/- 1/10 gram. Take a complete assembly for one cylinder, for instance a piston, ring set, wrist pin, wrist pin locks, if applicable, etc. and weight the whole assembly. Do this for all the cylinders, and find the lightest one. Use this as the standard, and file the balance bosses on the other pistons until every assembly is the same weight. Be sure not to mix up components as the engine is assembled. A dollar bill weighs one gram, so every set of reciprocating parts is equal within the weight range of 1/10 of a dollar bill.

Rods are suspended at one end, and the other end is weighed, so both big and small ends get weight-matched individually. Be sure to include the bearing in the measurement of the big end, and keep that bearing with the rod during assembly.

Inline 6-cylinder crankshafts are inherently balanced, so little if any dynamic balancing is required. 4-cylinders, V-8's, and V-6's need "bob weights" attached to the rod journals that represent all the "reciprocating weight",(Pistons & rings, upper rod, etc.) and a certain percentage of the "rotating weight"(Rod big end and bearings) before getting a spin-up. A "fudge factor" is added for the weight of the oil in the assembly. Weight is either removed from the counterweights by drilling, or added by welding to dampen out any vibrations. Add-on parts like the flywheel, clutch assembly, or harmonic balancer can benefit from a spin-balance procedure if the equipment is available and the cost isn't excessive.
Jerry

all the "reciprocating weight",(Pistons & rings, upper rod, etc.) and a certain percentage of the "rotating weight"(Rod big end and bearings) before getting a spin-up

I think you typoed that, it's all the "rotating weight", (Rod big end and bearings) and a certain percentage of the "reciprocating weight" (Pistons & rings, upper rod, etc.) before getting a spin-up.

An L6 is in theoretical balance because of the firing order.
However, the extra weight for both balance and inertial purposes is contained completely in the counterweights, but it’s simply not possible for the weight to be positioned directly opposite the component itself (it’s alongside for cylinders #3 & 4, but three cylinders away for # 2 & 5, and on opposite ends of the crankshaft for #1 & 6), so there is some flexing and bending force caused by the rotation of the eccentric weight as the crankshaft rotates that cannot be removed or fully compensated.
This isn't a matter of accuracy or quality control: it's not possible.

I've been trying to make sense of the process of separating the rod weight into reciprocating and rotating bits, with no luck.
Obviously, the bottom rotates with the rod journal, but technically the rod cap and lower bearing half are moving the other way. The pin eye is (almost) pure reciprocating weight, but why is the dividing line between reciprocating and rotating weight located at the center of gravity?

Before we get too academic here, what's the difference between "static" and "dynamic" balance?

Yep, I swapped recip. and rotate! The setup instructions for the balancing machine I used to use had a formula for calculating rod weights, and assigning a value for upper and lower weights when computing the bob weight amounts. It's been some time since I did my own balancing- - - -one of my former students manages a machine shop where I send all my balance work now, and he does a great job for me. I've found that the main advantage to a well-balanced engine is in smoothness of operastion and long life. The owner of one race car we set up got a piece of shift linkage upside down and had the transmission in 3rd. gear instead of 4th. on a sportsman car at the Nashville track one season-opening night. When the rev limiter started shutting him down at 7500 RPM, he maxed it out at 10,000, and commenced to run 4 laps around the 5/8 mile track with the limiter cycling and resetting between 9,500 and 10,000 5 or 6 times a lap. (No tach- - - -the cable was on backorder) We got him flagged down after 4 laps, and shut down for the rest of the night. On inspection, there was NO engine damage, not even a stretched rod bolt! We changed rod bearings and ran the engine all season with no problems.
Jerry

Static balance refers to weight-matching individual parts. Dynamic is "moving" balance, the vibration caused by spinning a part that has a heavy spot somewhere.

In the case of wheel balancing, a static imbalance can be corrected by finding the heavy spot and installing a compensating weight directly opposite it. Dynamic balance comes in when the heavy spot is off-center, and induces a shake as the wheel turns. A static-balanced wheel and tire (bubble balancer) can still shake when it's in motion due to dynamic imbalance. Compensation of dynamic wheel imbalanced is done by installing the weight diagonally from the heavy spot. Modern computer balancers figure static and dynamic balance at the same time, and compensate with only one weight on each side of the wheel. Ditto for weight-matched engine parts when the crankshaft is out of balance, weight has to be added or removed to cancel out the shake after the parts are weight matched. The 120 degree separation of weight masses on an inline six make it much more likely to be in balance from the factory.
Jerry

The reason I brought the subject us is that the engine I started with had recently been "rebuilt" but on closer inspection it wasn't all right - scored bearings and cylinder walls and a replacement rod that was lighter than the others so the rebuilder did a dynamic balance.

I thought dynamic would involve spinning the whole assembly like a wheel balance and I just couldn't envision how you could spin an assembly of rods, pistons and crank. Now I know what's involved and my curiosity is satisfied.