# Engine Balancing, Part 5

“Bobweight Calculation” By Ted Eaton

The previous articles in this series have expounded upon match weighing the pistons as well as the connecting rod small and big ends. Now it’s just time to start thinking about the crankshaft bobweight calculation. The bobweight will be a specifically weighted fixture that attaches to each of the connecting rod journals for electronic spin balancing purposes and will in turn simulate the rod and piston assembly weights for those mass characteristics necessary for a perfectly balanced engine. Like most V8 engines, the venerable Y-Block will require four of these bobweights, one on each rod journal. Each bobweight will take care of the rotating and reciprocating mass requirements for two connecting rod and piston assemblies along with their respective rod bearing and piston ring packages.

With the weights of the pistons and each end of the connecting rods already recorded on the balance job worksheet, there are still some miscellaneous weights required before calculating what the total weight requirement will be for the bobweights. At this time, the weights of the piston rings and connecting rod bearings for one cylinder are needed. This is a simple matter of weighing these pieces on a gram scale and recording their values on the same work sheet or balance card. Piston pin locks are also weighed and recorded if being required on the engine being balanced.

All parts continue to be weighed in grams due to the increased resolution garnered by this measurement system versus that of using ounces. As a for instance, there are 28.35 grams in an ounce and for a point of reference, a typical dollar bill weighs a gram. Saying a dollar bill weighs a gram is much simpler than saying it weighs 3½% of an ounce or 3/85th’s of an ounce. Thus it is grams as they can then be further broken down as fractions or tenths for additional detail or resolution.

The final value required for the bobweight calculation will be a nominal value in grams for the estimated amount of residual oil that resides at any given time within the crankshaft and on any given pair of piston and rod assemblies. Although industry standard for this oil is 2-4 grams, different shops will add an additional amount based on their experience or preference. Some engine designs will even mandate a much higher value due to its engineering attributes that has the crankshaft or its attached components holding more oil than the standard amount within them. An example would be hollow crankshaft rod journals that hold additional oil either by function or machining ease during the crankshafts manufacture. The Flathead Ford V8 crank would be a good example for simplifying the manufacturing process by using oil reservoirs in the crank pins while the 427 Ford steel crank would have even larger crank pin oil reservoirs designed specifically for stored oil in the event of momentary oil pump starvation. The Ford Y-Block crankshaft design is such that the industry standard could be used but an increase in the oil value may be required to simulate some of the other weight variables that can work their way into the mix.

Adding a specific amount of weight for a given bobweight in excess of what is initially called for would be referred to as heavy balancing or being over-balanced. This is done in instances where anticipated weights or forces will be changing either during the course of an engines life or if the rotating and reciprocating mass characteristics are expected to change at a given rpm range or condition.

If a carbon build-up on the piston top was anticipated over the long haul, then this could be also added to the oil value at this point. If you have a preference for a different oil value to be used on your rotating assembly upon getting it balanced, then talk this over with your shop and get their input on this. Most shops will be agreeable to sutle changes in the bobweight values if you have specific preferences.

There are a variety of other conditions which would require “overbalancing” as part of the balancing process. A change in rod lengths or crankshaft stroke can benefit from a given amount of overbalance depending upon the amount of change in rod/stroke ratio. The use of nitrous oxide, superchargers, or turbo chargers typically also requires a certain amount of overbalance. Using nitro methane in conjunction with a blower is likely the worse case scenario as cylinder pressures are extremely high under detonation which artificially increases the piston weight by a more than a normal amount. Any form of blown engine will benefit from a given amount of overbalance simply due to the weight of the piston averaging artificially heavier not only from the increase in cylinder pressure at ignition, but the increase in cylinder pressure taking place while the cylinder is also filling during the intake stroke. In this instance, the piston is averaging an overall heavier weight when running at speed. A normally aspirated engine has a given amount of pressure counterbalance in that the piston is subjected to negative pressure when the cylinder is filling but is under increased pressure during compression and ignition. If an aspirated engine is working with an extremely well designed induction system and is benefiting from a ramming effect to fill the cylinders at the upper rpm ranges, then overbalancing also helps here. And then there’s the rpm factor. Balancing is linear up to a point throughout the rpm range but depending upon the masses at work within your particular assembly, there is a point in which the crankshaft rpm starts to out run the dynamics of the existing state of balance. Overbalance allows these dynamics to stay in tune or “caught up” to the rpm’s of the crankshaft. There are proprietary formulas that calculate these amounts of overbalance for all the different variables and will vary somewhat from shop to shop. Again, talk with your balance shop regarding overbalancing and determine if this would be best applied to your application.

Now that all the rotating assembly’s component pieces have been weighed, it’s time to calculate the amount each bobweight will weigh before building them and attaching them to the crankshaft. To repeat what was stated in an earlier article, a 90° V8 engine will normally require a bobweight that simulates 100% of the rotating mass and 50% of the reciprocating mass. Because a single bobweight is being used for each V8 journal and represents a pair of connecting rod and piston assemblies, the weight of one piston with its pin, ring set, and a single rod small end will be added to the weight of two connecting rod big end weights along with the weight of two complete rod bearings. This in effect will give the required 50% reciprocating (that which goes up and down) and 100% of the rotating mass. The appropriate amount of oil and desired overbalancing is also added at this point.

With the bobweight calculation now being complete, it’s then just a matter of assembling the bobweights on a grams scale to replicate the calculated weights and then attaching these bobweights to the crankshaft in preparation for spin balancing. The next article in this series will cover exactly this. Special thanks goes to Ernie “Bounty Hunter” Phillips in allowing the use of his balance card for his racing Y as an example. Until then, happy motoring.

Originally published in Y-Block Magazine, Jan-Feb 2005, Vol 12, No. 1,  Issue #66