Talking Crankshafts with Tom Molnar – Part 3

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Over the past couple of issues we’ve enjoyed “talking crankshafts” with Tom Molnar of Molnar Technologies (www.molnartechnologies.com). As you’ve probably guessed by now, Molnar is a wealth of information. Equally important, he’s a straight shooter and doesn’t pull any punches when it comes to crankshaft technology. Everyone can learn from Tom’s knowledge and expertise. Check out our final half dozen questions and answers below:

(13) How does the fillet radius affect performance?

Fillet radii increase crankshaft strength.

(14) Are narrow rod bearings required with your crankshafts?

Most bearing manufacturers offer narrow or chamfered bearings that can be used without modifying the bearings.

(15) Are special main bearings required on your crankshafts?

Not in most applications due to the bearings that are available. It is always important to confirm the bearing does not make contact with the fillet radius, as this will shut off the oil flow to the bearing.

(16) On some race engines, a slight overbalance condition is used (for example, 51 to 52% of reciprocating weight). Does this actually work or is a neutral balance better?

The two main things we deal with in regard to balancing are rotating and reciprocating forces. If all of the forces were rotating, it would be simple. You have rotating weight on the rod pin, you put an equal amount of weight on the other side of the crank in the form of counterweight to counteract these forces. The problem comes in when the piston is at TDC and it changes direction, which puts a very high pulling (tension) load on the crank. Since this force only takes place at TDC and then is gone, and we cannot add and remove weight from the counterweights while the engine is running, we add 50% of the reciprocating weight to the counterweights to help reduce this tension load. Once the piston comes off from TDC, the pulling load is gone, but you still have the extra 50% weight on the counterweights for the rest of the 359 degrees of rotation. Adding another 1 or 2% to a crank that already has an extra 50% weight for 359 degrees of rotation does nothing but increase bending of the crank.

(17) Do engines with high amounts of boost (turbo charged, supercharged or nitrous injection) require a different crankshaft design than a normally aspirated engine?

Sometimes, yes.

(18) Today, many companies have their crankshafts forged overseas in countries such as China or Korea. Is it possible to control material and production standards with overseas foundries?

Yes, it is possible, but those who go looking for nothing more than a cheap place to get parts get just that: cheap parts.

What you’re looking at is the generous fillet radius found on a Molnar crankshaft. This increases the strength of the crankshaft.

When working with a crank featuring a large fillet radius, be sure to specify bearings designed for this feature. Most of the popular high performance/race bearing manufacturers offer special narrowed bearings.

Balancing is important, but don’t get too caught up in tricks of the week like overbalancing. Tom Molnar has a lot to say about this in the Tech Q&A.

Most custom crankshafts are supplied non-balanced. The reason is the crank manufacturer doesn’t know what weight piston-rod-ring-wrist pin combination you’re using and they need the exact parts in hand to balance the engine.

Certain applications where the engine is turbocharged or supercharged, or if it relies on a lot of nitrous, may require a custom crank. It’s best to talk to your crank manufacturer about this before making the purchase.