CRANKSHAFT BALANCE.

In a low speed drag race engine, what assets do an internal balanced crankshaft offer compared to the same crankshaft which is originally designed to be balanced externally. Does the internal balanced part offer consistent bearing load at all five main bearing locations whereas an external balanced part may accelerate the wear of the front and rear main bearings? Is changing the balance method cost efficient?

Dave.

Dave,

You said in a low speed application, and imo there is no advantage that an internal has over an external balanced engine. Is it worth it? Not imo. There are a couple million or more oem engines out in the real world, that were externally balanced from the factory. That includes various cars from all of the big three.

Charlie

Dave,
The bearing loads are really very similar between the two types of balance - that's not the concern area. What's different are the torsional loads on the crank. External balancing relies on the correction at the ends of the (two) balance planes, while internal balancing is performed at a shorter distance, resulting in lower torsional loads. Depending on what material you're working with for the crank (forged or cast), power levels, and the amount of external correction, the peak fatigue stress will be recognized at either main/pin fillets or the post fillet.

Now that the fatigue question has been answered, the other question: "Does it matter on a low speed engine?" If low speed (under 6000rpm) is the applicaiton, probably not. As stress goes up at the cube of speed, under 6K is likely safe.

All of our new engines are internally balanced for fatigue life and to keep NVH lower. As a crank engineer for Ford, with the Eco-Boost models and GT500 5.8L, we're seeing some pretty hot engines out now.

Mike

Unlike modern cars, even the internal balance on early engines, moves the balance point about 4" from the flywheel, and or balancer (or flying hatchet weight in the 460) to the first and last throughs on the crank internal to the engine.

Modern engines and or high dollar cranks have counterbalance at each through. The harmonic is eliminated at the source, instead of eventually.

I think that there may be some confusion stated after my initial post. I'll try to address the points:

Don't confuse torsional fatigue with bending fatigue - the two are different.

Modern high speed V8's utilize full counterweights on every throw to minimize bending fatigue, not for torsionals.

Crank balancing on is done with two planes of correction - generally mains 1 & 5. Whether correction is done at the flywheel or at the counterweight is of little significance with respect to balance as zero is still zero. It does have an impact on harmonic torsionals recognized by the engine at speed however, but this can be tuned out with the dampener.

Cranks always carry larger counterweights at each end as their effect on reducing couple (twist on an axis) is greater.

Dampener tuning is a tricky business and is unique to the application involved. Much time is spent to achieve favorable NVH characteristics. Selecting an aftermarket dampner is utilizing Kentucky windage and only gets you in a ballpark as no one but the OEM's go to the pains to instrument an application.

Mike