Pros/Cons of Lightened Crankshaft - 3 banger yammy

[tsrt]

I did alot of searching through older threads but never found any definitive pros and cons to lightening your crankshaft. I like the idea of shaving some weight from the engine but are there any effects of doing this and the motor revving quicker? Just looking for some examples. Also is this only beneficial if you also have other parts installed or is it all good on just about any engine ?

[Fastjeff57]

From what I've seen of those pressed together nightmares, lightening them could make things even worse.

Jeff

[Fast Fred]

press cranks are good if set up right. lighting of the assembly will only let it turn up faster, makes it jittery at low speed, less momentum is stored.

[Roflhat]

Is there any reason you would lighten the crank instead of the flywheel?

[Fast Fred]

makes the assembly weaker,the amount of gain is hard to measure ether way, cranks do not make horse power, or fly wheels, they just hang on for the ride, you get a similar effect by lighten the boat, a bunch of people been killed by flywheels cuzz they thought they knew what they were doin, but didn't.

[smittythewelder]

. . . lightening of the assembly will only let it turn up faster . . .

Fred, since you know this stuff, I'm guessing you meant "quicker" rather than "faster." Peak rpm wouldn't change, but acceleration MIGHT be slightly increased, . . . but as you said, other things are affected, not always to one's advantage. (Sorry if this is a quibble, Fred).

As to flywheels having killed drivers, I believe these were usually cast iron flywheels on old engines from the '30s and '40s, and maybe had been lightened. I don't know of any modern racing engines using cast iron flywheels, though there were a few that used cast aluminum. Forged steel flywheels probably shouldn't be lightened because a forging has a tough "skin" that ought not to be ground or polished or cut with a lathe tool, unless you are willing to have a shop shot-peen the surface after you've altered it. A flywheel of billet steel or aluminum could be turned down, but even then, shot-peening is not a bad idea. Maybe fabricating some sort of scatter-shield to go around the flywheel is not a bad idea either. Harrison had these for the cast aluminum flywheels they used for a while.