Technical Balancing an engine!

Find a good machine shop that can balance your assy. Tell them your plan - before you buy parts. Part selection in rotating assy can drive your balancing cost up alot. An experienced shop has probably seen whatever combo you dream up, or they've at least done the engine you want - and have info on how much impact a rod and piston weight combo would have on an assy.

That's all you need to know,

 

I'll try a simplified explanation.
If you looked down the centerline of the crank it could be balanced rotationally [like a car wheel] but the crank might not be balanced longitudinally.
So it could be heavier at each end but diagonally opposite. The crank would get a harmonic shake [similar motion to a Kayak paddle]

Normal inline engines have up/down cylinders adjacent to each other at each end [sixes are 120 deg apart so they need a small counterweight]

A V8 has 2 issues
1: the crank throws are 90 deg apart so you don't get adjacent crank throws counteracting each other
2: each crank throw has reciprocating mass that is 90 deg apart [from the "V" configuration].

That is why the reciprocating mass needs to be balanced [then added as a bobweight] and then the rotating mass needs to be balanced [with the reciprocating mass] .
It is also why there is a counterweight at each end [either internally or externally]

 

If you want an eye opener price out Mallory heavy metal.

 

I used Copper Tungsten, heaviest stuff I could find other than lead. Beats me why I didn't use lead...

 

Very interesting topic.

My OT pickup truck has an I-5 in it, I can’t even figure out how the firing order works let alone how to balance it!!!

On a more serious note many radial aircraft engines have rotating balance shafts built into them, if you’ve been around a Pratt & Whitney R-985 when it shuts down you can hear them rattling and banging as the weights change position due to loss of centrifugal force.

 

I built a 348 SBC using Ford 300 6cyl rods Because of the extra weight of the rods I should have went with after market rods because of the malory metal it took to balance it. But it was a sweet engine.

 

My experience: years ago, I decided to build a 383 SBC “claimer” racing engine. My builder really didn’t want to mess with it, because the cheap, cast pistons were so heavy. Lighter pistons make the assembly easier to balance, requiring less Monel metal.

Another SBC, I supplied a new 3.75 crank. I told them this one was internally balanced. My previous setup was externally balanced and they had my old front balancer and flywheel. They took a long time. They called and complained that they had already put a lot of heavy metal into it, and it it didn’t look like it would ever balance out. Guess what happened and what I should not have needed to explain to them.

 

Go ahead, tell us that don’t know

 

When they perform the balancing procedure, they bolt all the parts together and spin it on a machine. The original front balancer and flywheel had weights attached to them for the original external balance setup. My new high performance racing crankshaft had been manufactured as an internal balance setup. Maybe someone could take the time to explain the difference between internal and external crankshaft balance?

They should have known better than to bolt these parts together and then try to balance them. I told them to remove the counter weight from the harmonic (front) balancer and just buy a new, correct (internally balanced type) flywheel. The counter weights are welded to the flex plate on a Chevy.

 

And the length of the connection rod has an impact on this. The shorter the rod, the more dramatic this effect.

 

I balanced a single cylinder engine for GTP..I complained to engine balancer that it still vibrated bad at 1800 rpm ; at 1500 idle it was fine, at 2000 up it was fine. Did ya guess? He said set the idle at 2k.

 

Sounds like that age old doctors reply when saying to him "my arm hurts when I do such and such".
His reply is, well, then don't do that!

 

Ducati used to make a single cylinder engine that was basically a V-twin engine only using 1 cylinder, but keeping the second connecting rod that wasn't connected to any piston, just to help quell vibrations.

 

The only engine I balanced myself