Engine balance

How many angels can fit on the head of a pin? All of those engines can have perfect primary and secondary balance. On the V and flat 12 or 16 engines, the bank angle makes no difference to balance as they are just either two inline-6s sharing a common crankshaft or two inline-8s sharing a common crankshaft. The true Boxers are inherently balanced regardless of the number of cylinders, but the crankshaft for a Boxer-12 or Boxer-16 is even more of a spaghetti noodle than a straight-8 crankshaft = never used on anything practical. If you include "perceived smoothness" in the criteria, the more cylinders = more cylinder firing events per rev = smoother, and equally-spaced firing events = smoother (so a 60 deg V-12 or 180 deg V-12 is very slightly "smoother" than a 65 deg V-12). With that, a V-16, Flat-16, or Boxer-16 with equally-spaced firing events would be the "smoothest".

 

These engines aside from the V8 found on the lancia Thema are flat plane. Compared to a cross plane balancing these crankshafts is much more tricky as the mass you have to balance is not on the same axis, but you get less mass in rotation which has some obvious benefits at higher RPMs.

Also as Steve said, the more cylinders, the less rotation required to produces the same number of explosions. A V12 at 4000rpm will always feel more balanced than an inline 4 at 12000rpm.

 

Any backstory on why Ferrari bothered re-engineering their then core high volume engine for the Thema?

 

They went to the cross-plane crank on the 8.32 because that gives better mid-range torque in a 90 deg V8 at the expense of high RPM HP (which makes sense to do for a 4-seater and on the typical pushrod American V8 because it can't get to high RPM anyway because of the valvetrain and long stroke). Some of the engines in the OP's post are flat plane cranks, but some are not. The characteristic that they all share is alternate bank firing (a flat plane crank in a 90 deg V8 also gives alternate bank firing, but having a flat plane crank in other block types would not and wouldn't even give equal spaced firing which would be horrible).

 

Any motor can be balanced.. just not a 4 cylinder

Ray

 

The Renesis MSP 13B is the smoothest engine I have ever raced. Formula cars have solid mounted engines that serve as structural members between the tub and the gearbox so you can definitely feel everything.
In-line 4 cylinder and V6 powered formula cars I had to just “drive through” the vibrations. My PFM had very minimal vibrations.

 

I've driven modified 13B rotary engine powered cars... so nice.

Ray

 

More balance at lower rpm, and the crankshaft is not cast unlike other "production" engine so this was not overly expensive (for an already expensive crankshaft).

 

Audi did it with a 5 cylinder engine and later with a V-10 engine. I suspect with modern CAD design programs, the crank of any engine can have the balance weights located for a smooth operation.

 

Flat plane are harder to balance for this reason: less counterweights on the oposite side of the cylinder.

The issue however is: secondary" balance: it is dead easy to balance an engine for a given rpm, much more complex at different rpm ranges.Its just like musical instruments: any piano wire can produce three tones (base tone, octave and harmonic), tuning the first two is possible, the third (the harmonic) is another story. That's why you have some felt between the piano wires (in order to "mute" the hard to tune harmonic, and not have to care about it). No felt on a Steinway so you've got to fully tune it.

Same story for our engines: must be fully balanced across the whole rpm range.

(Forgive the so-so english).

 

Yeah. Toyota did a pretty good job with the 3RZ-FE using twin counter balance shafts (which rotate at twice the engine RPM's if I'm not mistaken). Most people remove those and throw them away when modifying the motor however.

Ray