Inliners International Forums General Discussion Engines Inline 6 Crank Needs Re-balance 4 lighter Pistons?

I’m sorry to ask this question, but I forgot exactly what I’d learned in college. I’m building an increased stroke 280 Mercedes (1960s) engine, had the crankshaft offset welded, ground and balanced. I had wondered why the grinder/ balancer didn’t need to know the weight of my much lightened piston and rod assembly, in order to remove the correct amount off the counter weights and he insisted that on an in-line 6 cylinder there was no need to change the balance of the crankshaft unless I was building a high revving racing engine (I’m not/ 6500rpm). While I didn’t notice any engine vibrations after assembling and testing this 1st version of the modified engine, the current 2nd version has custom pistons, rings and wrist pins that weight approximately 150 grams (20%) less than the original. Does the balance now have to be adapted to achieve a smooth running engine (or less stress on the crank pins)? Ross Pistons and my crankshaft grinder (Marine crankshaft) claim “NO”?, but don’t seem to be too familiar with the theory to back-up why...

I seem to remember that there can be several “orders” (primary, secondary...) of frequencies that occur on any engine and while it seems likely that on an in-line 6 cylinder, one of those orders is inherently balanced, if their were NO imbalances at all, on an in-line 6, it wouldn’t make much sense that the manufacturer added fairly sizable counter weights opposite most of the crank shaft journals, to balance the weight of (I’m assuming), the big end journal, the pistons and the oscillating mass of the connecting rods. They even take care to fine tune, (drill) these weights to achieve the accurate balance. This is what I am trying to understand. Why would it not matter if I change the weight of the pistons and rods.?

Much thanks, Tom

No. An inline motor is inherently balanced, so as long as the pistons are balanced, the rods are balanced, the crank is balanced, the balancer, clutch, etc, you don't have to rebalance when you change components. On a v-motor of any type, the 90 degree twist means you can never truly balance the motor for all rpm's, but they can use bob-weights to approximate the forces acting on the crank to minimize the vibrations in the rpm range you are primarily using the engine in. A good shop will find out what you are doing with the engine and balance it to suit on a V-6/V-8. Score one for the inlines!

The inline 6, flat 6, and V12 are inherently balanced. Translation: if all the components weight the same, and no crankshaft flexing occurs, all forces cancel for complete balance. In actual applications, crankshafts flex so nominal countweights are added.

The V8 can be completely balanced, requiring crankshaft counterweights.

Inline 4's and V6's engines require additional countershafts running at twice engine speed to balance forces.

The flat 4 engine's primary and secondary forces balance, but there is a resultant couple (torque).

The Bosch Automotive Handbook had a table of engine balance characteristics.

Also see http://en.wikipedia.org/wiki/Engine_balance

As I recall from vibrations class nearly fifty years ago, the inline 6 and 8 cylinder engines are the "best" as far being inherently balanced, as 56er points out above. According to Professor Taylor's textbook on IC engines, the 60-degree V-12 and 45-degree V-16 behave similarly. An inline four or a V-8 have an inherent out-of-balance problem.

For completely balancing an American style V8 (cross plane crankshaft):

http://craig.backfire.ca/pages/autos/v8-engines



Formula 1 V8's racing engines, along with Lotus and Ferrari V8 street engines have had flat plane crankshafts. The flat plane V8 crankshaft engines have more vibration, lighter crankshafts, and better exhaust tuning for each individual bank of cylinders, compared to cross plane V8 crankshafts.

The V8 can be completely balanced, requiring crankshaft counterweights.

Source of this information?

"The V8 can be completely balanced, requiring crankshaft counterweights.
Source of this information?"

Bosch Automotive Handbook has this information. My edition was from back around 1980, though new editions are currently available.

Also from http://craig.backfire.ca/pages/autos/v8-engines :

"The crossplane V8 would have first-order imbalance because of how the pistons are moved by the crankshaft, but the use of full-weight counterweights prevents this."

"Full-weight counterweights on the crankshaft are shaped and weighted in such a way that they counter the forces from the pistons and connecting rods on the nearest pin at the same time. Regular counterweights only counter the forces from the connecting rods and the crank pin. Only an engine with a 90° v-angle can utilize full-weight counterweights."

"Full-weight counterweights are placed on both ends of the crankshaft to oppose the rocking motion that would come from the forward most pistons moving in the opposite direction to the rearmost pistons. The middle two crank pins do not need full-weight counterweights because the piston motion forces are somewhat cancelled by the pistons on the other side of the engine. These counterweights are still very large compared to regular counterweights, however."

"The crossplane V8 has second-order balance, and that fact is the main reason for its existence. The odd-looking crankshaft moves the pistons in such a way that the net velocity of all of the pistons is always equal to zero, meaning that no second-order vibrations will be present."



An engine is generally considered balanced if the net primary and secondary forces and moments are zero, four conditions.
In actual applications, crankshaft flex and uneven firing orders on a cylinder bank may result in some vibration.