Inline 4 engines have perfect primary balance, but poor secondary imbalance. The longer the stroke, or the heavier the rotating assembly, the more second order vibration the engine generates. This stems from the difference in height of the pistons and rod cobination and the top and bottom of the power stroke in an engine. Crankshaft weighting can only be used to affect primary balance, and has little to no effect on secondary balance which is what this article will cover. 

At idle, these vibrations are low frequency maifest as vibration. As engine speeds increase, these vibrations shift into the audible range and it is amplified within the cabin of the vehicle. Some have described as the ticking sound of a diesel engine, but the sound is really subjective, the fact is that it will be present in some form.

To deal secondary order vibration that arises from this imbalance, a pair of balancer shafts is needed. They are driven by the crankshaft and rotate in directions opposite to each other, at twice the speed of the crankshaft. This is because there are two imbalacing points for every crank revolution. They are located on either side of the engine. Counter weights on the balancer shafts completely cancel the second order forces, reducing unwanted frequencies in sound and vibration.

The use of two balancer shafts instead of a large single one is because the vibration generated by the engine is mainly vertically . Two shafts rotating in opposite direction can cancel each other’s transverse force and result in a vertical force which balances the vibration.

Balance shafts cancel out engine vibrations at all engine speeds, not just at idle. Above idle those vibrations will manifest as cabin noise that increases in volume with engine speed. This is mainly done to improve noise, vibration and harshness levels in cars. Here is an anecdotal example that was found on a Honda forum:



“Prior to the swap (so K20A2 with no balance shafts) with the hard motor mounts these secondary vibrations resonated the cabin to the tune of ~87-88 dBA just cruising on the freeway at 3000 RPM (this increased to up over 90 dBA at higher RPMs). I played around a bit and confirmed that this measurement was unrelated to throttle position, it was pretty much purely a function of engine speed. Also my exhaust system was completely stock at the time.

With the K24 (with balance shafts), this noise is massively reduced. Now I measure around 74-75 dBA at the same freeway cruising speed. While previously the noise in my car was dominated by the cabin vibration, now it's mostly wind noise. I also now have a race header (with high flow cat) and an 05 RSX type S exhaust.”

In the case of the Honda K-Series engines, it is interesting to study the Japanese market FD2R K20A and the CL7 K20A engines which were produced around the same time and installed in the Civic Type R and the Accord Euro R respectively. 

Power 

2011 FD2R 165kW［225PS］/ 8,000rpm

2008 CL7 - 162kW［220PS］/ 8,000rpm

Torque  

2011 FD2R - 215N・m［21.9kgf・m］/ 6,100rpm

2008 CL7 - 206N・m［21.0kg・m］/ 6,000rpm

TODA, a respected Japanese tuning company sells a balance shaft delete kit for Honda K-series engined cars that use them. They note that there is a 5hp cost and a 4 kg weight penalty when using them, but the benefits are a more refined engine and smoother engine suitable for daily use. The picture below shows what the balance shaft delete kit looks like. 

For comparison, below shows a K-series engine with balance shafts installed.

And a picture of the housing containing the balance shafts removed. 

Concluding this article, balance shafts play a roll in reducing unwanted harmonics and frequencies in the K-series Honda engines. The engines will run without them for a marginal gain in torque and power, but at the cost of noise, vibration and harshness. Longer stroke engines like the Honda K24A or engines with heavy reciprocating masses would benefit more from having these balancing shafts.