I am going to go out on a limb here and say that many years ago when consulting for an auto business one task was looking at future engine specs for their FWD platforms. The I6, V6, and I5 were considered. My memory isn't all it could be given the timeframe, however the I5 delivers a harmonic of one higher magnitude than the I6. This combined with the even firing interval for the crank config made the engine very smooth. It also provided a good balance of bore spacing vs engine width (trans space being critical)

The end choice was an I6 purely because the marketing viewpoint won and trans engineers said they could do a compact box). The 90deg V6 was a nightmare except if offered manufacturing modularity for future expansion into larger engines.

For an ultimate affirmation of the I6 however, look at truck engines. These have worst case scenario for vibes etc due to reciprocating masses (some are now at 17litres and 650hp) and are almost exclusively I6s. Higher end are now returning back to V8s.

Now, if only I could find my calcs for the I5???

Inline-6 vs. V6, the real deal!

The I-6 and V12 are the only "naturally balanced" engines configurations. The Inline-6 is definetely making a comeback. Check out the all new 4.2 liter GM Vortec Inline-6. It has 270hp/257lb.ft of torque. It has been getting rave reviews thus far. The 2.8 liter twin-turbo Inline-6 in the Volvo S80 T6 and the new 3.0 liter Inline-6 in the new BMW 330i and 530i are all marvelous engines.

The I-6 engine is balanced end-to-end and requires no balancer shaft. The vertical and transverse forces generated by the individual cylinders, no matter first order or second order, are completely balanced by one another. The resultant vibration is nearly zero, thus the Inline-6 is virtually a perfect configuration. The Inline-6 is not the only configuration can deliver near perfect refinement, but it is the most compact one among them.

All Boxer engines are perfectly balanced, but they are two wide and require duplicate of blocks, heads and valve gears. This results in less efficiency versus an Inline-6. V12 engines also achieve perfect balance, but are obviously out of the reach of most mass production cars. Automotive engineers knew that long ago, that’s why you can see most of the best classic engines were Inline-6, such as the Rolls-Royce Silver Ghost, Bentley Speed Six, Mercedes SSK, many Bugattis, Jaguar XK-series and BMW’s and Volvo's various models.

As space efficiency becomes more and more important, most car makers favour the V6. The most influential V6 was perhaps Alfa Romeo’s 2.5-litre 60° V6 used in the GTV6. It established a reputation for a V6 that it can be compact, powerful, and smooth. An equivalent inline-6 would have never fit the small and sloping engine compartment of that car. Compare the shape of a BMW with an Alfa and you’ll know the packaging advantages of a V6.

When comparing a 60° or 90° V6, it is important to realize that they have somewhat end-to-end vibration like 3-cylinder engines. The is especially true for the 90° V6. It needs a counter-rotating single balancer shaft, at crank speed, to suppress the vibration. The balancer shaft is located inside the V-valley, so it is not space engaging. On the other hand, the 90° V6 has a decisive advantage in a production point of view. It can be machined in a V8’s production line because both of them are 90°. Unlike a V6, a V8 can only be optimised at 90°. This saves a lot of production cost. An example is Mercedes’ fairly smooth 2.4 to 3.2-litre V6s, which share the same architecture with their V8s, but added with a balancer shaft.

Inline-6 engines are nearly impossible to be used in front-wheel drive cars as well. Even a car as wide as Volvo S80 had to introduce the world’s shortest gearbox in order to make space for the 2.9-litre Inline-6 mounted transversely in the engine compartment. Longitudinal mounted Inline-6 doesn’t have such problems, but it engages too much space in north-south direction, thus engage some space which would have contributed to cockpit room.

However, BMW and Volvo are still loyal to Inline-6 engines. Ultimately, the Inline-6 engine is more efficient yet smoother. The V6 has more energy loss because it duplicates valve gears and camshafts, which increases frictional loss, while the use of 2 cylinder banks leads to more heat loss. In terms of production cost, although a V6 has 3 fewer main bearings, it has more valve gears, which is getting more and more costly these days, with the introduction of twin-cam, hydraulic tappets/finger follower, and variable valve timing. The Inline-6 is going to be cheaper than equivalent V6. The volumetric size versus horspower and torque of a well designed Inline-6 will always be better than a well designed 60°or 90° V6.

SwedeSpeed

-DrewSRX

Alex,

I don't recall anyone claiming 5 cylinders as the smoothest. It is pretty much agreed in the auto world that inline sixes are pretty much the smoothest and most reliable of engines around. Just look at BMW for example and there use of inline sixes...

No 5 cyl of any manufacturer is smoother than an I-6 or a 60 deg V6. 5 cyl's fire every 72 degrees. 4 cyl's are also unbalanced. 90 deg. V6's are also unbalanced unless the crank journals are offset to compensate for the inherent odd firing order.

My 965's engine is as smooth as silk.