The problem with the straight 8's was not having a main bearing between each crank throw like they would do now. The inline six Mercury outboards had the same problem. The racing versions had 3 more main bearings one between each throw, you could turn them over 8,000rpms with no crank flex, I only got 300 more rpm from a V-6 and that was because of better exhaust scavanging. The rods only push straight down on the crank at TDC and all engines do this. This has nothing to do with making a engine vibration free. They also didn't do very good balance jobs back then on the straight 8s because the motors turned less than 4000rpm max. If you have an 8 cylinder motor you have 4 power pulses per 360 degrees of rotation(every 90 degrees), regardless of the configuration of the cylinders. The V twins and the inline 2s both have 1 power pulse per 360 degrees(4 stroke). All engines have vibration, some worse than others, balancing carefully belps most. The more pulse per revolution the smoother the motor. Like a 2 stroke compared to 4 stroke with the same number of cylinders, you have twice the power pulse per revolution and that's why they will be smoother and more powerful. I've been around engines with as many as 36 cylinders, almost no feel of vibration.

 

Both Bear and Polar are correct about the firing pulses and the torsional vibration of the crankshaft. One other type of vibration comes into play here. The vibration of the engine itself due to the acceleration and deceleration of the pistons and con rods. For the inline engines, the vibration forces are aligned vertically along the axis of the cylinders. For the v-engines, the vibration is aligned along each bank at the same time. This means that each cylinder has a partial force component of vibration that directly opposes the other cylinder. The more angle between cylinders, the less engine vibration. The smoothest twin is an opposing twin. This explains why a v-twin would feel smoother, because of the lower engine vibration. Crankshaft torsional vibration is the same for each engine (unless it is a Harley which has non even firing cylinders). Another interresting thing about torsional vibration is that the torsional vibration of the twin due to inertial effects of the pistons and con rods at high engine speeds is considerably higher than even the combustion forces.

 

BryceGTX - well said.

Long live the boxer motor for smooth - you got that right. Porsche motors, Subarus, bmw bike motors, the old air cooled vw bug motors, lycoming and continental airplane motors!!!!!!!