Thread: Flat plane cranks

Why do they sound better?
The Shelby GT350 R sounds excellent - BBC Top Gear

V8's with cross plane cranks are better balanced than those with flat plane ones because they do not have secondary vibrations. However, with a 4 stroke motor, it is not possible to have a firing order with cylinders on each bank firing evenly. Instead it is right left right right left right left left which gives the characteristic V8 rumble. Especially with twin exhaust systems and no balance pipe between them. Can easily check firing order by looking at distributor cap and where leads from it go.


With a flat plane crank, up and down secondary vibrations at twice engine speed occur for each bank as in an in line four cylinder motor. Note how this crank is symmetrical around the centre main bearing. If instead pistons one and three went up and down together and pistons two and for did likewise, would create primary vibration of engine rocking up and down from end to end at engine speed.


Now to explain what creates the twice engine speed secondary vibration produced by the in line 4 arrangement: Note that when pistons 1and 4 are on top dead centre and pistons 2 and 3 are on bottom dead centre, their combined centre of gravity is exactly half way up the cylinder. When 2 and 3 are on top and 1and 4 on bottom, C of G is also exactly half way. However, when crank is a quarter turn / 90 degrees round from this, with four pistons all at the same height with two on the way up and two on the way down, because conrods swing in an arc from the gudgeons, they are BELOW half way up the cylinders. The position the crank has to be for a piston to be half way is when there is a right angle between the main bearing, big end and gudgeon when looking from the end.


The overall result of this is the centre of gravity of the pistons of an in line 4 cylinder motor (and also the secondary vibration in a !80 degree twin) bouncing up and down at twice engine speed. In some applications such as many tractors, this can be largely cancelled with two secondary balance shafts. These are weighted off centre and run in opposite directions at twice engine speed. Are timed so that along the line between the two shafts, the vibrations from them cancel each other out but at right angles to this, they add together. So with this balancer setup eg in the sump, it is timed to bounce up and down in opposite directions to the combined piston secondary vibrations. The motor then produces minimal vibration.


With horizontally opposed two and four cylinder arrangements, the secondary vibration problem does not occur because when the cranks are 90 degrees from top and bottom dead centres, pistons are opposite each other in the same places below half way. There is a bit of primary /engine speed rocking because opposing cranks and their cylinders are not exactly
opposite. Regarding the characteristic note of flat four motors, eg Volkswagen and Subaru, it is due to there being effectively two 180 degree twin motors. Two cylinders one side fire, then the two on the other.