Uneven Firing Sequence
What is it about the firing sequence of V-Twins, V4’s and Cross-plane crank 4 (CPC4) which seems to give them better rider feel, and better early drive? Inertial torque plays a part as we discussed above, but does the firing sequence also come into play?
The 90 degree V-twin has a firing sequence with a delay of 270 degrees and 450 degrees. As both cylinders are joined to the crankshaft at the same point. The first cylinder fires, and the second fires three quarters of a rotation later, then the first cylinder fires one and a quarter turn later at 450. (In the following examples each entry is 90 degrees of crankshaft turn, 1 is a fire, 0 is no fire)
90 degree V-Twin: (1-0-0-1-0-0-0-0-1-0-0-1-0-0-0-0-1-0-0-1-0-0-0-0-)
The tyre has a break of four large ‘gaps’ in the power between the second and first cylinder firing, in a screamer engine, the firing sequence looks like this, firing every 180 degrees.
Inline 4: (1-0-1-0-1-0-1-0-1-0-1-0-1-0-1-0-1-0-1-0-1-0-1-0-)
The V-4 engine as installed in the Honda VFR800 series fires as follows (180, 90, 180, 270)
A tyre is elastic, and the way it grips the road and exerts a force on it, is down to the movement that the tyre makes at the level of contact with the road. The movement is within the tyre’s construction, as well as being a certain amount of slip at the contact with the road surface.
There is a theory that if the tyre is being pounded by even firing sequences, it never gets a ‘breather’ from the pummelling; never gets a chance to relax back to rest position so it can key in to the road surface. Static friction is much greater than dynamic friction, one the traction and the tyre is spinning, it is much easier to keep it spinning. If you have ever been on a ride in low grip conditions you will have appreciated this. You want the tyre to spin back down to normal grip conditions rather than lock into the road suddenly. Chopping the throttle suddenly will give the tyre a chance to grip, and this is where a high side occurs. Electronics ‘think’ much quicker than human reactions and can help in this regard, as can a rider who isn’t freaked out about the rear end sliding about.
V4 and a V-twin firing sequences give the rear tyre a rest (red zero’s in the sequences above) but whether this improves the tyres ability to grip is not easy to prove without getting into the complex world of tyres and how they behave. Theoretically, if someone could build a 4 cylinder engine with a regular firing sequence but also with zero inertial torque, this could be compared directly with the standard flatplane crank I-4.
The 90 degree V-4 is essentially two 90 degree V-twins side by side. Each pair of cylinders shares the same crankpin, and they are 180 degrees (flat plane crank) out of sync.
A flat-plane crank as employed in a 90 degree V-engine (including V8’s that Ferrari and other exotic manufacturers use) has the same zero inertial torque characteristics as a cross-plane crank in an inline 4 engine.
To prove that a V4 is two V-twins, let’s take two traces of the V-Twin above and overlay them offset by 180 degrees (each 1 or 0 represents 90degrees in the sequence):
= 1-0-1–1-0-1-0-0- 1-0-1–1– 0-1-0-0-
This is the V4 VFR800 firing sequence
Out of the two; inertial torque and uneven firing, the most important contribution to improved drive is zero inertial torque.