tl;dr - It's a rule of thumb that uses gas speed to size valves
This gem of a document does a decent job of explaining the Lovell factor and its significance. It is my source for most of the information presented in this answer.
What's in a Lovell?
The Lovell factor refers to the average (mean) speed of the air-fuel mixture at the intake valves.
Brian Lovell realized/popularized the fact that naturally-aspirated, gasoline engines almost universally churn out peak power around 70-80 m/s mean inlet gas velocity; which makes the Lovell factor a very useful, quick-'n-dirty tool for sizing the intake valves regardless of engine size, bore, stroke, RPM or peak power:
It was, as I understand it, the late Brian Lovell of Weslake who conceived Mean Gas Velocity with respect to intake valves...
I should point out that Brian Lovell proposed Mean Gas Velocity as a means of comparing engines for which precious little data was available in a design era populated with slide rules and not computers; more complex calculations were definitely not on the menu.
The same concept applies to exhaust valves
The target is roughly 300 m/s.
Moreover, if one extends Mean Gas Velocity thinking to the exhaust valves of an engine, where the speed of sound in the elevated temperatures of exhaust gas is some 600 m/s, there the Mach number criterion of 0.5 translates to (if computed at maximum piston speed) a Mean Gas Velocity of 300 m/s.
Hence, it seems feasible to extend the Mean Gas Velocity concept to the exhaust valves as well; this is important as the relative sizing of the exhaust and intake valves is a critical design factor
Why 70-80 m/s for intake and 300 m/s for exhaust?
For reasons that are semi-explained in the document, it's all about trying to achieve Mach 0.5, because that's where the engine "breathes" best.
A ‘perfect’ design is considered to have a maximum particle velocity in the exhaust and intake ducts where the Mach number is 0.5.
Mach = Gas Velocity / Speed of Sound in that gas
Since the speed of sound will change depending on gas pressure, temperature and composition, the ideal speed will vary between the inlet and exhaust valves.
The Mach value will also vary with engine load since the valve diameter is fixed, so it is up to the engine designer to strike a suitable compromise between low-end and high-end performance.
As a side, this also explains why exhaust valves are (usually) smaller than intake valves.
How is it calculated?
Compute the mean piston speed (Cp) at the RPM of interest:
Cp = 2 * STROKE * RPM / 60
Compute the area ratio (Kiv) of the intake valve opening to the cylinder bore:
Kiv = INTAKE VALVE AREA / CYLINDER AREA
Compute Lovell factor (KL):
KL = Cp / Kiv
This metric will not, by itself, help you design other parts of the engine. I'll let the quote speak for itself:
I regret to say that, while this is a design criterion for the dimensions of an intake valve and is doubtless helpful in that regard, further assistance is not forthcoming for the rest of an engine design.
What are some possible use-cases for the Lovell factor?
The following spring to mind (most will involve resizing valve diameter(s)):
- Redesigning an engine for a different power level
- Altering the trade-off between low-end vs high-end performance
- Assessing the impact of a stroker kit on cylinder head re-design
- Deciding on the number of valves during engine design phase
- Manifold resizing
- Exhaust redesign to benefit from scavenging
- Redesign naturally-aspirated cylinder-head for forced-induction