What is the Lovell Factor and how does it apply to engines and engine design?

EDIT: How is it calculated and what do the calculated numbers indicate? How is it useful in engine design? (I'm looking for more specifics about what it means and how it is utilized.)

2 Answers 2


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:

  • 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
  • Nicely done +1! Commented Nov 20, 2015 at 11:53
  • Spot on Mate...
    – Shobin P
    Commented Nov 20, 2015 at 13:03

According to this Lovell Factor is the "mean gas speed of the inlet valves"

Calculating this number allows you to design the intake and exhaust valves to accommodate the higher RPM potential of the engine.

You can use the following method to calculate the Lovell Factor of any engine as long as you know the key parameters below:

  • Bore = B
  • Stroke = S
  • RPM @ which you want the lovell factor. = RPM
  • Number of valves per cylinder = valve
  • Diameter of the inlet valve = Dia

    Divide B by 2
    Multiply the result with 2
    areaofbore = above result*3.14 or PI
    total spin= RPM * S
    total spin=total spin*2
    total spin=total spin / 60000
    Dia= Dia/ 2  
    Dia= Dia* Dia
    Dia=Dia* 3.14 or PI
    valve = valve * Dia
    ratio = valve / areaofbore 
    total spin= total spin/ ratio
    lovell factor = total spin

Final Lovell factor is value of "Total Spin"

Most of the time, this is a silly number and should not be something you have to consider while building an engine for example in Formula-1 where the RPMs go upwards of 15000 to 19000 you need to be extremely precise on each and every detail this is where the Lovell Factor actually matters.

  • Formula-1 Car has a lovell factor of 77m/s @ 19000 RPM
  • Nissan GTR has a lovell factor of 52.43m/s @ 7500 RPM

It basically means how fast your engine can breath.

As per my limited knowledge the Lovell Factor is only usually considered while designing high performance engines like F1 where minute details in the engine design determine which car wins,

I am not sure if there is a "Bad" number since each engine is different and unless you have anything to compare with the lovell factor of your engine might be high for the RPM it is currently in which means that the valves are too small thus increasing the Mean gas speed.

For example having a high rev limit & low comparative Lovell factor combination is desired.

  • Please see edits. Commented Nov 9, 2015 at 16:49
  • @Paulster2 Updated answer
    – Shobin P
    Commented Nov 12, 2015 at 13:20
  • Where did you get this algorithm/equation from?
    – Zaid
    Commented Nov 12, 2015 at 13:22
  • took me a couple of hours but from a website.
    – Shobin P
    Commented Nov 12, 2015 at 13:23
  • You should quote the website you obtained it from
    – Zaid
    Commented Nov 12, 2015 at 13:24

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