To accurately compare any two or more devices, we often resort to the numbers. We assume that superior technical specifications, or more accurately “the larger“ numbers, are a sign of a better product. Yet when it comes to leaf blowers, this assumption may lead you to a bitter surprise.
What is CFM and MPH?
The two specifications that characterize a leaf blower’s “ability to blow” are the cubic feet per minute (CFM) and the miles per hour (MPH).
CFM (Cubic feet per minute)
- The unit of volumetric flow. That is the sum of the cubic feet of wind that flow through the nozzle of the leaf blower each minute.
MPH (Miles per hour)
- The unit of velocity (a fancy word for speed) of the wind that is being ejected by the device at any given moment.
At first sight, many of us may be baffled by the necessity for listing both specs. Aren‘t they basically the same thing? No! Moreover, both are needed to characterize the leaf blower’s ability to do its job.
A brief rundown of the leaf blower’s function.
- The engine of a leaf blower creates force.
- The force acts upon the surface of wind – in other words, pressure is created.
- The force is a constant – it does not depend on the size of the surface.
This results in the CFM and MPH.
- The force creates a constant volumetric flow (CFM), independent on the surface that it acts upon.
- The velocity (MPH) is on the other hand dependent both on the force and the diameter of the pipe.
If you are still confused by the difference between the two, consider the following example.
Imagine that you are blowing out candles on a birthday cake. While you may easily be able to do it in one blow just with your mouth, try doing it with a straw. Two things happen:
- Even if you blow with the same force, you will notice a dramatic increase in velocity of the air exiting the straw.
- You have decreased the area through which the air is flowing. Even though you are now blowing faster, you will have tough time reaching all the candles on the cake.
Does this mean that velocity is therefore a useless metric? No! In order to actually move the leaves, you need to apply efficient velocity. In fact, the heavier the object, the larger MPH is required to move it.
We can therefore conclude, that:
- We may use CFM to determine the power of the engine and the rate at which it can blow air.
- We may use MPH to specifically determine how heavy objects can be blown.
- We may also use MPH to determine the effective distance at which we can blow.
For further information about air flow, visit https://en.wikipedia.org/wiki/Fluid_dynamics (opens in a new tab).
Considering the CFM/MPH ratio
The specifics of what we consider a high/low MPH and CFM depend on the size of a device and its intended application, yet we can still draw some general conclusions just based on the ratio of the two metrics alone.
Low CFM/High MPH
- In order to achieve a decent velocity with decent diameters, a sufficient CFM is needed. Thus, if the CFM seems suspiciously low compared to MPH, the manufacturer most likely boosted the MPH by decreasing the diameter of the pipe. This results in the above listed straw effect.
- A blower of this sort may have some utility for household purposes – such as cleaning a computer fan. Still, it is pretty much useless when it comes to blowing leaves.
High CFM/Low MPH
- Assuming a decent diameter, these leaf blowers would be sufficient for blowing dry leaves, hay, maybe even smaller small rocks.
- The diameter has been most likely increased for larger angular range at the expense of velocity.
- When dealing with wet leaves, a broom might be a better choice.
High CFM/High MPH
- Assuming a decent diameter, this is the most optimal combination.
- The disadvantage of these devices is their price – In the end, you will always have to pay for quality.
These examples clearly illustrate, that the value of CFM should always be as high as possible while the MPH is a variable parameter. Even though it is essential to effective leaf blowing, it may be compromised based on the intended application of the device if your budget calls for it.
Most importantly, we must always consider the diameter of the pipe!
As we have mentioned, both metrics are ultimately dependent on the power of the engine. Thus, some manufacturers choose to characterize their devices in this manner. This would be the case for Stihl leaf blowers.
Engines can be characterized in multiple ways. While power is a good indicator of the work generated by the engine, force may be more useful when comparing the ability to move objects. The unit of force is newton – a metric unit that characterizes the acceleration of an object of a certain mass.
Blowing force (opens in a new tab) is an effective way to compare leaf blowers because of the effect of the pipe on CFM. Even though we have mentioned that strictly speaking CFM does not depend on the diameter of the tube, its physical presence does in fact restrict the movement of air. This is due to the friction that the inner surface of the pipe applies to the air. Thus, many manufacturers boost their CFM by measuring it without the tube!
Even though we recommend comparing devices based on multiple metrics, as demonstrated in this article, we would like to present several examples just to show how high their values can get.
We have utilized our database of over 300 leaf blowers to bring you the following champions. Currently, the leaders in their respective categories are:
For the highest CFM, the Billy Goat F1802V, with a CFM of 2900.
For the highest MPH, the Husqvarna 150BT, with an MPH of 270.
- CFM tells us how much air flows through a leaf blower.
- MPH tells us how fast the air exits a leaf blower.
- None of these characteristics alone tell us about the quality of the leaf blower – their combination must be considered.
What do they tell us?
- CFM is characteristic of a leaf blower’s engine power.
- MPH tells us how heavy objects can be moved by the ejected air.
- CFM is not indicative of the angular range of the device – Tube diameter must be considered.
- MPH is indicative of the radial range (how far does the air travel) of the device.
- CFM may be measured in various ways with different results.
- Blowing force is a more objective way to characterize the power of a leaf blower.