What is CFM? Do you know? Of course not, if you did, you wouldn’t be here. Luckily you’ve come to the right place. You have questions and we have answers.
Believe it or not, here are a few common questions people often ask on forums are : What is CFM? What does CFM mean? CFM vs MPH: What’s the difference?
Table of Contents...
- 1 What is CFM?
- 2 Backpack Leaf Blowers
What is CFM?
CFM is the most common way to measure airflow, it means Cubic Feet per Minute. It’s a measure in cubic feet, of the volume of air going past a certain point in one minute. To visualize it we may consider a vessel of 1000 cubic feet with an extractor fan that replaces all the air in the vessel every minute. That fan has a 1000 CFM rating and it can be replaced with a backpack leaf blower of 1000 CFM.
The blower should also replace the air in the vessel within 1 minute. In doing so, it will exhaust 1000 CFM through its blow pipe. Therefore, CFM is a good measure of how much air goes through the blower, but it does not tell us how fast the air moves. To determine how fast the volume of air moves we need a standard method to measure its speed. That standard is miles per hour (MPH).
MPH measures speed by the distance in miles something travels in one hour. We relate to it as the speed at which something moves past a point. Therefore, the MPH rating indicates how fast the air moves out of the blower. The two values, CFM and MPH combined, creates the force that moves the leaves and debris. It’s therefore wrong to weigh the one against the other.
I’m often asked what’s more important when it comes to leaf blowers, CFM or MPH? My answer is that it is wrong to compare CFM to MPH, it’s equally wrong to argue the merits of CFM vs MPH. CFM is not more important than MPH and the opposite is also not true. CFM is a measure of volume per minute and MPH a measure of distance per minute. You should use both figures to understand how powerful the blower is. Both matter, that is why the suppliers quote CFM and MPH. The debate about which is better or more desirable should not exist.
VIDEO | The Math of CFM and HVAC
The following example will help us understand what happens when we use a blower. But instead of using air, let’s imagine a vessel holding 1000 cubic feet water with a 2-inch valve at the bottom. Attached to the valve is a short piece of 2-inch diameter pipe. Our goal is to open the valve and see if it will empty the vessel in one minute. When we open the valve, we find to our disappointment it takes a lot longer than 1 minute to let all the water out. What we have here is a volume of 1000 cubic feet of water in the vessel. But we fail to move 1000 cubic feet of water per minute (CFM).
If you hold your hand in the water flowing from the pipe it will push against your hand but you will easily resist the pressure. To achieve our one-minute goal, we attach a pump to the valve and force the water through the pipe at high speed. We also adjust the water flow with the valve to empty the vessel in one minute. Now you will find it very hard to hold your hand in the water flow because the water is moving faster.
The blower in our first example moves the 1000 cubic feet air through the blower pipe. If that pipe has a nominal diameter of 3 inches it needs to be 5097 feet long to have a volume of 1000 cubic feet. However, the pipe is not that long, it is normally about 47 inches long. What we can imagine is moving our 3-inch sausage of air that’s 5097 feet long, past a point in the pipe within one minute. That’s 5097 feet per minute or 57.92 miles per hour. With a pipe diameter of 2 inches the speed increases to 11420 feet per minute or 129.77 miles per hour. This example ignores all frictional and other losses in the system. It merely explains the principle of moving a volume of air per minute at miles per hour.
Note : It’s important to clarify that CFM is not an indicator of volume. The term “cubic foot” is often misinterpreted as a measure of volume, but in this context it actually refers to “the mass of a cubic foot of atmospheric air,” — a measure of mass. This language apparently goes as far back to the age of steam power in the 19th century, which, as with many things, is still with us today.
SCFM vs CFM : What’s the difference?
Air density changes when either the pressure or temperature of the air changes. When a company quotes volumetric flow rate as CFM, it implies they measured the flow rate at the actual pressure and temperature of the day. SCFM is the Standard Cubic Feet per Minute. It’s a measure of the airflow corrected to standardized conditions. It makes it easy to compare air flow, and certain calculations are simpler. In serious comparison tests CFM is considered an unqualified term and should only be used in a general sense, not accounting for environmental conditions.
What is Newton? What does Newton mean?
So far, we discussed what CFM and MPU mean in relation to blower power. But there is another issue you should be aware of when discussing or even comparing these figures. These figures are not always accurate and manufacturers can choose how these figures are calculated to best suit the product. Therefore, some of the manufacturers calculate the CFM in the blower housing and claim that as the maximum CFM. By doing this it is possible to calculate a figure for a handheld blower nearly as high as the professional-grade Stihl BR 430. Others, like Stihl, quote the CFM figure at the exit of the tube, which is where matters when blowing leaves.
To curb this situation a new measurement method (ANSI B175.2) was introduced by the American National Standard Institute. This standard determines how certain blower specs are obtained and it does not rely on calculations. The ANSI B175.2 specification measures the actual airstream force generated by the blower, and so improves testing accuracy and consistency.
A Newton is a unit of force, it is measured in actual standardized tests that determine one comparable unit of measure. It is no longer necessary to try and comprehend the effect of air velocity and air volume which are calculated values. One newton is equal to 1-kilogram meter per second squared. Therefore, a newton is the force used to accelerate an object with a mass of 1 kilogram at 1 meter per second, per second.
Since it is a tested value, I cannot calculate it for reviews and therefore I must rely on the manufacturer to supply the test results. Unfortunately, it’s not freely available, but I will use it to compare leaf blowers whenever I find it.
Husqvarna 580BTS : 941 CFM in the pipe
The new Husqvarna 580BTS (967 85 13-01), delivers 941 CFM in the pipe at 206 MPH. The previous model 966 62 96-02 delivers 908 CFM at 206.2 MPH. My immediate observation is that the air inlet shroud of the fan looks different. It has a more aggressive look, and the diameter is larger, so changes to the fan design are obvious. The air flow in the housing increased from the 1024 CFM to 1077 CFM, supporting my observations. Husqvarna use the same motor; the specifications remain the same and the air filter looks the same. The only difference is a rise in fuel consumption which increased from 440 g/kWh to 464 g/kWh for the new model. It suggests the engine is working harder to drive the increased airflow, but that is a small negative for such a large increase in force.
VIDEO | The Power of 908 CFM — Watch it Blow Snow
If you want the new model with its higher air flow, you must be careful when shopping around. The visible changes on the blower are the new air intake shroud design, a Mark II nameplate on the air filter, and the model number. The blower tube is also different, the new model has Husqvarna’s 3-section tube with adjustable length. They still sell the older model with the previous air delivery tube design that has 4 sections. I prefer the new model because of these changes and consider the increased fuel consumption a small price to pay.
All Husqvarna 580BTS models are loud and the new Mark II is even louder. There is a 4-decibel increase to 104 dB(A) at the operator’s ears. It means good quality ear protection is necessary when using these backpack leaf blowers, therefore the increase will not affect you much. It may upset the neighbors and passers-by though. Keeping in mind that you do not normally use a backpack blower of this size on small properties, it should not be of much concern.
The Blowing Force of 40 N for the previous model was already impressive, so I’m keen to see the new rating. Unfortunately, it is not available on the Husqvarna website, I tried the operator’s manual and found nothing.
The engine and its controls are the same. You use a primer bulb to prime the fuel line with fuel, close the choke when it’s cold and pull the cord. All user feedback about the engine and starting is positive, and although it’s noisy, some liked the sound of the engine. It is a 2-stroke engine so you still use fuel mixed with two-stroke oil.
You should use a good quality oil that is formulated for air-cooled engines. Do not mistakenly use two-stroke oil for water-cooled outboard engines, it is not effective in air cooled engines. Husqvarna recommends using unleaded fuel with a minimum octane number of 90 RON and with less than 10% ethanol concentration (E10).
The air cleaner of the Husqvarna 580BTS engine is mounted on top of the engine. Husqvarna did this to increase the size of the filter so that it will last longer in dusty conditions. The filter uses dual filter cartridges, the one is a washable prefilter for a long operating time and trouble-free use.
The easily adjustable soft-grip control handle is mounted next to, and to the right of the tube. It’s designed this way to reduce strain on the arm and reduce fatigue. The handle has a trigger that you can use to control the speed of the engine as you wish. To enable you to set the engine speed without holding on to the trigger a cruise control lever is used. You pull it back to accelerate the engine and to stop the engine, you push the cruise control lever forward to the stop position. The cruise control is also used during starting to open the throttle somewhat. It is a handy feature that is easy to use and you do not have to hold on to the trigger while you start the engine.
The exhaust muffler is fitted with a spark arrestor screen which you can remove for cleaning and inspection, that’s within easy reach. The exhaust muffler outlet points to the back, away from the user, to reduce noise and direct the heat away.
The engine and fan assembly are mounted on rubber mounts to the frame. They allow the engine assembly to move about and effectively absorb all vibrations. The padded frame feels comfortable on your back and absorbs more of the engine vibrations. The frame design allows some of the fan’s inlet air to circulate past your back and cool you down. It is a welcome feature on those warm days of leaf blowing. The harness is shaped to be comfortable when wearing the backpack and the shoulder straps are padded and wide to further reduce fatigue.
The Husqvarna 580BTS weighs 25.8 pounds and some users find it a bit on the heavy side. Despite its weight it is easy to swing onto your back, and the harness waist and chest straps fastens conveniently. The ergonomic design of the frame and the wide shoulder straps lets you wear it comfortably from one fuel stop to the next.
The see-through fuel tank holds enough fuel for an hour’s use on a tank. You will find that its impressive blowing power will enable you to clear a large area during that period. This blower packs a huge punch and it has a reputation of being a reliable machine that is easy to start and use. With its additional 33 CFM it is also the most powerful backpack leaf blower on the market. Perfect for those who use their blowers commercially or to clear large properties.
What is A Good CFM for a Leaf Blower?
It depends on what you want to do with the blower. To my mind, the most valuable blower will be one that is maneuverable, light, and with the highest possible CFM. I also feel an important requirement is that its blowing power must be variable. The reason I say that is because it depends a lot on the size and layout of the area where you want to use it. A blower that is all-powerful may even blow some plants from the soil and remove all the mulch and strip the flowers.
A handheld blower is not all-powerful but fully maneuverable, you can use it effectively in a bedding of plants to clear out fallen leaves. You can take it up a flight of stairs or clear a small area with ease. It will move leaves in an open area of a reasonable size quite satisfactorily. You can move it in-between and behind things to remove leaves. It’s handy to clear the load area of your truck of sand and debris. However, you will find it too slow and inefficient to clear a few acres. I use a small handheld corded blower, 220-CFM at 125-MPH, to clear out the large flower beds on our property. I blow them out into the open and then I use the lawnmower to pick up the leaves and bag it.
434 CFM Blower — Husqvarna 150BT
A backpack leaf blower moves more leaves in a shorter period. If I owned one, I could do all the work with the blower. But you must be able to slow down the blower when using it in a bedding of plants or else you will damage some of the plants. Backpack leaf blowers are more powerful, but the long pipe makes it impractical to blow in-between and from behind things.
It could be a better all-around solution for my property but it is more expensive to own. A backpack leaf blower like the Husqvarna 580BTS, has a dry weight of almost 26 pounds. Not as easy to carry up a flight of stairs and to use in a small enclosed space as my Ryobi. It is noisy, and most backpack models use a gas engine for power.
A walk behind blower is most effective at moving leaves but you can only use it on flat open ground. You cannot take it up a flight of stairs and use it to clear out the beddings. The Little Wonder Optimax with a Vanguard engine is a prime example of an all-powerful leaf blower. But I will not be able to use it on my property, it’s too powerful and heavy and runs on wheels.
What’s the Highest CFM Leaf Blower?
The CFM ratings for blowers vary a lot and it depends on the size and design goals of the blower. Handheld blowers are meant to be light and maneuverable and therefore not very powerful. Backpack blowers are more powerful and heavier and you must carry it on your back because of that. The highest CFM rating is achieved by the walk behind leaf blowers. A blower like the Little Wonder Optimax with a Vanguard 570 cc engine delivers 2850 CFM at 179 MPH. In an open field test, it beats anything else hands down.
The machines I quote below are meant to be examples of powerful blowers from reputable companies. I make no claim that they deliver the highest CFM in their class.
VIDEO | Ultra-High CFM Push Leaf Blowers
Stihl makes a range of corded handheld blowers. The BGE 71 can deliver up to 300 CFM and 148 MPH and it has a low and high speed selector. Makita makes the UB1103 Variable speed that has a dust bag for vacuuming and it has a delivery of 144.79 CFM
The new 36 Volt Stihl BGA 100 delivers 447 CFM at a speed of 167 MPH. The Husqvarna 436LiB delivers 388.46 CFM at 105.14 MPH. The Makita DUB362Z Cordless Blower delivers 243.67 to 473.22 CFM at up to 120.8 MPH.
The Husqvarna 525BX handheld leaf blower delivers 459 CFM. The Stihl BG 86 also moves 459 CFM at a speed of 154 MPH (190 MPH with the flat nozzle).
Backpack Leaf Blowers
The air flow in the housing is 1024 CFM, however, it is not a useful figure. It merely indicates how much air it can move per minute without the pipe attached. It holds no value to us since that is not how you use a leaf blower. Husqvarna quotes it for comparison purposes just to give you another frame of reference. Unfortunately, some manufacturers quote the “in pipe” figure only. My feeling is that more information is always better, as long as you understand the relevance of the data.
A reputable company will always quote the air flow in the pipe, which is 941 CFM for the Husqvarna 580BTS Mark II.
494 CFM — Husqvarna 350BT : Gas Backpack Blower
Fitted with a flat nozzle you will get the highest speed (209 MPH). Most people use it with the round nozzle and with that fitted the speed is 206 MPH.
The Husqvarna 580BTS delivers a 40 Newton punch and the Stihl BR600 Magnum’s force is 32 Newton
How does CFM relate to —
leaf blowers, fans, air compressors, air conditioners, and range hoods?
We already explained how CFM relates to blowers. Now, let’s consider CFM ratings for fans, air compressors, air conditioners, and range hoods.
Once again, CFM is how much air a fan moves past a certain point in one minute. The difference between and blower is that the fan does not force the air through a pipe. An open fan without a shroud is also less effective than a shrouded fan because some of the air curls back.
The number depends on a few things: the fan’s motor design, the blade pitch and the length and design of the fan blades. When you buy a fan the same rules apply, more CFM means more air. Ceiling fans deliver high volumes air at slow speeds but most extractor fans run at high speeds.
When a compressor pumps 10 CFM its intake port inhales 10 cubic foot of free air per minute (air at atmospheric pressure). It does not specify the air delivery to the tools but the rate at which air is collected for compression. Most air tools will need 5 to 7 CFM but some larger industrial tools need more than 10 CFM at 100 to 120 psi. More than one air tool can be used at the same time if the air delivery system and CFM rating of the compressor is suitable. Some four wheel portable compressors that you sometimes see working at construction sites deliver as much as 1687 CFM at 145 psi.
The CFM rating of an air conditioner defines the amount of air passing across the evaporator coil. You need 350 to 400 CFM per ton of cooling for a properly operating air conditioning system. If the air flow is too low the evaporator may ice up and allow liquid refrigerant to enter the compressor. It will damage the compressor and that is why it is so important to keep the air conditioner filters clean.
A range hood typically moves 1000 cubic feet air per minute. Its calculated to allow the range hood to exchange all the air in the kitchen within a predetermined period. A hood with ducting to the outside does not force the air through a narrow pipe therefore it does not need the power of a blower. A range hood that recirculates the air through filters also do not need lots of power and therefore the fans used are quiet.