I just don't understand it......

[Bill Huber]

I have an old, 10 years, Sears shop vac.
The HP rating on it is 2.5, it has all kinds of suck.
Now I see all the newer shop vacs are like 6.5 HP, but when I try one I see no real difference then mine. I can put my hand over it and it doesn't suck the blood right out of me.

So what is the deal here, have they change the way they report the HP or what am I missing.

Take the new Bosch, it is rated at 6.5 HP but only has a 130 CFM, I have seen I think one of the Ridigids rated at 6.5 with 220 CFM.

I just don't understand.

[Jim Becker]

"6.5 hp" and it probably plugs in to a 120v outlet, too, eh? It's NOT 6.5 hp unless the motor is on fire. Even that old one was not really 2.5hp if you do the math on the amperage, etc... Welcome to the world of creative marketing and "peak horsepower".

[Brad Schmid]

"6.5 hp" and it probably plugs in to a 120v outlet, too, eh? It's NOT 6.5 hp unless the motor is on fire. Even that old one was not really 2.5hp if you do the math on the amperage, etc... Welcome to the world of creative marketing and "peak horsepower".

Well... maybe it really is 6.5 hp... You know, if 6 1/2 Clydesdales all inhaled at the same time... oh never mind, i'm bored tonight:D:D :p

[Bill Huber]

"6.5 hp" and it probably plugs in to a 120v outlet, too, eh? It's NOT 6.5 hp unless the motor is on fire. Even that old one was not really 2.5hp if you do the math on the amperage, etc... Welcome to the world of creative marketing and "peak horsepower".

So that is what I thought.

If I am going to get a new shop vac I want one that has the highest CFM, and the heck with the HP and amps. Would that be correct?

[Bill Huber]

Well... maybe it really is 6.5 hp... You know, if 6 1/2 Clydesdales all inhaled at the same time... oh never mind, i'm bored tonight:D:D :p


The problem is what if one of the Clydesdales has been smoking, will that lower the HP?

But then how would this effect the CFMs?

[Howard Acheson]

With vacuum cleaners the spec you want to optimize is static pressure or suction. That's what lifts things. CFM is air speed or movement. CFM is what you want to optimize for dust collection as it is air speed that keeps dust particles in suspension.

Dust collectors have relatively little suction but lots of CFM. Vacuums have lots of suction but relatively little CFM. That's the reason vacuums do not do a very good job collecting dust.

[Dale Lesak]

Let's start with "CFM", Cubic foot a minute. the higher the "CFM" the more air flow you will have. the air flow is what moves the dirt up the hose and into the sweeper. You can have a high CFM and still have low vacuum.The vacuum is what sucks your hand to the end of the hose. the higher the vacuum the harder the suck. most shop vacs will only pull a couple of inches of vacuum, A vacuum pump used in auto AC repair will pull 27 or 28 inches of vacuum but only has a CFM of 1 or 2. A poorly designed sweeper with a big motor won't work any better that a good design with a smaller motor. HP is just a selling point, a lot of people thing that big HP means more power. not true, just using up more electricity A good vac will have a high CFM with a good vacuum The CFM moves the dirt and the vacuum is how big of a gob of dirt it'll move. HTH,

[Brad Schmid]

The problem is what if one of the Clydesdales has been smoking, will that lower the HP?

But then how would this effect the CFMs?

Depends on if he inhaled or not :D :D :D

[Michael Weber]

Back in the early 70's I was selling stereo equipment. The industry was going through a competition to see who could rate their amplifiers the highest. Came up with all kinds of rating classifcations to enhance their products. One I remember was "Instantanious Peak to Peak Power". An amp might actually a 5 watt RMS amp but for one microsecond it would put out 1000 watts based on the peak to peak amplitude. Of course it would blow the transistors but hey for that microsecond it was putting out 1000 watts. It got so out of control some mfgs. got together and established rating standards. Anyway, I suspect this is whats going on now with the HP ratings of equipment. Pretty soon we might see Sears selling a 100 HP compressors no larger than their current size. ;)

[John Bush]

You must consider the Clydsdale's exhaust as well.

[Jimmy Newman]

They still do the stereo amp overrating thing, but mostly only with car stereo equipment - which is why you can get a "thousand watt" car amp which runs off of 12V and doesn't pop any fuses :).

Also, in terms purely of power with no system losses (friction, motor inefficiency, etc.), 5HP is about 3700 watts - or about 34 amps at 110V. Just something to think about.

[Tom Veatch]

...
I just don't understand.


Well, it's really not that hard to understand - a little unethical in my view. No, I'll go further than that. I consider it dishonest.

Those "pseudo-HP" ratings are calculated from the amperage that the motor draws with a locked rotor. It will draw a whopping amount of current and develop a very high torque trying to turn the rotor - for a little while at least, until the windings burn out.

If you take that locked rotor current and multiply by the voltage, you come up with a wattage. Divide the wattage by 746 (watts/HP) and you get an equivalent HP. Of course, the motor under actual operating conditions won't develop that much power.

Now, power output is porportional to the product of the torque developed at the output shaft times RPM of the shaft. But, the rotor is locked so the RPM is zero. The motor will develop a whopping amount of torque under those conditions, and may even consume the "advertised" power (wattage) from the wall socket. But, the POWER it is producing at the output shaft is ... ZERO!

Just for information purposes, that 6.5HP shop-vac, if it truly developed 6.5HP in operation, would be drawing no less than 44 amps from a 110v circuit - and that's if the motor was 100% electrically and mechanically efficient. And that just ain't gonna happen!

A better way to compare would be to look at the amperage rating on the motor dataplate. That will be closer to the "real" power the motor will develop - but, they can play games with that number, also.

[Greg Funk]

They still do the stereo amp overrating thing, but mostly only with car stereo equipment - which is why you can get a "thousand watt" car amp which runs off of 12V and doesn't pop any fuses :).

Sadly they do may car audio amps capable of putting out well in excess of 1000 watts RMS. They draw power directly from the battery with 1/0 AWG wire and 250 Amp fuses...

[Andrew Williams]

Sadly they do may car audio amps capable of putting out well in excess of 1000 watts RMS. They draw power directly from the battery with 1/0 AWG wire and 250 Amp fuses...

And if you are lucky enough to witness it, one boom from the bass is enough to blow out all of the car's windows! (followed by cheers from the neighbors)

[Paul Greathouse]

The problem is what if one of the Clydesdales has been smoking, will that lower the HP?

But then how would this effect the CFMs?


I don't know about Clydesdales but Per Swenson has a cat that smokes. I don't believe I have ever heard of catpower being disputed though.

[Cliff Rohrabacher]

I have an old, 10 years, Sears shop vac. [...]
I can put my hand over it and it doesn't suck the blood right out of me.


Not for nothing but this is a family forum. ;-)


It might be the impeller that is moving more air with fewer ponies.

[Dennis Peacock]

You must consider the Clydsdale's exhaust as well.


LOL!!!! Now HOW did I know THAT was coming. ;)

[Gary Keedwell]

You must consider the Clydsdale's exhaust as well.
:rolleyes: :D :D Funny...

Gary K.

[Jimmy Newman]

Sadly they do may car audio amps capable of putting out well in excess of 1000 watts RMS. They draw power directly from the battery with 1/0 AWG wire and 250 Amp fuses...
Oh, I'm sure there are some that can actually do it - but I've seen a number of amps that are about $100 and advertise as 1000W - and then if you read the fine print, say about 50W RMS :).

[Noah Katz]

" Those "pseudo-HP" ratings are calculated from the amperage that the motor draws with a locked rotor. "

What's especially dishonest is that unlike other tools, there's no operating condition under which it's possible to do that.