# Anyone know the RPM of modern MTD/Cub Cadet 2-stage?



## DennisP (Jan 10, 2016)

Hey guys, in finishing off things for the year I got a chance to run the 2013 Cub Cadet 524SWE next to the rebuilt 1999 Craftsman 28"..

One of the big things I noticed is that the Cub Cadet seems to have quite a bit more impeller air volume out of the chute than the 1999 Craftsman 28". They have the same size impellers with the exact same design and dimensions.

My question is, without my measuring the pulleys on the 2013 Cub Cadet, does anyone know the impeller speed?

I know the speed on my 1999 Craftsman 28" is roughly 1000rpm with the engine at 3600rpm.

I am just wondering if the better throwing performance of the newer machines is all about impeller speed more than anything else. If so I have no problem putting custom pulleys on my 1999 Craftsman 28" and upping the impeller speed. Both snowblowers have the exact same gearbox and bushing/bearing setup, so I am certain that if I use this increased impeller speed on my 1999 machine I will be just fine. I have increased the engine output from the 9hp it originally came with to a new 11hp engine, so that only helps in this.


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## Town (Jan 31, 2015)

When I increased the impeller speed on my machine from 1040 rpm at 5,000 engine rpm to 1100 rpm at 3,700 engine rpm there was a big increase in snow blowing performance. In particular with water and very wet sloppy EOD conditions, it became a much better pumper of water.

You will need to measure the diameter of the two pulleys and get their ratio to determine impeller rpm related to engine rpm. The actual ratio is based on the belt position on the pulleys (mid point of belt depth) and not the actual outside diameter of the pulleys.

The circumference of the pulley is measured as "diameter times a constant (which is 3.14)". So the ratio is impeller pulley diameter divided by engine impeller pulley diameter but you can ignore the constant. This however is not the actual ratio you need since you want the belt position diameter at 1/2 the belt depth which for a 1/2" wide belt is usually 5/16; so 5/32" approx. If the belt top rides at the outside diameter of the pulley then the diameter you want is 2x5/32" = 5/16" less than pulley diameter.

To give you an example: my engine pulley diameter is 2.75" and impeller pulley diameter is 8.5". The belt runs at the top of the pulleys. The effective diameters are 2.75" less 5/16" = 2.4375" and 8.5" less 5/16" = 8.1875". The pulley ratio is 8.1875/2.4375 = 3.358; So at 3700 engine rpm the impeller should run at 3700/3.358 = 1102 rpm. The actual for my engine is 3700 rpm and the actual for my impeller is 1100. 

I was helped a lot in coming to this understanding by Toro-8-2-4 who provided lots of helpful info. Hope this helps you and good luck.


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## DennisP (Jan 10, 2016)

Well, I looked up the part numbers. The pulley set for the 524SWE is listed as a 8.1" x 2.6" combo. The pulley set for the 1999 28" is listed as a 8" x 2.6". Which means for the same engine speed the 28" impeller should be spinning slightly faster.

I am just wondering if the chute diameter and chute adapter on the 524SWE isn't smaller? That would allow more velocity out of the chute. I am also thinking the small lip they designed into the newer chute adapter acts as a scoop as well diverting air (and ultimately snow) into the chute.

Which just brings a different thought to mind. What if, instead of using a paddle kit, I just modified the stock reinforcement lip on the 1999 design and built-in a small scoop?

That should do the same as the impeller kit would, yet allow for even tighter clearance at only one spot. Now, with that in mind it would have to be built out of metal, and ultimately should have almost a blade-like design against the impeller edge to scoop and divert the snow up into the chute.

I do believe I have another project on my hands...


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## Town (Jan 31, 2015)

DennisP said:


> Well, I looked up the part numbers. The pulley set for the 524SWE is listed as a 8.1" x 2.6" combo. The pulley set for the 1999 28" is listed as a 8" x 2.6". Which means for the same engine speed the 28" impeller should be spinning slightly faster.
> 
> I am just wondering if the chute diameter and chute adapter on the 524SWE isn't smaller? That would allow more velocity out of the chute. I am also thinking the small lip they designed into the newer chute adapter acts as a scoop as well diverting air (and ultimately snow) into the chute.
> 
> ...


The snow is powered off the impeller by centrifugal force and the efficiency of the impeller will change dramatically given a straight impeller blade or a forward curved blade or a backward curved blade. It is largely rpm determined so you need to research the efficiency curves for each or stick with what the manufacturer determined the optimum to be.

You may have some considerations that the specs are not revealing to you now. Clearance of impeller to impeller housing; belt widths and running distance to the outside diameter of the pulleys. Since the spec diameters do not match your engine and impeller speeds. 

In your original post you said the Craftsman turned the impeller at 1,000 rpm when the engine was running at 3,600. So the impeller pulleys actual drive belt ratio is 3.6:1; but the spec in your second post identified the Craftsman's 8.0" and 2.6" pulley diameters. Assuming you have a 1/2" belt where the top rides at the outer diameter of each pulley the effective diameters are 7.6875" and 2.2875" giving a ratio of 3.36:1 which at 3.600 engine rpm will drive the impeller at 1,071 rpm. 

Your specs for the Cub (8.1" and 2.6" pulley diameters) would give 7.7875 and 2.2875 effective belt diameters of the pulleys assuming a 1/2" belt running at the outer diameter of both pulleys. The effective ratio is 3.40:1 so at 3,600 engine rpm the Cub would turn the impeller at 1,059 rpm.

A snowblower impeller does not move very much air. Even a 14" impeller driven at 1,100 rpm is hardly noticeable to me. It does not feel like an air delivery fan, more like a person blowing hard. 

Good luck with your project, but I would be taking another look at your pulleys and belts and where they ride and what is the clearance from impeller to housing.


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## Spectrum (Jan 6, 2013)

DennisP said:


> I am just wondering if the better throwing performance of the newer machines is all about impeller speed more than anything else.


I would not ignore the worm drive ratio. It's one thing to change pulley ratios and get the front end going faster which will help. I have been very impressed by machines with mist set governors.

If I wanted to really optimize things I would consider upping the impeller speed and decrease the typical worm drive ratio a bit to make full RPM easier to maintain. 

Worm ratios are easy to get just by rotating the impeller by hand (disconnect the spark plug) and counting what it takes to get an auger revolution. If you go by gear tooth counts keep in mind that the worm may be a twin lead. A 21 tooth worm wheel may need 10.5 input revolutions to go 360 degrees. This could play into performance differences you are seeing in your machines.

Pete


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## DennisP (Jan 10, 2016)

Actually, in both of these cases, the impellers are both 12" and the impeller housings are identical in diameter, the design of the impellers are the same (the center mounting method is the only difference, as they went with a slip-on collar design on the newer one to facilitate easier and cheaper builds compared to the old design that was a slip-fit with a pair of spring pins), identical clearance to the housing from behind and from impeller blade to housing, with the only difference being the chute adapter between them. Yet the newer Cub Cadet has a forceful blast of air out of the chute. And I am certain that blast of air correlates to a more forceful exiting of snow out of the chute.

And, in the first post I said "roughly", as in I never measured the speed, but know that it is supposed to be "roughly" around 1000rpm, which is exactly what anything less than 1100rpm would be. But in the second I correlated that the Craftsman is in-fact spinning the impeller even faster than the Cub Cadet, so it didn't matter.

I still believe the excessive clearance at the chute reinforcement area is just allowing the air (and snow) to be flung around the inside of the impeller housing, now ejected out the chute.

It is just like lower-end centrifugal supercharger designs that have excess clearance between the impeller and the housing to limit boost. Put on a new housing with less clearance and the same supercharger produces huge increases in boost and efficiency.

Again it is no different than putting an impeller mod kit on. Those kits basically also increase the impeller tip velocity by effectively moving the impeller tip out further, with this going in the opposite direction of limiting the amount of material (be it air or snow) that can bypass the chute. And I can definitely see how that would make a big difference with solid and/or wet snow/slush.


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## DennisP (Jan 10, 2016)

Spectrum, the worm ratio is identical between machines. Actually the internals of both gearboxes are the same. Only the shafts themselves are different in regards to the pulley connection on the impeller shaft. The number of turns on the worm gear are the same. I counted them when I had them both torn apart. The brass gear is the same part number for both machines.


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