21 vs 16 cc Pumps-Power Drain Question

[af73bn]

Hi,
I'm hoping some of the folks here can help me clear up some technical confusion.

I spoke to a gentleman at the Scag factory today. Nice guy and very willing to answer all of my questions.
He said Scag prefers using 16cc hydraulic pumps, even on their Cat diesel unit, because they only draw about 4 horsepower each (8 horsepower total drain on the engine).
He said the 21cc pumps draw 7.5 horsepower each off of the motor (15 horsepower total drain on the engine).

It was kind of shocking to hear that the pumps draw that much HP off the motor. Over the last couple of weeks, I asked my Ferris and Grasshopper dealer the same thing, and they said it's something like 1.5 or 2 total HP loss on the 16cc, and about 3 HP loss with the 21cc pumps.
Does anybody have personal first hand knowledge of how much HP these respective pumps pull off of the engine?

[Outdoors_Unlimited]

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[StanWilhite]

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[cgaengineer]

Sounds awfully high to me...I dont hear the engine dying down very much when using hydro without blades engaged...hydros should be pretty efficient.
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[Richard Martin]

Originally Posted by cgaengineer

Sounds awfully high to me...I dont hear the engine dying down very much when using hydro without blades engaged...hydros should be pretty efficient.[/size]

You shouldn't notice it under those circumstances. The engine has plenty of power to run the hydros. You'll notice it when you're cutting grass and hit thick grass. My Dixie has 18cc wheel motors and they really hurt the mower. The speed is nice but the 28 EFI Kohler just lacks power when cutting in thick grass. There is no way a 28 horse engine should have any trouble cutting thick grass.

Dixie mismatched the pumps and wheel motors on a few hundred mowers as they were just starting to make the really fast mowers. Mine is one of them. We GPSed it at over 16 MPH@3800 RPM. They have a smaller wheel motor for mine that fixes the horse power issue but it also slows the mower down a bit. If and when one of my wheel motors goes bad I plan to replace both wheel motors with the smaller units.

[puppypaws]

Originally Posted by af73bn

Hi,
I'm hoping some of the folks here can help me clear up some technical confusion.

I spoke to a gentleman at the Scag factory today. Nice guy and very willing to answer all of my questions.
He said Scag prefers using 16cc hydraulic pumps, even on their Cat diesel unit, because they only draw about 4 horsepower each (8 horsepower total drain on the engine).
He said the 21cc pumps draw 7.5 horsepower each off of the motor (15 horsepower total drain on the engine).

It was kind of shocking to hear that the pumps draw that much HP off the motor. Over the last couple of weeks, I asked my Ferris and Grasshopper dealer the same thing, and they said it's something like 1.5 or 2 total HP loss on the 16cc, and about 3 HP loss with the 21cc pumps.
Does anybody have personal first hand knowledge of how much HP these respective pumps pull off of the engine?

You will give up pulling power for speed with the pumps used by most manufacturers, especially the Hustler Super Z when utilizing the 21cc pumps. I think Hustler has solved this problem with the Sauer Danfoss DDC-20 industrial grade slipper piston pumps, and to warranty the hydraulic system for 3000 hrs. or 5 yrs. they've got to feel it is totally bullet proof. I will find out next week, I've got a new 35/72 Super Z that will be here on Wednesday.

The larger pumps (21cc) will definitely pull more hp, and will really be noticed when mowing up-grade. How much hp each pump pulls I do not know, but you can definitely tell a difference between the smaller and larger pumps in power consumption.

[af73bn]

The one thing I forgot to ask the guy at the Scag factory was:
Do the pumps draw the same amount of HP off the engine regardless of what position you have the forward levers in...half-way, three quarter-way, etc.? (assuming of course your on flat ground).

Perhaps he was trying to tell me that the pumps pull a constant amount of HP at all times.

I guess what I'm really driving at is: Why should I for instance, purchase a 30 HP 430D Grasshopper, only to have half the power robbed by the 21cc pumps?
It would almost be better to get a 25 HP 325D Grasshopper with 16cc pumps. The 325D would essentially be putting more HP to the blades, instead of wasting it on the hydro.

Just a thought.

[puppypaws]

Originally Posted by af73bn

The one thing I forgot to ask the guy at the Scag factory was:
Do the pumps draw the same amount of HP off the engine regardless of what position you have the forward levers in...half-way, three quarter-way, etc.? (assuming of course your on flat ground).

Perhaps he was trying to tell me that the pumps pull a constant amount of HP at all times.

I guess what I'm really driving at is: Why should I for instance, purchase a 30 HP 430D Grasshopper, only to have half the power robbed by the 21cc pumps?
It would almost be better to get a 25 HP 325D Grasshopper with 16cc pumps. The 325D would essentially be putting more HP to the blades, instead of wasting it on the hydro.
Just a thought.

The configuration of wheel motor versus pump has a great deal to do with power loss. Hustler had at one time what was called a "High Torque" model Super Z, they continued to use their 21cc pump but implemented a different White wheel motor to increase pulling power. This cut the speed from 15 mph to 11 mph, but increased torque for customers cutting more hilly terrain.

You need to know what wheel motors are being used, and if they are attempting to increase speed, increase torque, or reach a happy medium between the configuration. The 21cc pumps will definitely pull more power, but depending on the design of wheel motors being fed by the bigger pump is what determines how much power is robbed.

If you know anything about gear reduction this will give you an idea of what is happening, but through hydraulics. Take a drag car and put what is considered a low gear (456) rear end into the vehicle, this will give you very fast acceleration for a short distance, and once it reaches that maximum speed in that 1/4 mile distance it goes no faster. Take the same car and install a 373 gear (considered higher) rear end and you lose the fast short distance speed and torque, but gain a tremendous amount of top end speed.

The 21cc pump can be configured to dump more fluid into a wheel motor designed to achieve speed, which in return robs power, or it can be configured to push less fluid into a wheel motor to achieve torque (pulling power), while robbing less power from your engine. Most 21cc pumps are used for achieving speed, if the speed is not needed as with many, then they stick with a 16cc pump and compatible wheel motor. Does this make sense to you?

You are only getting 1 mph more speed from the 430 versus the 325 which is very insignificant. This also tells me they are using a wheel motor to hit a happy medium between speed and torque.

[djagusch]

Originally Posted by puppypaws

The configuration of wheel motor versus pump has a great deal to do with power loss. Hustler had at one time what was called a "High Torque" model Super Z, they continued to use their 21cc pump but implemented a different White wheel motor to increase pulling power. This cut the speed from 15 mph to 11 mph, but increased torque for customers cutting more hilly terrain.

You need to know what wheel motors are being used, and if they are attempting to increase speed, increase torque, or reach a happy medium between the configuration. The 21cc pumps will definitely pull more power, but depending on the design of wheel motors being fed by the bigger pump is what determines how much power is robbed.

If you know anything about gear reduction this will give you an idea of what is happening, but through hydraulics. Take a drag car and put what is considered a low gear (456) rear end into the vehicle, this will give you very fast acceleration for a short distance, and once it reaches that maximum speed in that 1/4 mile distance it goes no faster. Take the same car and install a 373 gear (considered higher) rear end and you lose the fast short distance speed and torque, but gain a tremendous amount of top end speed.

The 21cc pump can be configured to dump more fluid into a wheel motor designed to achieve speed, which in return robs power, or it can be configured to push less fluid into a wheel motor to achieve torque (pulling power), while robbing less power from your engine. Most 21cc pumps are used for achieving speed, if the speed is not needed as with many, then they stick with a 16cc pump and compatible wheel motor. Does this make sense to you?

You are only getting 1 mph more speed from the 430 versus the 325 which is very insignificant. This also tells me they are using a wheel motor to hit a happy medium between speed and torque.

You are making this more complicated than necessary. 21cc pump pushes 21cc's of fluid per rev at certain rpm's. Depending on wheel motor size it affects the revs of the wheel motor.

The major difference in hydro system design besides pump/wheel motor size is the pulley's and what rpm the mfg's are designing the pump to run at. Some mfg's have it set conservatively and some have it close to redline of the pump.

[cgaengineer]

Originally Posted by djagusch

You are making this more complicated than necessary. 21cc pump pushes 21cc's of fluid per rev at certain rpm's. Depending on wheel motor size it affects the revs of the wheel motor.

The major difference in hydro system design besides pump/wheel motor size is the pulley's and what rpm the mfg's are designing the pump to run at. Some mfg's have it set conservatively and some have it close to redline of the pump.

I would imagine like anything else, the faster you spin them the more the efficiency drops...I'm guessing when run full speed frictional losses become high...heat will than be a problem.
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