Flkow rate for this pool pump?

I have googled everything and cant seem to find much of anything...this unit is 6-10 years old. I am sizing a sand filter for a friend, her current motor/pump is a Magnatek Centurion and as best as I can read( very faded), part number 12(or15)-177080-24 type Cx, rpm 3450 sf 1.65.
I am trying to determine the flow rate of this unit so as to purchase the correct size sand filter( considering a 21 inch Hayward, 36 gpm I believe), Her pool is 8400 gallons.
Can anyone enlighten me at all on the possible flow rate with the info I have? Thanks
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Small correction, the filter being considered has a flow rate of 44gpm with effective filtration of 2.20 sqft

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you could use a garbage can or large bucket container with gallon capacity on its label. pump hose into container, power on, how many gallons per minute? example: our pool pump is 1.5 hp for 12,000 gallons of above ground 24ft round pool x 4 ft walls, climate = buffalo ny. your task is for your climate, determine the filter size so you don't work the pump so hard that the backpressure is high. as your pump gets old and noisy or self-destructs from going dry consider a 2-speed pump. we use a 2-speed motor so as to be able to shock or vacuum or skim on high versus use low for routine chlorine circulation. your desirable water turnover thru the filter is faster with the larger sand filter of course versus a smaller one. see hayward filters: http://www.poolian.com/detail.asp?product_id=S166T without the amperage info, you may only have a small 1/2 hp. test motor on a gfi with an ammeter? see similar pumps at: http://www.poolian.com/detail.asp?product_id=AOSSQ hidden factors in pool care could be nearby leaves, bather load, blowing sand or dirt?
GotBonus wrote:

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GotBonus writes:

Meter the current and figure the watts and then the HP of the motor. I would guess 3/4 HP. Learn GPM for a given HP pool pump via Google.
If you don't have a clamp-on ammeter, you could get clever and read the utility meter with a stopwatch, turning the pump on and off.
Bucketing the water output is a very rough approximation, since you have to alter the plumbing and head.
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I appreciate the replies, the motor most definitely is a 1 hp unit, I could make that out for sure, so that is not an issue.
I have google'd everything I could regarding what I know about this set-up and found nothing specific, a few things close, but that doesn't count.
One thing I do know is that a 1 hp pump could range dramatically in flow rate depending on the impeller and whether this unit is a high,med or low head unit. THAT is what I am trying to find out
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