MY #1 CHOICE WOULD BE A NEU MOTOR . NEU MOTORS ARE A BIT COSTLY BUT HOLD A LOT OF RECORDS . LEOPARD MOTORS ARE NICE TOO .BOTH MOTORS HAVE WATER JACKETS FOR THEM . I PUT A NEW 1515 IN A 27" CATAMARAN RUNNING 4S AND GOT 68 MPH ON THE GPS. ALSO I HAVE AN OLD DUMAS SKI DADDLE 36" RUNNING A LEOPARD LBP3674 - 2650KV WITH 3S AND GET ABOUT 40 MPH . I AM NOW WORKING ON A 36" GENISIS CAT WITH TWO LEOPARDS . PERFORMANCE TO BE SEEN !!!! I HOPE THIS HELPS YOU OUT SOME .

if you use a "y" lead to couple 2 ESCs ( or if you use 2 different channels) make sure you remove one of the red signal leads from the ESC that way you do not risk you receiver ( this presumes your ESC(s) have BEC)

HI robhenry
The L5055-700 are 1600watts on 8S, so only about/less than half that on 4S.
I am installing only one in my crash tender with a 2 blade 52mm CNC alloy prop to start with and can upgrade later.
I bought this one without running gear and single shaft.
They only did 28 knots.
Canabus

Hi Canubus
Am running 2 9.6c packs in series.this gives about 18v on load.
Rob

Hi robhenry
Your motor would be only running at about 9500rpm 0n 9.6 volts.
The closest brushless motor would be a Hobbyking Turnigy L5055C-700kv on 4S Lipo batteries (10360 rpm), these are 50mm diameter, 6mm shaft.
Note you require a separate ESC for each motor and a Y connector to link them to your throttle channel.
Personally I would use the Hobbyking car 100A ESC's (HK-100A).
Also both the motors and ESCs require no water cooling.
Canabus

Hi Canabus
Thanks for your response. Very helpful.
Currently the 820's are running on two 9.6v Nimh
packs in series. Have not clocked amps in running conditions.
I assume that I am not exceeding 20v x 20amps rated
by Graupner spec sheet.
Best regards
Rob

Hi robhenry
What volt battery are you running at present ??
12 or 24 volts.
If 24 volts those motors spec at 24,000rpm.
Power is 511 Watts on 24 volts.
24,000 divide by 14.8(4S Lipo batteries) equal 1612 kv.
So the closest motor would be 1700kv, eg water cooled 3660SL-1700kv which has a 5mm shaft and 44mm diameter with a length of 60mm( from Hobbyking).
Power rated at 1800 Watts on 6S, so it would be lower than that.
But if you are running 12 volts on the Graupner motor it would be about half the rpms.
These brushless motors are far more powerful so check out the specs on the motors you are looking at !!!
Canabus