HP-Watts to crank over a big twin?

Doing some fantasy figuring so need some realistic data points to leap from.
What is the expected current draw in amps or watts that a good Commando starter should draw to spin a Commando with ease? Any idea of the motor shaft torque before its geared?

from dynodave's site- stock starter
Norton 06-4791/Prestolite MGD4111 Starter
stock 2 brush 21 segment single wrap armature-about 3.5 gauss/amp in series with 2 - 6 turn series fields
*Long out of productions. Some NOS units are reported to still be available for around $600.
**Draws about 300amps stall, 200-266amps normal starting current, 120amps once engine is turning.
Voltage depression at battery during current surge, 9.41-9.77v typical.
***Impression- Functionally adequate if all factory stock systems are up to snuff.
****10 tooth armature gear
*****6-1/2" installed length

4 brush and aftermarket draw more
No idea of shaft torque

Used to work for Lucas and back in the late 80's we designed a 4 pole PM motor to replace the basic MKW starter. With 4 field coils in series the MKW made about 6ft/lbs stall (lock) torque, depending on volts available. The two coil would be lower, maybe 4-5ft/lbs on a good day. Bear in mind it takes appx 20ft/lbs at the crank to turn over a 9:1 750 with 40 weight oil at around 60F. (last time I did a measurement). That's less than 2ft/lbs at the starter on normal Mk3 gearing with say 90% gear efficiency. The MKW would be about 2000-2250 rpm at that point again depending on available volts (speed) and amps (torque). That's around 640 watts at 2250 rpm and 140 amps at 9v with a best of 50% starter efficiency at that point. Sort of adds up and remember as the engine spins up the load will reduce.

Ok your two reports provide a ball park range for me to plug in equations
300 amp x 12 v = .6 hp , 300a x 9v = .5 hp
640 watts at 2250 rpm and 140 amps at 9v with a best of 50% starter efficiency = .86 hp juice fed in for .43 hp

These seem to be cream of the crop for the cost and size and power range there should be enough stored power on board to spin a big twin to start.
http://www.etoolsnow.com/manufacturer4. ... manufid=22

640 watts at 2250 rpm and 140 amps at 9v with a best of 50% starter efficiency = .86 hp juice fed in for .43 hp

Click to expand...

That's starter output, hence 640w/9v=70A@50%=140A. BTW I miscalculated, didn't properly convert Lbs/Ft to Nm when I did the watts calculation. So 2lbs/ft at starter@90% geartrain eff. and 2250 rpm = 590W. (20lbs/ft at crank/12:1=1.67lbs/ft /0.74=2.25Nm/.9=2.5Nm@starter=590W@2250rpm or whatever the starter can deliver dependent on voltage etc.). It's still ballpark stuff though. You need at least 3/4hp at enough cranking speed to fire.

Ok Kieth thanks for re-calibrating my figuring. Takes a good hunk of lead to dump 3/4 hp/640 watts power, more than I want to carry around or have room for on Peel.
Peel swing arm air tank is hold only 0.045 833 cubic ft/79/2 cubic inches. I've regulator set for 100 PSI shut off. Air tool start may be another dead end idea.
But water injection and suspension tucking will be on tap.

Harley Sporster starters are 1.4Kw, that means about 2HP if my math is correct (742w = 1 HP) and that is what Old Brits use.

Jean

PS Hobot, an air starter would be a great idea, the top tube could be used as an air reservoir if it hadn't been destined to another use no?

Peel's spine will carry oil as do both the frame loops and only holds 31 cubic inches. Swing arm not a whole lot more. Maybe another 20 ci tapping tall handle bars and rear frame loop volume. I could add more air tanks, and may for a record attempt, removing 20 lb worth of air station stuff and alternator, for a few lbs of air power to work suspension and water spray. Would only need one good air shot as we all know Commandos should start on first or 2nd piston passing TDC. I don't think I'll have enough air to blow much an air horn.
I don't desire-need air shifter with AMC kill button ease, but actually think I may not have to shift much.

I'll pump up swing arm tank to planned 100 PSI and run a hose to air tool and see what happens, BEEEZEE or BEzzeeshhhhh ...

I'm no wimp and fully expect to kick over 10.5 920, when feeling fine and fresh, its the other times and just plain fun to have option of no strain starting.