RPM of small gas engine

At a guess I'd say 1800 ought to work. Gen sets with Vibration & Stratton engines seem to spin at that rate.

The standard governed speed for 4 cycle engine like lawn mowers is 3600.

Standard small engine (single cylinder, 4 cycle, 3 to 14 hp) rpm is 3600, some later engines run a few hundred RPM faster.
Most of the pumps I found were rated at 3450 to 3600 RPM for full output. Note that you can get trash pumps, semi-trash pumps, clear water pumps, and high pressure pumps.
Yours appears to be a semi trash variety. This is good for some soft solids like an occasional leaf or bird dropping, high volume, lower pressure.
Karl Townsend wrote:

:I just bought a trash pump without the gas engine. : :What RPM do I spin it at? : :FWIW, I hate small gas engines. Nothing but trouble. I'll use a 3 phase :motor for this job.
Looks like a 3600 RPM electric motor should be a good match.

The 4 cyl. Continental on my SA 200 welder runs at 1450 rpm.
Steve

How about a VFD?

For long life you want most common engines to run in the 1400rpm to 2000 rpm range (depending on size). Ditto for noise, faster = noisier. 60HZ alternators need to be locked at 1200rpm, 1800 rpm, or 3600 rpm for 6 pole, 4 pole, and 2 pole alternator heads. Yours is a DC unit, they can build the windings to produce optimal output at whatever the engine wants. 1400 rpm in a 4 cylinder flathead is a long life setup.
SteveB wrote:

99.99% of 4 stroke small gas engines are governed and rated at 3600 RPM

But that is not a "small engine"

Sorry, but previous posters included 4 cyl engines.

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3000 RPM .
Need to start on Propnae . Exh is routed around to heat liquid fuel and to "regenerate" heat into the intake . Then switch to liquid , turn off propane .
Japan cant sell machinery in U.S. , Jap' makes motors in China , China trademark . missing the Jap name
Price is highly subsidized . 13HP motor has a balance shaft and retards camshaft to lower cranking PSI , for easy starting .

A 3600 RPM 5 hp. may be hard to find. I'll start shopping eBay for a 5hp. VFD. Didn't really want to put this much $ into this job though.
Karl

DAGS
Powerhorse+pump+RPM
3400-5500 RPM depending upon size and engine used, based upon a quick glance at the search results.

A 3450 RPM should be easy to find, and work well enough. The 150 drop in RPM will not affect the GPM enough to notice. Plus you not need the VFD. Greg

If you want to use an electric motor you can go a bit smaller - If it calls for 5.5 HP gas engine, you can probably drop to a real 3 HP electric motor - a 5 HP would spin it without breaking a sweat.
For some reason small gas engines are rated at gross HP, and when you subtract the power used to run the cooling fan...
And find out the optimum speed for the pump head - probably 2,400 RPM to 3,600 RPM range common for gas engines.
This style pump head is meant to direct-mount onto a certain tapered crankshaft tail normally found on gasoline engines - which you are NOT going to find on regular electric motors. The pump impeller has the female of the tapered shaft with the lockbolt, and the tolerances when it's all bolted together is critical.
Pool pumps and stationary pressure pumps are built to bolt to a standard C-face pump duty motor with straight keyway shaft, not these. If you search, you can probably find a semi-trash style pump made for an electric drive - just like you can find prerssure washer pumps built both ways.
If you really want to go electric with that particular pump head, you need a pedestal adapter for the pump. It has the proper tapered shaft with center bolt, a pedestal in the middle with two shaft support bearings and and bolting face at one end to mount the pump, a mounting foot built into the casting, and the other shaft end has a straight shaft with keyway to mount a standard drive pulley or a Lovejoy Coupling.
Thern you vary the pulley ratios to drop the speed a bit from the 3,450 RPM of the electric motor to the ~2,400 RPM that the pump head really wants.
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Here is one for example, EBay #120391225187 Greg

Karl, I have repaired a ton of small engine powered pumps over the years, 3-8 HP, and I have ONLY seen engines running at 3600 RPM. No other RPM's, just 3600. Look for a electric motor running about 3450 it will work fine running direct drive. No belts, no VFD needed. You will more than likely need to build a mount and shaft setup to mimic the gas engine that would normally be supplied with that pump. Greg

Not really, we have run these small engines on a legitimate dyno at one of the major lawnmower mfg facilities. (They paint their stuff red) The engines tend to put out less than the engine mfg lists: something like 95% of the nameplate number down to not much over 50%
But the real reason you need a larger gas engine than electric is that the gas engine has no additional torque capability, an electric motor does. A 20% overload for 1 second will kill a gas engine cold. Same overload on an electric motor will cause a surge in input power, not much else.
I'd expect ANY pump in this class to be set up for 3600 rpm. It's the standard speed for both gas and electric motors. The OP listing clearly states "3/4 inch straight shaft with keyway" A lot of work with no benefit.
It has the proper tapered

O Rly?
I read the eBay listing referenced as a tapered shaft with an end hole for a lock bolt. Sure you looked at the right page? They :do: come both ways.
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