How long can you run a 150cc engine on a test stand before it overheats and what maximum rpm...assuming it has a prop on it? Will the airflow from the prop be enough to keep the engine cool on the test stand for an extended period of time?

If the heads are not too close to the test bench firewall, you shouldn't have a problem

Roger

How long depends on a number of variables for any engine, test stand or airframe for ground running. You need to get yourself a remote sensing thermometer and run it a little bit, check the temps and do it again. Then you'll have a better idea if there is going to be a problem with heat and how long you can ground run one before it starts getting too warm. You don't want to overheat the engine, any size engine, on the ground or in the air...

I like to run in engines on the test stand in short bursts, because It allows me to monitor the temperature and other things more closely.
If the engine heats up too much, or shows any signs of sagging, the throttle can be chopped right away for a cooling down period.
After a liter or two, and no more sagging is experienced, the engine is free enough for the prolonged work in a plane where monitoring is not so easy.

Exactly... [8D]

While I'm relatively new to the RC stuff, I've been around all sorts of engines for a very long time. I can't see throwing a new engine on a model, setting the needle valves and go flying.

I want to run it a bit on the ground and make sure everything is kosher giving the engine a little break in time with the heating and cooling between runs to help start the break in process. As long as you don't overheat it, I can't see any harm in doing it. Then watch it carefully for the first few flights for the signs of overheating, and I like to check the temps right after landing the first few times as well to make sure it's running cool enough.

As a matter of fact I'm ground running one right now, and posting here while my new twin is cooling off outside...

On a twin test setup-- make windows or cut away the board to allow air to flow past the heads

Would it help to have the front end of the plane on the engine (cowl) to allow more directed airflow to the heads?

Do a serch on engine cowling
Edit: or better still, engine baffling

NACA engineers spent a lot of effort to get reliable engine operation and cooling, without reducing airplane performance. There is a wealth of information there.

The inportance of this issue, still hot after 80 years, can be gathered from the [link=http://history.nasa.gov/SP-4219/Chapter1.html]naca files history [/link] from which I quote:
"
As most successful research programs do, the NACA cowling investigation started with a question: "Is it possible to extend a cowling outward over the exposed cylinders of a radial-air-cooled engine without interfering too much with the cooling?" It is significant for NACA history that the question, which brought the breakthrough counterintuitive answer, was asked at the NACA's first annual manufacturers' conference, which was held at Langley Memorial Aeronautical Laboratory on May 24, 1926. This event became the NACA's "rite of spring." A combined technical meeting and public relations extravaganza, the annual conference gave the NACA research staff an opportunity to ascertain the problems deemed most vital by the aircraft industry so that it could incorporate them as far as possible into its research programs. At the same time, the conference gave the staff a chance to publicize its recent accomplishments before individuals who rarely had the time to read the NACA's published technical reports but who needed, and wanted, to know what the NACA was doing. The conference also gave the research staff at Langley a chance to bang a big drum before congressmen and other public officials who "had neither the time nor the qualifications to read the technical reports" but who played critical roles in the appropriations of government money. The event started in 1926 as a modest and relaxed one-day affair, but it soon grew into an elaborately staged pageant that took weeks of preparation by the NACA staffs both at Langley and in Washington. By 1936, the spectacle lasted two days, the first day for executives of the aircraft industries and government officials, the second "for personnel of the government agencies using aircraft, representatives of engineering societies, and members of professional schools." In 1926, only forty-six attended the conference; ten years later, more than 300 people were attending each session, including aviation writers who reported fully on the laboratory's presentations in newspapers and journals.9
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