Hi - I have an OS 91 and I see that the charts call for one of these: 13x8 or 15x6 or 16x5 or 14x6 - But what if I put a lightly smaller prop? let's say 12x8?

also, which one of the recommended props should I use?

If it is a two stroke, here is a thread with some info http://www.rcuniverse.com/forum/fb.asp?m=1983653

Using a lighter prop lets the engine spin up faster and/or at higher RPM. Using too light of a prop will allow your engine to spin faster than the recommended RPM range and could cause damage.

Sometimes a slightly smaller prop is used for break-in. Engines have a tendency to develop more power after being run for a few gallons.

The right prop is one that lets your engine spin in the right RPM range and gives you the performance YOU want for that particular plane.

Larger diameter props pull harder and smaller diameter props spin faster. When you increase diameter you decrease pitch, and vice versa.

Out the props you listed, the 13x8 would give you the highest top speed, and the 16x5 would give you the best vertical performance.

Ditto, 4-strokes are made for power, not speed. You don't want it to turn more than maybe 9500 RPM

Hi - I just checked the specs of the OS91FX on the OS website and it says 16,000 RPM did I read it wrong?
here they are:
PECIFICATIONS
Stock Number: OSMG0591
Displacement: 0.912 cu in (15.0 cc)
Bore: 1.091 in (27.7 mm)
Stroke: 0.976 in (24.8 mm)
Practical rpm: 2,000-16,000
Output: 2.8 hp @ 15,000 rpm
Weight: 19.3 oz (550 g)
Includes: Muffler, Safety Propeller Locknut Assembly, #8 glow plug
Requires: fuel, mount & prop
Recommended Props: 13x8, 13x9

printed rpm are just about worthless and unatainable in real life.if you want to use a 12 inch prop try a 12-9 or a 12-8 3 blade.

Using ThrustHP as a calculator, it would take
2.754 horsepower to turn a 13x4 prop at 15,000 rpm.
5.508 horsepower to turn a 13x8 prop at 15,000 rpm.
3.999 horsepower to turn a 12x8 prop at 15,000 rpm.

I do not know if these numbers are exact, but, I do think they show that you'll never see the "advertised" rpm using a prop that is in the recommended range.

For at least one "real world" number for a 91FX, here is the text from the link I posted:
"12x8 APC - 11,700 peak, idle around 2200-2300 after a gallon of break in.
- stock muffler, powermaster 15%, OS #8 plug, 75F and humid at around 500ft ASL"

Sorry, I thought you were talking about a 91 4-Stroke.

But even a 2-stroke is not going to get 16,000 RPM unless you are running it in a ducted fan jet - and then you can count on it lasting about a year before it burns itself up at that speed.

You should shoot to get 11,000 to 11,500 RPM out of it and prop accordingly.