on my 91fx... i am not getting the rpm that i would like to get with a 13x8 prop... this is recommended prop... what would happen if I go to lower prop like a 12x8 or so? would it have too much revs? is it bad to under prop an engine but still tach below the max rpm of the manufacturer? I want to get closer to 15K rpm... I am using the stock muffler which came with the engine...

You SHOULD BE OK. You need to be sure you are AT LEAST 500 rpm under max rpm (800 - 1,000 rpm would be better).
The reason is that the engine is going to "unload" in the air and your max rpms will increase.

You don't say how you want to fly the plane. If you are doing sport/scale flying you may want to try a 13x5 or 6 - if you will be doing 3D try a 13x4.

its for sport/aerobatic flight... ok thanks for advice... the engine just sounds much better and more powerful when its revving more... like its effortless...

Switch_639,

its a trade-off, the prop size... If you install a larger prop, you will get less RPM and a smaller prop will give more RPM. But this not mean you will get more thrust from the smaller as compared to the larger and even the reverse may not be the case. Rather than evaluating by the sound of the engine, you should see how the airplane feels flying with different props. Try them all and select the one you are most comfortable flying with. For example, I have following props: 11x9, 12x4, 12x8, 13x4, 13x6, 14x4, 14x8, 15x6, 15x4 and just two operating engines: 55AX and 91FS. I am only using 13x4 and 15x4 respectively on my airplanes as both are 3D. Remaining are stock I have tried and kept aside so I can use on my pattern ships later.

You may want to note that the lower RPM on a larger prop will be better for bearing life but will increase spool-up time marginally. Note that prop pitch also plays a role in this. Also, if you under-prop your engine, you risk overspeeding damage. So select carefully. For the record, i feel 12x8 is too small for that engine

Ameyam

i feel comfortable when a plane is flying fast... rather than having thrust, I want revs... I have 11x9, 13x8, 14x8 and 15x6... the 14x8 and 15x6 give me below 10K rpms... and OS suggests its overpropped then... must at least be getting 10500 revs... the 13x8 does give me more, but I feel I need a bit more...

This graphic was made for Power Point propellers, but it can be a guide for the range of propellers diameter-pitch that better suit each engine displacement:

http://www.top-flite.com/accys/topq5000a.html

The top curve would be the selection that matches your personal preferences.

What is the "recommended RPM range" of any engine? The manufacturers specs go as far as stating, for instance, 3.2 HP at 16000 RPM. Who would fly a sport engine at 16000 RPM? It is just not recommended.

The best thing to do is to fly the plane with various props and use the one that gives you the best performance for your needs and don't worry about "recommended" props. They are usually starting points, and give you a range to "play" with while trying to find a match for your plane and flying style.

CGr.

If you want more speed just get more pitch and less diameter. Higher speeds eat your bearing quicker and wear your piston and liner quicker. As stated above, sound has nothing to do with how your engine is performing. It's in air performance that matters, so prop the plane to fly like you want it to in the air.

How do you know when your engine is "under performing"? Let's say that Switch's 91FX turns a 13x8 at a max rpm of 9000. Are you concerned? If so what steps do you take to improve the engine's performance? Ipicked 13x8 because it's at the bottom of the displacement / prop graph for a .91.

For 2C engines:
When the manufacturers give a HP rating at a certain RPM, it just means that the engine will produce that amount of power at that single point. It doesn't mean that is the upper limit to the RPM it can operate at, nor is it the RPM you want to prop for on the ground. Because when the model reaches flying speed, the prop allows the engine to "unload" and reach higher RPM than what it can reach static on the ground.

For most sport models, the amount of unload has to do with the engine timing, exhaust system and drag of the model. Generally most sport models will unload somewhere between 1,000 to 2,000 RPM, so the highest you would prop for would be no more than the peak HP RPM less 2,000.

It is due to this unload raising the flight RPM, you also set the needle on the rich side of peak RPM, to keep from going lean in the air.

People seem to think that the higher RPM operation is hard on the life of the engine. Perhaps, though ball bearings on the crankshaft are not the only consideration. The rod has bushings that float on a film of oil, and lugging the engine at low RPM is quite hard on the bushings. One of the reasons that CL engines typically used 22% oil or higher, while RC engines typically operate with 17-18% oil. Meanwhile most people damage the bearings by operating in dirty environments or leaving unburned fuel in the engine. And of course, crashing and not tearing down the engine to clean it, which in some circles might be called a hard landing. If you break a prop, chances are good that the engine might have ingested a bit of grit through the carb or around the front bearing.

If it is a 4C engine, the rules are different again, as you need to stay below the RPM where the valves float.

Good - luck!

As others have said, Manufacturers may make claims of top speed being 16,000rpm, but you'll only get that high if you're turning a ducted fan with it. A propeller is a whole different enchilada.

At best you might get 10,000 - 10,500 tops

ok so I am not reaching 10500 rpms with 14x8... and I am overpropping if I only get 8000rpm? the engine is not getting hot or lean... its very quiet 4stroke like sound at those low rpms with the stock muffler... its underperforming at 8000rpm? is it bad to be at such low revs? the engine has loads of pulling power though...

yep a lot
bolt a 14x6 and your engine will have a lot more power, right now im guessing a .91 four stroke could turn a 14x8 just as fast if not faster (still overproping the .91 fs too btw) . a .91 four stroke has the power of a .65ish two stroke.
change your prop from a 8 to a 6 and go from a .65-75ish engine to a .91 engine

I'd run a 13x6 on a .91

wouldn't go smaller than that though. And, these guys have it right....the sound the plane and engine is making is not a good barometer. Some brands of props are noisier than others; doens't necessarily mean they're performing better.

If you really want to go faster, put a 1.20 on it with a 13x8 or 9

Hi,
You can run a 12x8 APC on a that engine no prob.
91FX + stock muffler + 15% nitro + 12x8 APC = 12k RPM
91FX + Jett muffler + 15% nitro + 12x8 APC = 12.6-7k RPM
The latter is my current setup. I like to go fast too.
I can add also that a 12x6 is underpropping the engine. The 12x6 makes a lot of noise with no performance gain.
Cheers.

How do you evaluate performance when in the air? Qualitative or quantitative? What do you use for measurement?

TIA

I am running a 14 x 6 on my 91 fx and it did great on my 60 size stick. I mean it would haul freight. Not long ago I stuck the engine on my Revolver 70" but does not quite perfom as good. It acts real sluggish. I plan on dropping down to a 13 x 8 or 13 x 7 and give that a go to see what happens.

It is amazing how taking an engine from one plane to another changes everything. The whole setup needs to perform well together.

I run a 14x6 on my .91FX in my Hellcat. Using 15% nitro, at an altitude of 5300 feet, I get about 10,800 max but usually take it down to 10,300-500 before I take her off. 20% total oil with 4% being castor. Runs good, runs strong, never overheats.