OOOPS! Brain f@rt!

The 13x6 was on the 72, I'm running 14x6 on the Saito 91. both engines are broken in, rings fully seated with stupendous compression. Still, 14 is less than 16 not more.

Yup 10,000 is what i saw o my Tach. only breifly because as i said, i was just breaking it in, so i only peaked it for a second. Also I hear not all tachs are not the same, so ther might be no reason to envy. Not sure , but what you said earlier about not getting the same RPMS as other folks says yours might be reading lower than most?

Dick,
I'f you like, i'll PM you when i have the FX all tweaked out, in case you are interested in seeing one with a Mousse can. I fly at S.A.M. on weekends. There is also a great guy there who flies the dog house and loves it you might enjoy seeing another smiling face flying your creation.

Best regards,
Joe

I think my tach is ok. I check it with the flourescent lighting like the directions said. I was losing rpm because my valves had enough clearance to shove a toothpick in there.

I still would like to see a 91FX turn a 16x6 at 10,000 rpm. My Webra works pretty hard to turn a 16x6 APC at 9,600. With more nitro I've seen 9,800. If I let it whail like a banshee it might get to 10,000. If your 91FX does that, you got a good one.

The rpm range for a 91FX is 2,000-16,000 and I believe the peak power is made at 15,000 rpm. If you are only turning 10k then it sounds like you have it overpropped. The engine isn't designed to make peak power at that rpm. If you only want to turn 10k it sounds like you need to get a 4 banger. As far as the torque issue, there's no doubt that the 4-stroke excels here. Why do you think so many people love the 4-stroke for hovering? Good throttle response and high torque. A nice thing about a 4-stroke is that it not only has great torque at higher throttle settings but it's exceptional at lower throttle settings as well. I'm not knocking two-strokes, but by design they make their best power at higher rpm's. Sure, a mousse can will help broaden that range but the 4-stroke has an advantage at lower rpms with it's more linear power output. I know a guy with a UCD3D with a 91FX and it does a great job. Many guys love the Saito 100 on this plane as well. Everyone has their own theories about 2-stroke versus 4, this is mine. I may be wrong. Sorry about rambling on about this!

I can't imagine a 91FX at 15,000 rpm. What prop is that... a 10x6?

Years ago, OS said right on their website that the 91FX was recommended up to a 16x6 prop. Now they have changed that. The 91FX is a somewhatly short stroke motor, and does prefer a bit higher rpm. That's why it's recommended to use a smaller prop. But I can't see a 91FX lasting too long at 15,000 rpm???

I think the 91FX is a good motor, but I'd stick with a 14x6 to 15x6. A 16 inch prop seems much, and anything that runs that engine at 15,000+ rpm is way too small for my taste.

When Spaceman said he was turning 10k with his then I checked the rpm range of the motor. I thought that wasn't many rpms for a 91. OS has the peak power listed at 15k for the 91FX. I'd be concerned about lugging the engine and overheating with that much prop.

OS probably changed it because that's quite a bit of prop for a 91.

This is from the OS website.

Assuming the engine is fully broken in, you are using good quality fuel with at least 18% oil, and that there is no fuel foaming occurring, it is possible you are overpropping the engine. With use here in the U.S. at our altitudes, fuels, etc, we have found that this engine prefers slightly smaller props - 13x8 or 13x9 specifically - and turning at higher RPMs. If your engine's max RPM is under 10,500 then the engine is working too hard and not making peak performance.

I read it this way... Correct me if you see something different...

Use a 13x6 to 13x9 and shoot for at least 10,500 rpm. Are they saying that perhaps anything beyond a 13x9 would likely not break 10,500 rpm? If not, what are they saying here?

I think they are saying to not overprop the engine so much that it will knock you out of the peak powerband and also risk overheating. Looks like they want 10,500 and up. I'm not saying that you have to prop to turn 15 or 16k, just that 10k is a little low for this engine. I'd be more comfortable running this engine up at least 12-13k. Once again, since they have the range up to 16k with peak at 15k, it wouldn't bother me to prop it to run at 15k. Don't worry, it won't blow up, it's designed to take it! 2-strokes are tough. I hope the rpm range on their site isn't a misprint. I figure it isn't. Now a Saito 100 is a totally different deal. If you exceed the max rpm on a 4-stroke then you are asking for problems. The paperwork on my 100 says up to 11k. I personally don't want to see over 10k on the ground, 9500 would make me happy.

Yup, my Saito and YS stuff say stop at 10,000 also.

A 15x6 seems to be a good choice for the Saito 100. Seems like several people are also running the APC 16x4 wide blade with good results. I'll ask the guy with the UCD w/91FX what prop he's running and at what rpm and let you guys know.

Hi Wade, Thanks again for the help with the Avitar.

The 16x6 might not be the optimum prop. Having no experience with the thing, i just looked at the manual, the biggest they recommended was the 17x6, so i just grabbed the 16x6. Once i get it mounted up to a plane, i'll experiment. I use a digital fish gage and measure static thrust, trying to optimize thrust and still have good throtle responce. For most of the smaller engines i've played with they do tend to come out on the large diameter side. 11x4 on a piped 36, and 12.25x3.75 on piped 47.... Both gained over 1000 RPM and behave far better at half throtle than they did with regular mufflers.

I agree that my saitos usually behave very consistently at half throttle, (so long as they stay running ) my point is that two strokes can have that too. it just takes a bit more tweaking. I admit i have limited experience, but so far a well tunned Mousse can looks like the great equalizer.

With typed words i probably can not convince you, but since Mr Smith resides here in Sunny Tucson, i just might be able to convince him, that 2 strokes are in fact a perfectly legitamate means of nitromethane consumption.

Spaceman, that avatar is pretty cool! Hey, I'm no expert either. I have several two-strokes as well. Hard to beat the simplicity of a two-stroke, but the 4-strokes sound cool and I like the low rpm torque they have.

I prefer 2 strokes actually. This season will be my first full season on 4 strokes. I think what is happening is the two strokes are gaining some midrange and 4 strokes are getting better at top end. My Rossi was good, just a little left stick work was in use.

Thanks!

Definatly will take a look at smaller props for the FX.

I can't remember exactly what the guy was running on his 91FX. I was thinking maybe a 14x6. I'll find out though. He had a mousse can on it, ran great. Where did the guy that started this thread go? I don't think he's ever replied. Must be in Florida on vacation!

Wade

We have to get TailTwister an avatar. Let me know if you want one and I'll hook you up.

Wade

TailTwister, just keep messing with the 4-strokes and you'll line them out. If you don't then maybe Wayne Geffon can give you some pointers. I think he has tons of experience with 4-strokes.

Spiff, don't get me wrong, if your 91FX is turning a 16x6 at 10K, just check for overheating issues. Don't ditch it right away. My take on the OS "verbage" was that it was an excuse for changing the prop recommendation. For pattern or sport type flying, I'd go with a 13x8 to 14x6. For 3D I'd try a 15x6 or 15x4. Maybe with your motor, you'd get better than 10,500 on those. I had one that would not. I had to go to a 14x6 to break 10,500 on the ground. Four other pilots in my club did the same. Like I said, maybe you got a good one. I've also wondered about their mentioning of fuel as being different. Most overseas fuel is 0% nitro. Maybe the 91FX likes lower nitro. Some high compression motors try to detonate with high nitro and big props. Anybody try a 91FX on 5% or 0%?

Umm... Don't know how to say this, but it looks like i may have biffed pretty bad. I just put my FX back on the stand to verify RPM, I could not get it over 9000 even for a momentary peak and was loosing compression as i tried to push it. Not sure if the engine had a moment of brilliance, or i had a morning of stupidity. For now take the numbers with a grain of salt.

Many apologies

My asbestos suit is on.

I actually was pretty excited that OS may have finally delivered (to someone) what I felt I was promised. When they lowered the prop recommendations, I e-mailed them a very angry letter about the change. If I had read a 13 inch prop recommendation in the first place, I would have never owned a 91FX. Just for laughs, go to the OS website and compare prop recommendations for the 91FX and the 65LA. To me that's the funniest thing I've ever seen. Spiff, I have to say thank you for rechecking your data and clearing it up. There are those here who would have not done so. Now build that darn mousse can and lets se what it can do. Maybe you can get some real power from the 91.

Twister, You are one cool dude.

Crow is nasty stuff,

but you just made it taste a lot more like chicken.

No prob' dude. I'm not here to piss people off.