model pilot 14

I own An Avistar with an OS LA 40 in it, seems a little under powered, when I bought the corret size exhaus deflector, it was too big on the exhaust pipe, I returned it for the next smaller size, fits a little better but my question is: will that reduction in size of the exhaust power cause a loss of power and if so, could that be a possible reason why it seems a little underpowered?

Thanks

Nick

David Cutler

It's true back pressure from the exhaust pipe can reduce power, but I doubt if the pipe you have added will provide enough to make any noticably difference in the power, providing it's not too long, and if you are just using it to deflect the exhaust, it won't be.

What fuel are you using? Try a little more nitro. It'll also might make the 40 LA a little more flexible.

-David C.

model pilot 14

I have been using 20% nitro does it go any higher?

NIck

guille2006

If you're using a commercial exhaust deflector, you don't have to worry about power decrease. Perhaps you fell the plane underpowered because of the sound....
Anyway, I think that 20% nitro is too much for a OS 40 LA... I'd better use n more than 10%...

model pilot 14

thats a good point I used 10% for the first year or so and then bumped it up to 20% and it has seemed suggish, alos tok baffle out of the muffler to make more power

NIck

David Cutler

Oh, and don't forget you might have to lean it out slightly more than without the deflector as the slight increase in back pressure it will increase the pressure in the tank slightly, thus making it run a little bit richer (which might explain the reduction in power).

-David C.

saramos

I started with an OS .46LA on my Tower trainer, and then moved it to my 4* 40. It's performance was less than stellar. One thing that really helped though, was switching out the Master Airscrew that I started with, for an APC. My subjective observation would say I experienced an almost 10% improvement in performance. As for nitro content, I run 10%.

Scott

fledermaus

I also found that APC props gave my LT-40 (with a .46) much more solid climb-out than Master Airscrew. I think the choice of prop (not only diameter and pitch but manufacturer) will have a bigger effect than small changes in the muffler.

redfox435cat

first off an la 40 is not known as a power house of a motor. 20% nitro is too high for this motor especially if your not using the right plug. the higher the nitro the colder the plug you need because more nitro ignites at a cooler temp. If you really think you need to run 20% use the right plug and take the head off and add another shim to decrease compression. Otherwise switch to 10%. The defector has nothing to do with your power loss, it probably contributing to about 10-20 RPM loss. The simple though of more nitro means more power is simply wrong thinking. Motors that are running 30% plus are set up for it and they don't last very long doing it. If you need more power get a more powerful engine. I don't recommend removing the baffle since it decreases the back pressure to the tank which leads to further leaning of the engine which will ruin it.

Also porting and polishing the inlet and exhaust ports on the sleeve help a little, don't do this unless you know what your doing or are willing sacrifice the motor for experience with porting and polishing. a perry carb works wonders, and aJett exhaust adds power but it just make more sense to get a better motor if your resorting to this.

I'd recommend a Tower 46 or evolution 46, good cheap power. With the tower 46 just add some Vaseline to the carb outlet and both needle valves and its a great engine

Flypaper 2

I have two OS 25 FPs that have run on 25% nitro since the early 70s and they work as well now as they did then. They are the predecessor to the LA . They are a low comp. engine to start with so there is no need to add shims. With lower comp. all you are acomplishing is dropping the power down to what it would be with thw 10%. They are an economy engine so don't expect to much. If you want more poop get the FX of which I have two of also and they run on 25% too with no problems.

britbrat

Anything more that 10% nitro is wasted money with that engine. You can gain a smidgen of power by removing the muffler baffle (as you have already done), plus drilling out the exhaust stinger one drill-size larger than stock. Ditch the deflector & wipe the goo off when you have finished flying.

What prop are you using? The OS 40 LA is pretty soft & it needs to be correctly propped for the particular airframe & intended use to get the most out of it (not that there is a lot to get).

jaka

Hi!
Yeah!
Don't use more than 10-15% nitro in the OS! The OS .40 LA is perfect for all the high winged trainers on the market provided you choose the right prop manufacturer (APC,Bolly, RAM or Graupner) and prop size.
As the OS LA series of engines are no high rewing racing engines they really do like larger props and low rews.
Put on a 11x5 or 11x6 APC prop if you fly at sea level and see the difference.
I recommend a 8oz tank (uni-flow set up with 2 clunks), an OS 8 or Enya 3 glowplug, 5-10% nitro and 20% oil (whatever oil is fine).

Regards!
Jan K
Sweden

kdheath

The stock LA mufflers are pretty restrictive to start with. Adding the stinger makes it some worse. Toss the deflector, drill the exhust tube on the muffler out as much as you can, and experiment with props. The LAs don't have the Scheurle boost port-cheaper to machine that way. So they won't rev like an FX or whatever. Use a larger prop with lower pitch than you might think to. The 11.5-4 APC is a good starting point for LA 40s.

MinnFlyer

My Feedback: (4)

Just to throw in my .02, I used one of those commercial exhaust deflectors once and got a noticable decrease in power.

The best thing I can say is... Try it for a flight with and without and see if there's a difference

britbrat

The dyno-tested power peak on that engine is @ 13,400 RPM, with peak torque @ 12,700. The torque peak is essentially flat from ~12,100 to the power peak @ 13,400, so a prop that gives you 12,000 + rpm will work well. If you prop for peak power, beware that you don't exceed 13,500 rpm, because power & torque fall dramatically from there. This means that you must anticipate unloading -- which with an Avistar won't be too wild, since it isnt a particularly clean airplane.

I suggest an APC 10-6, which should give you ~12,100 rpm @ sea level on 10% fuel, thus giving you an unloading margine of 1,300 RPM.