We flew my Sundowner and Evolution 26 combo yesterday and it appeared that the engine was restricted in it's rpm. It was almost like there was a brake of some type. We were spinning an APC 15X10 at 8700 on the ground. OK, not too bad. But the problem was that coming through a dive or turn the engine did not noticably encrease in rpm. If the plane seemed any faster through a dive it would almost immediately loose its speed as soon as it leveled out. The best we could guess was that the Bisson muffler was resricting the engine rpm. I have to assume something is wrong if I enter a dive at 100mph and come out of it at near the same speed.

Any thoughts?

Thank You.

Phall,
The 15x10 seems a little small to me. I would use a 16x8 at least. You can cut the exhaust tips off of the muffler to reduce back pressure. This will result is an increase in power with little or no change in noise level.

Dave

You don't pick up power in a dive -the engine is actually making far less power - just revving without load
but I guess you knew that -just want to hear it rev?
some guys lean out their enines so that they will rev more in a dive - BAD idea
set like that - it is far too lean for level and especially, pulling uphill
the prop size is ok but yes that muffler -tho quiet -does restrict power .
onracing tuned setups -such as were used on control line - the engines would -when the planes reached enough speed to hit correct resonant tuning -- allow the tuned length and richer mix to create more power and lots more revs
your setup is not like that .

Dick
I was there with Phall flying the plane for him while he handled the stalker radar gun. Motor was needled correctly, no sagging thru turns or uplines. The muffler simply acted like govener, the plane would go up almost as fast as it would go in a shallow dive. While the tuned pipe would be the best thing for this engine. the airframe really isn't setup for it. What would be your recomendations for a better in cowl muffler?
We know the sound you are talking about when the tuned pipe comes on and stages, it sounds like it is shifting gears. We have experienced that setting up nitro monos and outrigger hydros for the past 10 years.

take care

zonk

Dave,
I do not know why I would drop in pitch. I did the numbers and I would have to do about 2200 more rpm to achieve the same speed. That would put me around 11,000 rpm which might be a little much for that engine. Or not? We did open up the ends of the stacks and increase 200-300 rpm.


Dick,
I was not really wanting to increase power in a dive I was only providing that information to what I think is the muffler not allowing the engine to rev past a certain point. It seems with any other plane coming down at full throttle that you can definitely notice something is happening. Speed , noise......something. This plane seems the same no matter what. So without going to a tuned setup, is there a more prefered muffler?

Prop for lower rpm, like 7500, and the muffler will perform a lot better. It has restricted gas flow, so cannot handle the increased flow at 8500 and above. That means it will just limit power output even more than it already does if you go too high in rpm.

The Evolution muffler -which I used on one of my 26 engines -isn't too bad -tho nowhere the power of the factory tuned setup.
The reason I mentioned the dive and pick up rpm thing - is that a couple of weks ago -I saw a guy with a early style pattern model and a glow engine go thu the "make it run as fast as possible thing" in dives
he kept leaning it out till th noise was a real pain in the ask - it never went any better in level flight of course but he got the prop screetch and thrill he was after -in dives.
I am sure the thought it was making more power .
The Sundowner -being a lookalike racer has guys wanting as much speed as they can get and with the 26 that is never going to be very fast .
The real problem being that the drag increase exceeds the power increases -and it takes 4 times to power to go twice as fast -which means each additional mph takes a dis porportionally bigger chunk of power.
-the prop which makes it go as fast as possible may be larger in diameter and or larger or smaller in pitch - -
you can calculate that one till you are out of ink but it ends up being a cut n try .
The ONLY fer shere change is to increase -- you guessed it - torque at speed and the in cowl mufflers won't/ can't do it
the best power in cowl will end up being a staight stack which is slightly larger at the end than where it attaches - and it will be "a little noisy".
Or clip the wings on the model
Or reduce the landing gear size etc and reduce cross section.
Or run alky /5% nitro and 20% Klotz-in a modified carb and a compression increased .
50 years ago we ran 102 in the 1/4 mile - stock bodied coupe, 283 cubes -etc..
increasing speed even 1/2 mph was a real trick -and it always required more torque--to make more hp.. ask anybody with any racing background - increases are always hard fought .
In the 26, there is a finite (max -rate -of- flow ) which is reduced once max torque is reached - that means - once you exceed maximum torque the hp may increase but torque goes down.

The exhaust system regulates how much fuel charge comes in -which means wide open exhaust is best muffler - then an extractor type then a tuned type- each one pulls in a bigger charge of fuel -which strangely enough- is where the power comes from- go figure -

So what has to be done to the carb for alcohol? And what compression would I need?

I'll kick myself later.

I will not get into that -except to say the carb has to be drilled in the feed passages and compression needs to be same as th glo engine 26 - It would be easier to use a 26 glow engine and fit the head for the small plug and use a smalltuned can with exhaust pressure - the Pitts type is still a worthless piece of ****. I ran ST2300 with this setup and ground rpm was typically 9000 on 18x8 APC--and it wasn't even a racing setup -it was an aerobatic models. which throttled beautifully and never ran lean.