TimC

I finished the break-in today and here are the numbers. It was about 105° today at 4000' elevation so the density altitude must be up there. Using an APC 13x4W, 15% Omega and an O.S. .91FX muffler with the baffle removed, it peaked at 11,040 rpm. Transition is good and the idle is very low. Using a .61FX muffler it peaked at 10,700. This is after five 16oz. tanks of fuel. The Jett-Lite muffler is too loud for running in town. Oncet I saw my neighbor Clevis eat a live chicken.

bob27s

My Feedback: (19)

sounds pretty good, considering the altitude and conditions.

Sport_Pilot

Yeah, 4000 feet at 105 degrees must have been bordering on the height that aircraft pilots would have to use oxygen.

Iflyglow

My Feedback: (79)

My cheapo Magnum .61 four stroke turns a 13-4W @ 10,100 on WC 15%.

TimC

I guess that shows I'm pretty stupid for buying a Jett and not a Magnum, huh. If this means I can't be in your Genius and Thrifty RC'er Club, I guess I'll just have to learn to live with that.

Gringo Flyer

My Feedback: (18)

Tim have you at least tried it with the muffler? I would be interested to see what boost you could get with it. Maybe it was bc of the heat and elevation but I would think you could get a little more out of that engine. I am getting 11k+ with a ST90 and jett muffler on a 13X6 APC. I would think you could get at least comparable numbers out of the jett.

Also if possible I would check your db levels and make sure they exceed with the other muffler. Its a shame not to use that muffler. When I switched to the jett muffler on my ST90 I didnt really pay attention but I didnt notice a huge change in noise level.... but maybe I am just deaf.[X(]

TimC

Gringo Flyer, the muffer that came with my .76 is the Jett Lite. It's hardly any muffler at all. I changed to the Jett muffler for actual flying. I don't think a bored and stroked .40 size engine will run with a .90. Peak rpm is not what this engine is all about anyway. I just posted to share with anyone curious.

Gringo Flyer

My Feedback: (18)

I guess I showed my ignorance on that one. I was not aware that that engine was a bored out 40. If thats the case and its the same size and weight as a 40 its a monster.

TimC

Yes, it's got the guts of a .82 Saito (without the shake), and weighs a little less than an O.S. .46.

Gringo Flyer

My Feedback: (18)

I just took a look at the jett website and dont see that engine. I see the 60L which weighs about as much as a ttpro 46.

TimC

It's one of the BSE signature engines. Click on BSE Signature, scroll down and then click on .40-.76L.[link=http://www.jettengineering.com/]Jett Link[/link]

bob27s

My Feedback: (19)

Just for clarification, yes, the BSE-76L is the same size, mounting and appx weight of an OS.46 engine (similar to Saito 82 as well), so it is indeed a "40" size engine, but this engine is not a modified .40 of any kind - it was designed from the ground up as a .76L, meant to turn big props at lower rom (10K rpm target). It does so very well, using the untuned lite-jett muffler, and with a turbo-jett muffler you can get another 600-800 rpm on a 14x6. (mine turns an 14x4W at 11,500 peak).

Of noted, the engine was originally developed for CL stunt, per their new displacement rules. It worked so well there, that the timing and engine fit were further developed into the RC version shown here. The carb used is designed for the 3D/aerobatic/scale type applications - very linear throttle response.

The 60L is similar, and that too is available as a profile/3D engine, and known as a 60L "Hover" engine. In that case, the engine is indeed in the same crankcase as the Jett .40-.46-.50 engines.

Bob

MOJO65-9or10

Hey bob27, which engine is better suited to 40 3D stuff. The 76 or the 60L. They both seem aimed at the same market and I guess a better question is does one perform better in a 3D application or what. I really want to know whats the difference in flying one over the other in 40 size 3D stuff.

Just looking to figure them out. They SURE are SWEET engines for sure.
Dave

bob27s

My Feedback: (19)

Thanks for the good comments Dave.....

Let me see if I can summarize this...

The 60L has the "capability" for smaller props, and higher rpm. Essentially the same as what "can" become a 60LX engine - no rpm limit, huge prop usage range - so it is more versitile for just about any model you can imagine. Target rpm for the 60L is around 14,500 peak ground rpm - but runs well from 12,500 up to as high as 17,000 (standard SJ-60L configuration)

In fact, quite a few of the BPA and Senior Pattern guys have looked to the SJ-60L for power - since they have a 10cc engine limit, and the engine will be installed in planes that were originally design for ST53, ST60-G, Veco and Webra Blackhead engines. Those were all light-weight engines. The 60L can out-perform any of these, using the jett-stream muffler (longer 60L size, lower rpm) - with up to an 11x6 prop on it...... and it will not cause the plane to end up nose heavy (as it might if you use a modern, heavy, .61 size engine). For the same reasons, it also makes an outstanding scale engine - power to weight is hard to beat.

For 3D or profile stuff, it loves the 12.25x3.75, 11x5, 11x6 and 12x4 props. And it can use any of the mufflers - jett-stream, turbojett, and also produces great power using the lite-jett untuned muffler. For pilots more familiar with an OS46, TT46, or similar - it runs "the same way", just puts out some better top end power - and the throttle response is excellent.

The 76L as you have seen is a different animal. This is a brute. But it is designed to be rpm limited. This engine uses big displacment to its advantage, and is very at home with a non-tuned exhaust. Target here is around 10,000 to 11,000 rpm. Engine can easily run as a high as 13,500 rpm but we rarely recommend propping it to run over 12,000 rpm on the ground. So this engine is best for heavy 12" props, 13", and 14x4W and 14x6 props. Runs more along the lines of an (80-size) 4-c engine - just with 2c smoothness, and no serious concern of a top end rpm limit. Big midrange torque. Perhaps the big difference is this engine runs down in midrange best of all of them. Again, the power to weight here is incredible, and it can use any of the exhaust systems.... but I prefer to run this with the std equipment lite-jett for most applications.

(also consider for a moment, that many ST90 and OS91FX owners have reported top end with a 14x6 somewhere in the 10K rpm range too.... then remember this is a "46" sized small block)

I have attached a few photos SJ-60L applications. In any of these, sure, you could have used a BSE-76L, but that would have come down to preference, and the prop you would want to use. For instance, for the TopCat here, I think using the 60L with a 12" prop may produce better over-all handling and slow flight than if it were flying with the 14" prop.

I hope this sorta explains things

Bob

MOJO65-9or10

Thanks for taking the time to answer my questions (you always have fantastic answers). This just put the 60L on my wish list for after tax returnn time.

Thanks for the answer,
Dave

DarZeelon

Tim,


Altitude doesn't matter at all to RPM.

Lower air density will reduce the torque produced by the engine by exactly the same amount that it reduces the drag the prop's blades encounter.

Even though RPM is the same at higher altitudes, the horsepower is much lower.

The higher temperature will adversely affect the RPM, however.
------------------

Is Clevis a fox, by any chance???

TimC

Dar, what is the difference between thin air from high altitude and thin air from high temperature (If air density doesn't affect rpm's)?
Clevis is my Neanderthal neighbor who doesn't like loud engines.

DarZeelon

Tim,


I don't remember which law of thermodynamics states this, but power output is a function of the temperature difference between the intake and the exhaust.

The colder the air entering the engine and the hotter it is exhausted, the more power the engine makes.

The air resistance to the prop's turning is only a function of altitude density and the power is a function of both altitude density and temperature, so higher temperatures will reduce power beyond the prop's lower drag.

So RPM will go down on the same prop.

TimC

Dar, most of the big boilers I've worked on have an air pre-heater to heat the combustion air for better efficiency. How does this figure with the cold in/ hot out theory?

DarZeelon

Tim,


There are planes that have emergency power boosting devices, for 'hot and high' conditions.
These include water/methanol injection, on both turboprops (Swearingen Metro III) and many WWII carrier aircraft, with piston engines.

The point is 'hot and high'. I.e. hot weather (or high altitude) reduces the power of an engine to a point that it cannot come close to its intended performance, without those devices.

The Metro III also has JATO in the tail.


Performance is hurt by hot air, but if you look in a carburettor equipped car engine, the intake manifold is heated to aid fuel atomization. Maybe very cold air does not allow the engine to combust all its fuel, hence a lower efficiency.


I am sure you have flown in hot weather and have clearly experienced the performance deficiency... It is very clear to see your model flies better when it is colder.

Sobrinof

[/quote]I don't remember which law of thermodynamics states this, but power output is a function of the temperature difference between the intake and the exhaust.[/quote]

Carnot law says that the maximum efficiency for a thermodynamic machine is a function of the difference of the hot source and the cold source

In an engine, the hot source temperature is the maximum temperature that the fluid reaches inside an engine (1800 K aprox), not in the exhaust (700 - 800K) The cold source is the environment and its temperature is aprox. 290 K, hence, the maximum efficiency is aprox (1800-290)/1800 = 84%

This maximum efficiency is not reachable at all because mechanical limitations. (An engine needs to be cooler not to be melt, there are mechanical wear, and it is not possible to send the exhaust gases cooler that 600 K) for instance, hence, the max. real performance for an engine is aprox. 35% for a truck diesel engine, 25% for a gas car engine, and perhaps less than 15% for a model engine...



[/quote]The colder the air entering the engine and the hotter it is exhausted, the more power the engine makes.[/quote]

This is true, but mainly not because Carnot law. (of course Carnot law justify it because the cold source is lower, but not very much)

The cold temperature makes the air more dense. Hence, it is possible to introduce more air (more mass) to be burn into the cylinder

It is because you need aprox. 15 times more air than fuel to burn in the engine, the more air you are able to introduce into the engine, the more the power and vice versa. Of course you need more fuel in proportion to the mass of air you introduce. That is the reason you need to adjust the high speed needle when the wheather is very hot or very cold...

When the weather is cold, the output power is higher a vice versa... and the fuel consumption is higher too...

Greetings

DarZeelon

Sobrinof,


This is true, but only in theory.

All gasoline car engines, whether with a carburettor, or fuel injected, heat the incoming air to aid atomization of the fuel.
Without it the engine will not manage to make much power, or will need a very rich mixture to run consistently.

gkamysz

My Feedback: (19)

Maybe you have an example of this. I've never seen it in the way you are implying it functions, not at all in a fuel injected engine. Atomization is handled by the carburetor or injectors. Vaporization occurs naturally. Now, I've seen historic texts describing heated intake manifolds for kerosene or alcohol fueled spark ignition engines, but this is going back 100 years or more.

Air density is the same whether it is due to temperature or pressure(altitude). Sobrinof is correct. I would say that two stroke glow engines are typically below 10% efficiency though.

DarZeelon

Greg,


Take a look at this photo.

This is an Edelbrock Performerâ„¢ manifold.
The red arrows point to the ports of the exhaust crossover, which is under the floor of the manifold, below the carburettor.

Inside the manifold there are 'fins' to dissipate the exhaust heat into the intake charge.


Of course there are higher performance items that keep the intake charge cool, but they are intended for applications, for which idling and smooth low-throttle performance are not a necessity...

Sobrinof

Hello Dar

Probably, this is done to assure that the mix gas-air is well done in order to burn the mixture completely in the engine. When doing this, you reduce the efficiency of the engine because of carnot law and reduce also the power output because you introduce less air into the engine as i said before, but surely, the designer of the engine, when balancing both effects, though this was the best solution


For instance. This solution will never been adapted in a diesel engine. The reason: mixture is made inside the cylinder, not outside

Diesel engines use to install intercoolers between air intake and the cylinders. Reasons for doing this: to reduce air temperature and when increasing its density it is possible to burn more fuel with the same engine size. Also the famous carnot law (you reduce the temperature of the cold source), but this effect is less important...

The intercooler is almost mandatory when talking about engines with turbocompressor because air temperature is increased highly and if the air is not cooler, the power output will be significativaly reduced...