What size prop should I use to test the .28 engines?

I recall reading on the RB Concept forums that Rody Roem uses a wood 7X4 or 7X6 for break-in purposes, but IMO testing an R/C car engine on a bench with a prop is not going to give you much useful data.

The best "Dyno" is the car itself...just drive it and see which one gives you more [X(][8D]

Ya, it would be interesting to see the load matched real close to the power at hand, like with the tractor pulls. It seems like they used to be on TV all the time and I haven't seen it the last few years.

I dont think this is going to work all that great unless you find prop that works with the car pipe you will use. You will need to buy a few props for this. Once you have a controlled set up with pipe & prop combo you will be okay. But if your testing different engines each engine runs different on different pipes and props so theres going to be a lot of gaps left open. It will be hard to determine if the engine has less power or the prop pipe combination is mismatched.

I am not really interested in installing and setting up three engines in the same car. My intent is to create a controlled environment that creates a level playing field for all three engines. The only results I expect are to see which one pulls the hardest and the RPM. I want them to be subject to the same challenge. I realize I am not going to be able to figure out HP.

It’s really hard to look at something and say “hey, my car is going about 50 mph”. I do think that many people do that, but I like data thats why I use a GPS. You know what they say “without data it’s just another opinion. The forums are full of “my engine is better than yours” And then we have companies with their so called "Clamed Horsepower rating" and their fancy packaging to further confuse everyone. I think my project would some things to rest. But then again I am sure someone will put a spin on it.

Let me ask, how do you gauge your performance upgrades, pipe, new carb or whatever?

If you can somehow figure out how to match the typical RC car load with an airplane prop, you will have it solved, plus the prop will help keep the engine cool while the engine is wailing away. Judging from the claimed rpm figures of car engines.......you will probably want to run a 5x3 prop on your .28

To determine changes in engine would be to use a tach. That may work out easier. Whatever combo allows the prop to spin faster would be an indication the upgrade was going in right direction. I swapped out carbs before in engines and noticed an increase in rpms after the tach was put on it.

Or you can do what i did with a car before. As crazy as it sounds those mobile police radar trailers things you see on side of road work. I drove my car against it and it picked up the cars speed. It was pretty consistent also. That would work out good if you happen to see one. Then try out the different parts.

Another tip off when working with small engines is that any move that forces the opening of the needle valve [in order to maintain the right mixture] is a good thing.

combatpigg was only joking when he said to use a 5x3 prop.

What you want is a prop that will load the engine typical to a airplane engine of the same size. Use the same prop on all three engines, to give you some idea of the differences between them. I suggest you use a 9x6 wood prop to start. If your engine is not getting enough rpm then step down to a 8x6, then maybe a 7x6. Be sure the prop is thick at the hub, so that it doesn't fly apart with high rpm. NEVER stand in front of the prop when the motor is at full power and don't stand inline with the prop arc either.

BTW, there was a 'motor break in' stand marketed not to long ago by Hudy/Serpent that was exactly what you are talking about here with the prop on a car engine.

http://www.twf8.ws/new/tech/engine/b...hudybench.html

Here is one like it:

http://rc-mushroom.com/product_info....oducts_id=4531

KMOT, I don't think a .28 car engine will have the right timing to turn a same sized airplane prop. I think car timing has the exhaust leaving too early for the engine to have the torque for a relatively big and constant load like a 8x4 prop [for example]. Car engines have their torque multyplied through the gear train, so that is why those engines are set up for ungodly rpm [at the expense of torque]. I had a car with a Picco .21, it sounded and revved just like a little Indy car....too much for me, you could only count to one as it sped away before it was time to turn around.

Kmot,

Will theses work indoor? It appears they work with sunlight?

Thanks!

pigg: The only 5x3 prop I know of is for Cox .049's so that is why I thought you were kidding. But even though a regular size prop might not allow the engine to achieve full rpm, it still would be indicative of the potential and also would show any differences between the three engines, imho. Also, most cars/trucks/buggies/truggies are not at WOT all the time. Mid-range power is usually more important if you are driving on a circuit.

ExtremeNitro: Do not run a nitro engine indoors. You can asphyxiate yourself.

There might not be too much left of a Cox 5x3 after boring a 1/4" hole in it. The class of engine in question will have a target rpm range to aim for. If the load allows the engines being tested to operate in that range, the testing should bear some fruit. There is a difference between max dyno results and best driveability down the track. If the shoe was on the other foot, would you want to test some Jett vs Nelson Q40 engines while they were mounted in RC monster trucks?

I have an exhaust system that goes to the pipe and another for the room, but thanks for looking out! Do I need sunlight for the tach to work?

Yep...that or a flashlight...as in DC / battery operated. AC indoor lights can "fool" the optic sensor.

If I may for a moment.....

Thrust only tells you static performance of an engine/prop combination. In a very indirect way you measure rpm.

You want to know what the engine is doing, and how it performs under a defined load.

Power of the engine known by knowing load and the rpm it turns that load.

An easy known load is a prop. APC props in particular are good because they are fairly consistant blade to blade
For your tests, you would actually use the very same prop.

An array of say 5 different size props might allow you to plot a performance curve based on load and rpm.

Put the engine on a solid test stand.
Pick a prop. Run the engine. Measure the rpm that prop turns on that engine, and what it also turns on your sample of engines.
The closer together you can complete the tests, the less temperature/weather will come in to play.

Exhaust system may come into play, and may sometimes be considered a variable.

Keep in mind, for a car engine, it is designed for 20K+ rpm and higher operation. Has little torque in its native state. So you will have to select sample props that allow the engine to get up to its usual operating rpm, or desired rpm range.

Also to consider here....
As aero guys, we are usually dealing with steady state, full throttle measurements. Cars rarely run at full throttle or max RPM for extended periods of time.

For a car, boat or even helicopter, engine acceleration is something that is often important. That is why the aero tuned pipes are designed differently than a car or heli pipe. Boats do tend to run WOT quit a bit, so the marine pipes are more similar to aero pipes.

What I might suggest, is looking into one of the Eagle Tree data systems. Combine that with your test stand and prop testing. Hands-free measurements and detailed recording of everything you want to know on engine performance.

They make great data collection systems for aircraft, cars and boats that are good in engine rpm and acceleration plots. You can also compair it against throttle position to see throttle response.

You can measure rpm, and plot that rpm vs time (acceleration) - and get a reference on torque and throttle response.

Worth noting, these will not be concrete HP or torque numbers, but if you are as an experiement evaluating and compairing the capabilities of a number of engines, you will get good, real-world, compairable, and consistant information.

Bob

bob27s,

Thanks for your input! You make a lot of good points.

what bob said. What he didnt mention is how you get hp from how much torque you have. If you could somehow measure the torque on your engine you would use this formula to get your horsepower number. example: If you had a engine that produced 0.88ft lbs of torque at 15,000rpms. You would take those two numbers and multiply them together and then divide that by 5252. this will give you your horsepower number.

.88 times 15000/5252=2.5hp

If you are curious this will work for any type of engine