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Old Jun 24, 2007, 09:10 AM
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2-Stroke porting/timing for torque?

I was reading a thread about a 2 stroke versus a 4 stroke and it occurred to me that if the porting, or timing, of a 2 stroke could be changed, that maybe it's maximum torque range could be lowered to an rpm comparable to the 4 stroke, and thus swing the same prop as the 4 stroke. ..... MAYBE .....

Anyone know of a site that has information on modifying a 2 stroke?
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Old Jun 24, 2007, 10:42 AM
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I don't know of any site that gives that information but there are many 2 strokes designed to give high torque at low revs. These are the engines used in CL stunt where we swing large props at quite low revs, usually around 8000 and in a 4 stroke mode. This type of engine is gradually creeping in to RC because it's the same kind of performance needed for 3D flying where large props are used for high thrust.

The main difference is that the exhaust port timing is lower than what's usual for RC engines. This shows up in the ST G51 where the RC version has the exhaust opening at about 74 degrees BBDC while the CL stunt version uses a different liner with a 70 degree exhaust. Apart from the obvious difference of a venturi, this is the only change to the engine.

There are guys who modify existing engines to give very high torque. One supplies an OS 35 (lapped piston) that can turn a 13" prop quite happily all day long.
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Old Jun 24, 2007, 12:43 PM
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Are there any gearboxes around for nitros? You'd get more thrust from a high reving higher HP 2 stroke geared down with a big prop than a low HP 4 stroke with the same prop.
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Old Jun 24, 2007, 12:52 PM
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Round Rock, Texas
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Quote:
Originally Posted by Ira NZ
Are there any gearboxes around for nitros? You'd get more thrust from a high reving higher HP 2 stroke geared down with a big prop than a low HP 4 stroke with the same prop.

Is that not what a low-pitch prop is all about? It allows for higher revs to get the engine into its power band. Jim
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Old Jun 24, 2007, 06:44 PM
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Everett Wa.
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DU,
As I understand it the lower exhaust ports were done to lower the blow down period. This results in an engine that has a very mild break from 4 stroking to 2 stroking and back down (eliminate run away). By the way this is also the philosophy for non-piped heli engines. This is a problem with some high performance engines that once on step the carb cannot modulate the power very well. (Gotta love those non-nickel liners)

Ira NZ,
Webra and OS both made gearboxs for their 10 cc engines. What a disaster, the same pinion tooth had to take the torque impulse every time the engine fired. The gears quickly wore out (broke).

Young gun,
A tuned pipe can be set up to act as a gearbox in that it can move the torque band all over the rpm range. It also can act as a governor or super charger.

What I have found is that the closing point of the crankcase intake valve controls the point where the engine developed its power. The sleeve timing controls the shape of this curve. (Very general statement here).

Low pitch to diameter props are very noisy and are very inefficient (poor) at converting power to thrust. True they can allow an engine to operate at its design point. But they don't convert this new power to thrust very well but you now have more power to waste.

I would point those who are interested in this low speed high torque stuff to read about full wave resonant tuned pipes. Tuned pipes are the secret to the 2-cycle engine.

Oh and don't bother try to get a nickel OS to handle this power. It can only lead to frustration

Friends don't let friends fly nickel,
Konrad
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Last edited by Konrad; Jun 24, 2007 at 08:20 PM.
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Old Jun 24, 2007, 07:23 PM
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It sounds like that IF you could lower the cylinder ports in relation to the piston top and head, that you could get the lower speed high torque.

Is that about right?

If that is right, any idea of how much lower?
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Old Jun 24, 2007, 08:17 PM
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Everett Wa.
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Nope.

Younggun,
What is your background in engines? The question you ask has filled many a library and corporations have spent countless man-hours on.

Friends don't let friends fly nickel,
Konrad
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Old Jun 24, 2007, 08:47 PM
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Here is some skinny on a MOKI .61 and the differences between it and its predecessor. I got this from the Just engines site. Just an example of what you're discussing. I have a MOKI .61 LongStroke I've had for several years, it is now available again.

MOKI 61 Bore and stroke==24x22 MOKI .61 LongStroke 23x24
11x7 prop rpm =12,900 rpm==12,800
11x8 rpm=11,450 rpm=12,100
12x8 rpm==9,950 rpm=10,300
14x6 rpm=8,400 rpm=9,000
The sleeve utilizes conventional Schnuerle plus boost port scavenging with a single transfer port on either side of the divided exhaust port and a single boost port directly opposite the exhaust. The exhaust was timed to open 74' before BDC and close 74' after B DC for an open duration of 148' and the transfer and boost ports timed to open 64' before BDC and close 64' after BDC for a total duration of 128'. Although the exhaust timing remains the same as the earlier engine, the duration of the transfer and boost ports has been lengthened by 12'; another contributing factor to the engine's increase in power
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Old Jun 24, 2007, 09:13 PM
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Everett Wa.
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If you look at the power curve of engines that have been long stroked (timing stays the same) you will see that the whole power curve is shifted up and that the peak power happens at a HIGHER rpm than the short stroke engine. Yes there is more power at the lower rpm band than the short stroke engine. This is because the cylinder ports are bigger for the same degree readings. Short stroke engines are used to keep piston speeds down (lower wear) and to try to keep the rod in one piece. The both of these reason is why OS went back to the short stroke engine. (Great Pains was being charged too much money to haul off all those SF engines (aluminum scrap) to the dump.

Friend don't let friends fly nickel,
Konrad
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Old Jun 24, 2007, 09:18 PM
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Part of the article I left out said that the rod length was increased by .120 inches in the LS version, however the engine is only .080 taller. They simply moved the wrist pin higher up and lengthened the rod to reduce rod angularity. A good move to reduce side thrust.
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Old Jun 24, 2007, 09:50 PM
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Australia, NSW, Cherrybrook
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I have a question that may or may not be applicable to this thread.

Enya have just released new .61 CX (ABN) motor in a 40 sized case. The stroke is the same as the 40 and 50 variants and the weight is actually lighter than the .50 (13.5oz bare or 18oz with muffler).

A clubmate has warned me that boring the motor out will actually reduce the size of the transfer ports and could possible cause issues. I also read somewhere that the new motor should be considered for torque rather than rpms.

Any thoughts?
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Old Jun 24, 2007, 10:31 PM
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Everett Wa.
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NO NO
Not another arrow through my heart. Enya is using ABN junk. Enya used AAC on their high performance engine. Have the market forces force ENYA to abandon quality?

All engineering is compromise. Small ports are not issues if you want to keep the velocity (kinetic energy) of the intake gas high. Ports that keep the velocity of the charge high give better transfer at lower speeds. OPS had a 10cc engine that did both small ports but many of them to keep the total gas transfer high.


PLEASE
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Konrad
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Old Jun 24, 2007, 10:57 PM
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Adelaide, South Australia
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Quote:
Originally Posted by CustomPC
I have a question that may or may not be applicable to this thread.
Actually it's very much applicable .
An engine designed for high torque at low revs obviously isn't one of these high speed screamers designed more to give impressive HP at revs no one ever uses them at. One of the things needed to get these high revs is transfer passages large enough to handle the volume of air. When an engine like that is run at lower revs then the velocity of the air slows down and doesn't give decent scavenging which drops it's efficiency and therefore its torque. This is one reason why the LA's are quite good at making low down torque because they've had the boost port blocked off so now all the air has to pass through only two ports which speeds up the velocity.

So smaller transfer passages helps keep up velocity along with having less area (because of lower timing) in the liner ports. The lower exhaust timing also keeps combustion pressure on the piston for a longer period which adds to the available torque as well but gives less time for the exhaust to escape. This doesn't matter because running at lower revs increases the time for the exhaust to escape.

None of my CL stunt engines has an exhaust timing more than 70 degrees while most RC engines will be 75 or more. The longest blow-down of stunt engines is 12 degrees (and would you believe that's with a Fox 35?) while most RC engines will be around 15 degrees. So a small difference in cylinder timing makes a big difference.

The same isn't true though of crankshaft timings. I can't make head or tail out of them because they range all over the place. There are huge variations in both opening and closing times and also huge variations in the total open times. This extreme variation is true of both CL and RC engines.
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Old Jun 24, 2007, 11:02 PM
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Konrad, here's the link to the motor

http://www.enya-engine.com/61CX%20TN_E0.html

So, is it worth getting the new .61? It should power a 40-50 sized model nicely don't you think?

Please elaborate what's wrong with ABN (nickel) motors. I'm curious to know.
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Last edited by CustomPC; Jun 24, 2007 at 11:08 PM.
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Old Jun 24, 2007, 11:09 PM
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Adelaide, South Australia
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Quote:
Originally Posted by Konrad
NO NO
Not another arrow through my heart. Enya is using ABN junk. Enya used AAC on their high performance engine. Have the market forces force ENYA to abandon quality?
I didn't see this until after I'd posted my last one. Well you may be able to rest easy. Although the Enya web site says ABN a couple of people have had emails direct from Ken Enya who says that it's actually a ceramic liner. If so then this is a step in an ideal direction, even better than the AAC they've used and still use in some engines .

CustomPC
If you want to get an Enya then I'd seriously suggest you contact Kaz Minato at http://homepage3.nifty.com/bluemax/MNT001.html and send him an email from that page or directly at bluemax.minato@nifty.ne.jp because he's offering 20% off the factory price. I bought the remanufactured Enya 60-IIIB through him a few weeks ago and the price with muffler and shipping was only a few dollars more than the list price for just the engine. It took 4 days to get to Australia. He speaks perfect English and there were no hassles whatsoever. An email to him will get you a quick reply giving a quote then use the order form from his web site above. Don't forget to order a muffler with it. With the current exchange rate for Aussie dollars it's almost exactly 100:1 so if something is 15,000 yen then it's $150.
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Last edited by downunder; Jun 24, 2007 at 11:25 PM.
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