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Old 01-30-2018, 10:32 AM
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DJ125 - Any results yet?

Hi All,

I'm curious about this engine. The new DJ125 is quite a pretty engine with piston ports, no reeds, and is being sold by MilehighRC at a reasonable price.

Has anyone used this engine yet, if anyone has any real world performance data, that would be great. The specs look really good.

Thanks





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Old 01-30-2018, 11:21 AM
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Lots of Zenoah influence in this design. After flying the 445 and GT80 Zenoahs for several years, I don't think I'd have much interest in this engine. For me, the reed valve engines are much more user friendly.
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Old 01-30-2018, 12:19 PM
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Don't have anything to offer Joe, but I like the idea of that intake design. Seems like we always get a slightly different mixture on one cylinder with reed valve induction...would be interesting to see if this design remedies that.

Interesting to note that the description on MileHigh's website says it is not intended for 3D.
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Old 01-30-2018, 03:09 PM
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How does that intake work? I understand reed valves and rotary valves, but not this one.
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Old 01-30-2018, 03:16 PM
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Quote:
Originally Posted by VinceY View Post
How does that intake work? I understand reed valves and rotary valves, but not this one.
Itís a piston port engine there are ports in the piston and as they move past the opening in the cylinder the fuel is drawn into the engine
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Old 01-30-2018, 03:23 PM
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This diagram from the Wallbro manual shows it.
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Old 01-30-2018, 03:27 PM
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For example, most chainsaw and trimmer engines are piston ported. Likewise most of the early rc engine brands were piston ported. Reed valve induction is much more popular today though the Zenoah line of piston ported engines continue to be very popular.
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Old 01-30-2018, 03:57 PM
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Piston ported engines are really reliable and are torquey. Piston ported engines have a strong intake pulse, because unlike reed valve engines, the intake pulse only occurs when the piston skirt clears the intake port.

Reed valve engines start their intake charge as soon as crankcase vacuum is great enough to suck through the reed.

Piston ported engines are great for things like chopsaws, generators, stationary engines, even chain saws when a constant throttle is applied. Reed engines are great for applications which require lots of variability on throttle, Dirt Bikes, Quads, etc.

Those are just general, but a properly ported piston ported engine can be made to perform really well. Hence I'm curious if anyone has had experience with these.
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Old 01-30-2018, 06:13 PM
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Both kinds of 2 stroke engines can be very decent engines. I'd have to believe the main reason for the popularity of piston ported engines is their reduced parts count. No reeds to get stuck open when a user leaves off an air filter and a wood chip gets sucked in.

I see where Mile High is recommending the DJ125 for warbird service. It could be a very good engine for that for those who want to hear an engine scream. The Zenoah twin was good for that kind of service.
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Old 01-31-2018, 09:23 AM
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Is the piston ported version louder?
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Old 01-31-2018, 09:26 AM
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Is the piston ported version louder?
No but they tend to develop their best power at a higher RPM than reed valve engines.
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Old 01-31-2018, 09:32 AM
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No but they tend to develop their best power at a higher RPM than reed valve engines.
This is where the magic comes in, it all depends on the port timing actually. If they mimicked the Zenoa's, then yes, it will want to scream.
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Old 01-31-2018, 11:34 AM
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I wouldn't expect a torquer design for a model engine. If it were a torquer, it would probably fail in the market place as most engines are compared using typical prop sizes at full throttle RPM's. If the engine favored a lower RPM range, it wouldn't measure up to expectations in the "public opinion" arena as it most likely wouldn't be compared to the competition in a fair "apples to apples" test.

Don't take me wrong here. I hope the DJ125 is a successful engine but I remain skeptical of the unusual design.
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Old 01-31-2018, 11:47 AM
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You are absolutely correct, public opinion = win or fail

The reality is, most of our model engines are a torquier design, since we do not rev them to the stratosphere. Most don't even know that Torque x RPM / 5252 = HP and the higher the rpm's torque peak (determined by port timing) = greater horsepower.

Most of our engines are designed to deliver power below 7K rpm, this serves many purposes, one of which is keep the prop tips quiet.

Back in the day when I worked for Suzuki, we used to get insane HP out of a little 125cc engine, but we revved them up to 22K rpm and port timed it accordingly. Even the little RM125 Dirt bike back in 1990 developed 38HP but we revved them to 11,250 RPM.

Our airplane engines are rated at 12+ hp at a much lower rpm.
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Old 01-31-2018, 11:59 AM
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+1

The only motorcycle (when I rode motorcycles) that ever really scared me was I believe a tiny little 50cc, liquid cooled dirt bike with a huge pipe hanging on it. At the time, I'd never ridden anything that came on like that thing did and wasn't expecting that kind of performance from such a tiny engine.
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