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Old 12-16-2014, 10:22 PM   #1
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Default Notched-piston mod?

I read in some tuning guide somewhere on the internet that one "common" mod for nitro engines is to notch the piston where it overlaps with the boost port. The idea is this will supposedly create a smoother surface for the intake charge to flow over, and also increase the effective size of the boost port because the piston won't be blocking as much of it.

However, this mod would also allow the boost port to open earlier, since the edge of the piston that's supposed to cover the boost port will have been moved down. Taken to an extreme (i.e. if you notch the piston so far down that the entire boost port is uncovered at bottom-dead-center), you could cause the boost port to open at the same time as the exhaust port, or possibly even open before the exhaust port.

I can't think of any scenario where the engine's performance would be improved by opening the boost port before the excess exhaust pressure has had an opportunity to vent, because it seems like that trapped pressure would just stall the flow of intake charge up through the crankcase. However, perhaps there is something I'm missing, like...maybe that problem doesn't really exist, because the boost port won't just open earlier, it will also stay open later...or something like that. I honestly have no idea.

So I guess what I'm asking is:

1) Does notching the piston even slightly destroy its ability to form a good seal in the pinch zone? Or would it be okay as long as the piston is notched prior to break-in, so the wear-pattern inside the sleeve would match the shape of the notched piston?

2) What is the actual effect of changing the timing of the boost port such that it opens earlier and closes later? How would it affect the engine's torque at low RPM vs. at high RPM?

If you want to link me to a pre-written port-timing guide instead of typing answers to these questions, that's perfectly fine; I wouldn't mind reading a comprehensive guide, I just can't seem to find one. Pretty much everyone just says "leave it to the pros", and that's cool, but I still want to understand what the pros are doing.
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Old 12-17-2014, 12:51 AM   #2
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http://www.amrca.com/tech/tuners.pdf
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Old 12-17-2014, 04:20 PM   #3
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Cool, thanks. I figured there had to be a guide like that somewhere, but judging by the original publication date (1973), it's not too hard to figure out why I couldn't find the information easily online.
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Old 12-22-2014, 04:37 PM   #4
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The section related to my question starts at Page 115 of the PDF you linked. I had forgotten all about the old-fashioned 2-stroke engines that had an upward ramp build into the piston crown, to direct the incoming fuel/air up into the top of the cylinder. Now that I think about it, designing the boost port on a Schneurle-type engine (like our nitro engines) so the edge of the piston partially obscures the boost port at bottom-dead-center means that the edge of the piston provides extra upward redirection for the incoming fuel/air, effectively combining the benefits of the Schneurle design and the ramped design, while avoiding the inherent efficiency problems of the ramped design. So it would seem that notching the piston would reduce its ability to direct the boost-port flow upward, and that loss of functionality would only be offset by the increase in exposed port area if the engine was badly designed to start with -- that is, if the transfer ports are WAY too small. In that case you'd be better-off just buying a better engine, unless you have the tools to widen the transfer ports without damaging the chrome or nickel plating (i.e. NOT a Dremel).

It also states something that I *had* suspected might be the case: because the intake charge is only slightly compressed by the downward-traveling piston while it's waiting in the crankcase, the pressure through the transfer ports is fairly low, so the exhaust port MUST open well in advance of the transfer ports, otherwise the pressure in the transfer ports will be totally overwhelmed by the exhaust pressure inside the cylinder. So notching the piston more than a tiny bit will *also* cause the boost port to open too soon, possibly allowing exhaust pressure to force its way down into the crankcase, blocking the flow of fresh fuel/air through the boost port. This may have the side effect of increasing the pressure flowing in through the Schneurle ports, but they won't work right if the boost port can't do its job and clear the remaining exhaust out of the upper part of the cylinder. Not to mention, the fuel/air coming in through the Schneurle ports will be contaminated with exhaust, meaning there will be less fuel and oxygen to burn in the next combustion cycle.

So basically, unless the engine is really badly designed, there doesn't seem to be any benefit to notching the piston at all, except to say you did it and the engine still runs. I'm going to file this one in the same category as nitro superchargers -- useful for bragging to people who don't understand how nitro engines work, but useless for improving performance.

- - -

On another note, I really enjoy the tone of older technical books like this one. It was written at a time when it was still okay for the author to sound more knowledgeable than the reader, and a little bit of "talking down" was perceived as making the material more accessible instead of being patronizing. Most "technical books for the layman" written in the past couple decades end up taking a "let's learn together!" approach, apparently to avoid offending the reader by suggesting the author already knows better than them.
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Old 12-22-2014, 05:17 PM   #5
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"Often, the most subtle changes yield very large differences not only in peak power, but in the shape of the entire power curve, and it is all too easy to deal an engine a considerable injury while performing some minor alteration with a steady hand and the best of intentions. In this respect, I think it most unfortunate that the two-stroke engine cannot be driven below a minimum level of operating efficiency by even the most awful butchery of its transfer ports, as an engine thus served will continue to run, and run fairly cleanly, after that kind of surgery.

How much better it would be if a serious departure from the optimum would produce a great fit of misfiring, or some other obvious evidence of distress. Unhappily, it will not, which means that an engine's transfer ports should be left strictly alone unless you have both the knowledge and the tools to make any modifications properly. The tools you will have to acquire on your own; knowledge is what I hope to provide with this book."


This is what I'm talking about, especially that last sentence. God, I love it. Someone who isn't afraid to sound like they know what they're talking about, but without feeling compelled to overcompensate and talk way too aggressively, like so many knowledgeable people on the internet do nowadays. Some days I think I was born a decade or two too late.
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Old 12-22-2014, 09:34 PM   #6
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oh so true
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Old 12-31-2014, 12:47 PM   #7
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Hi, this is my first post on this forum, after finding a link via google for tuning/modding nitro engines.
What a fantastic find the link above is, and a perfect example of how a 2 stroke motor is designed and how to get the best from it.
I look forward to finding more great reads on here, and if you guys have any other great resources for nitro modding/tuning, please share them
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Old 12-31-2014, 01:07 PM   #8
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Nice thsi will help in tuning my modded blaster quad that my son is going to be racing. good find. been kinda going off some info from friends. this will help greatly. and when and if i get back to nitro racing this will help also.
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