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Heads and Cylinders All about your porting, compression, rings, cylinder and piston modifications to your bicycles engine

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  #11  
Old 07-20-2016, 11:41 PM
StevenMain StevenMain is offline
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Default Re: My mods, and performance, how am I doing?

I'm addicted to learning, I'm always wanting to try and build new things. Two stroke engines were something i've never been inside, which is funny because I have plenty of automotive experience just sold my race car, a Turbocharged 2002 Trans Am with 1,000 rwhp to a gentleman in Qatar. I love to build.

I feel like I must be still laying alot out on the table though compared to some of these 50cc two strokes which make twice the power. I don't know where I can begin to look though since my two stroke knowlege is litterally nothing. My porting is what really made the thing scream and ill be honest. I was probably very lucky.

Id like to experiment with a 20% nitro blend RC fuel I have, but to run it I would need to figure out how to get precisely 4 times the amount of fuel in the engine for the same amount of air. I don't think my 20mm carb can be rejetted that big lol.
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  #12  
Old 07-21-2016, 03:36 PM
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sbest sbest is offline
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Default Re: My mods, and performance, how am I doing?

I'm with you on lifelong learning.
These engines are the best tool you could ever get to learn about 2 strokes.

I took a "one at a time" approach to mods.
Taking advantage of easy top end swap time, I stuck with stock components with slight mods to gauge the effects of each. Like you, I found opening up the port windows really helped. My goal was broad power and this little engine can do it from 500rpm to 10,000.

As for making BIG power, these engines need more window area to do it. The whole cylinder circumference under the the transfer ports needs to be open.

I'm playing with torroidal combustion chamber shape. It limits rpm somewhat but gives large low rpm torque. The hemi shape is still king for rpm.

Steve
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  #13  
Old 07-23-2016, 02:25 PM
StevenMain StevenMain is offline
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Default Re: My mods, and performance, how am I doing?

Im going to need you to stop being right about ****. I removed the muffler baffles, the bike ran slower. 38 mph it would pull like ****, then, almost as if it hit a rev limiter, stopped. I wonder if the Huffy Davidson has some sort of funky ignition curve to prevent the engine from going over x rpm. Which wouldn't make sense since I did already have it at 41 mph. Not sure a higher rpm is possible from this engine it just STOPS revving, stops from a decently strong pull no less.
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  #14  
Old 07-23-2016, 06:01 PM
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sbest sbest is offline
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Default Re: My mods, and performance, how am I doing?

Same experience here. Ported cylinder pulled strong and then quit like a rev limiter.
No more revs even on a downhill. It might be an ignition thing?

I did a bunch of exhaust experimenting this afternoon with stock cylinder 48cc and 66cc Grubees.
48cc (stock cylinder, converted back to single speed 56T) and 66cc (44t stock cylinder), both with modded heads.

I have a recently abandoned runway in my backyard.
Level but strong winds, saw speeds of 40-58 kph against the wind and then with it.
Interesting stuff, upwind/downwind, 48cc/66cc.
48cc is in a lightweight 10spd frame (with shifter frame still in place), 66cc in a heavy MTB and big tires.
stock pipe= 44/52, 44/57,
portmatched pipe= 46/52, 46/58
Straight pipe= 40/42, 42/44
Stock portmatched pipe, baffle removed= 42/48, 42/50

Not a lot of difference in power between the two stock engines as tested.

Steve
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  #15  
Old 07-23-2016, 10:35 PM
StevenMain StevenMain is offline
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Default Re: My mods, and performance, how am I doing?

looks like port matching is the best (maybe better scavenging?) but beyond that the restriction in the end is a good thing.
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  #16  
Old 07-24-2016, 02:13 PM
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Default Re: My mods, and performance, how am I doing?

I think the stock pipe is better tuned than we have been giving it credit.
Gordon Jennings talks a bit about the value of the stinger in tuning for top end.
The restriction of the stinger keeps the end gasses in the cylinder and not escaping out the pipe and tend to return higher speeds. To properly size your stinger, you keep making it smaller and smaller and watch the effect on top speed. So it is with the stock pipe on these China Girls. Not so much restriction as a tuned effect.

Port matching is simple on these pipes. Bolt the gasket to the pipe and plow a 3/4" drill into the flange at the same angle as the pipe. BANG! Portmatched!

Have been looking at the port measurements today. Looks like these cylinders are designed for a 42mm stroke. We can lop off some of the intake skirt for that side, but the exhaust port and transfers lower edge on most of these cylinders is 41-43mm down from the headgasket surface. I have bumped my basegasket up 1mm but can foresee raising it 3mm and shaving some off the top of the cylinder or making a recessed cylinder head. Might work on this today.

Steve
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  #17  
Old 07-24-2016, 03:17 PM
StevenMain StevenMain is offline
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Default Re: My mods, and performance, how am I doing?

For my components I bolted them to the cylinder, shot spray paint through the port, and unboltef and went to town with a dremel. The exhaust pipe isn't large enough diameter for a true port match on mine so I ran it out as close as possible. My pipe has a rectangular flange opening now.
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