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Old 01-11-2009, 10:37 AM
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sprocket sprocket is offline
Motorized Bicycle Newbie
Join Date: Nov 2008
Location: Amherst MA
Posts: 5
Default Re: Scratches in the bore

Originally Posted by Creative Engineering View Post
A picture is worth a thousand words...

Hi there, nice picture. Since you took the crankcases apart you could probably know what of the distance between the conrod centers. I needed that number to make some calculations. I estimated it to be about 88mm I think. I bought some computer software for tuning 2 stroke RC boats engine and I'll use it to do some mods to my HT engine. First thing I noticed when I took the cylinder apart were those superficial scratches on the bore, on not so superficial, you can feel it with your nail, I guess they leave debris inside the engine when the put it together.
So I noticed 3 areas that can be improved a lot in these engines, one is course compression ratio, I calculated mine to be 5.4:1, pathetic. The other thing is the blowdown time, i.e. the time in crankshaft degrees the exhaust remains open before the transfer ports start to open. A good figure for this is 30 degrees. I measured mine to be about 13 degrees only, that is pretty bad, an this value is as critic as compression ratio on a 2-stroke engine. If the pressure at the cylinder is above the pressure at the crankcase when the transfer ports open, then burned gas flows from the cylinder to the crankcase trough the transfer ports, instead of fresh gas flowing to the cylinder in the oposite direction, not good.
The other thing is the time in degrees the induction port remains open. I measure mine to remainopen for only about 75 degrees, that's also pretty bad, it can easily doubled or more. I've seen people dremel about 2mm of the bottom of the piston skirt on the induction side to increase this opening time. Acording to my calculations we could dremel a 12mm window on the bottom of the piston on the induction side, and still be on the safe side.
So my cylinder has 2-3 mm of piston clearance on the top, so what I plan to do is lower the cylinder 2mm, by removing material from the bottom of the cylinder of the top of the crankcases. That will remove about 3.5 cc of the combustion chamber and boost compression, and then by lifting the exhaust port about 5mm I will get the right blowdown timing, plus the window dremel on the piston skirt, that should improve the engine performance a lot. Also I noticed some metal protruding from the tranafer ports where thet meet the cylinder sleeve, very awful, all that needs to be smoothed.
I;ll let you know how it works ! (if it works ! hehe)
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