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Post by joshpsmsc on Feb 6, 2008 18:43:34 GMT
I want to better learn how to tune smallies. When measuring port timings, do you start with 0 degrees at TDC or BDC? I often read numbers for exhaust and transfers, I just don't know where they start.
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Post by adspeed on Feb 6, 2008 21:11:04 GMT
I always start with 0 deg at TDC.
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Post by joshpsmsc on Feb 6, 2008 22:16:55 GMT
Ok. Next question...I assume that the measurements go clockwise...when I see an exhaust port number of 170, does that mean 170 degrees from TDC that the exhaust port starts to open or is all the way open? I should probably buy a book.  |
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Post by tony on Feb 6, 2008 22:26:23 GMT
its port duration... how long its open for.
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Post by joshpsmsc on Feb 6, 2008 22:31:50 GMT
So if I see 125/175, what does that mean? Does it matter when it opens or just how long it is open? I'm sure that they both matter.
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Post by tony on Feb 6, 2008 22:33:57 GMT
125/170 is transfer/exhaust. As they open either side of tdc the timing is symetrical from there.
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Post by joshpsmsc on Feb 6, 2008 22:52:20 GMT
 Do they open at the same time? I'm definitely confused. I understand that 125 is how long (125 degrees) the transfer is open and 175 is how long the exhaust is open. What I don't understand is when they start to open, when they are completely open, and when they are completely closed. I know that at 180, or BDC, the piston starts to go up again, closing the ports.
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Post by joshpsmsc on Feb 6, 2008 22:56:54 GMT
Okay, after some thought, would 175 mean that the exhaust port opens at 92 degrees from TDC? 125 would mean the transfer starts to open at 117 degrees from TDC?
How do I know what a good port duration is? How long can the exhaust port be open? Opening which port gives me more torque?
I have a shit ton of questions, without the english slang. ;D
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Post by tony on Feb 7, 2008 0:53:43 GMT
yes plus the .5! you get the idea now.
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Post by monza on Feb 7, 2008 3:00:09 GMT
Josh. Order "2-stroke Tuning" by A. Graham Bell from Amazon. I bought it for like $25. Good book that gives all the basics on port timing and how changes in the timing affect the output.
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Post by joshpsmsc on Feb 7, 2008 3:09:40 GMT
I just ordered that book today, from Amazon. Tony, I left out the .5 but understand the concept now. Thanks for your help.  |
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Post by adspeed on Feb 7, 2008 9:09:40 GMT
Josh I have seen people set the disk to 0 deg when say the exhaust port opens, this way the disk reads the actual degrees of rotation. eg for a 180 exhaust it would read 90 at BDC and 180 as the exhaust port closes. Not the way I do it cos you have to reset it every time you measure a port....and for ports over 180 it gets a bit messy....I prefer to keep everything symetrical.......just a thought! |
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Post by tony on Feb 7, 2008 23:48:44 GMT
i'm working on a variable height cylinder that sits on a bloody great spring..as i rev it up the barrel moves up to increase my power band. ;D
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Post by joshpsmsc on Feb 8, 2008 1:27:51 GMT
 I'd like to hear more about that!!
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ozzy
Automatica
Posts: 4
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Post by ozzy on Feb 8, 2008 11:48:06 GMT
tony, nice idea, how do you do this without wild changes in transfer duration??
ozzy
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Post by tony on Feb 8, 2008 12:14:35 GMT
;D well raising the exhaust say 8 mm will have say 20 degrees influence on the timing but only say 10 or 12 on trans due to the piston being in the "faster" part of its stroke near the ex port. The problem is having a variable squish band that gives me a wide squish at low rpm up to a narrow one at hi rpm as well as retaining its .8mm squish clearance! ;D ;D
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Post by jonnysnatchsniffer on Feb 9, 2008 12:54:20 GMT
so why does no one try and incorperate an exhaust power valve or is it old hat now ? it cant be too expensive as the tzr 170 kit is only about £350 with the power valve , so why dont they make 1 for scoots ?
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