boon Posted October 15, 2012 Share Posted October 15, 2012 best bet if you wanna go faster is to get a bigger bike Link to comment Share on other sites More sharing options...
shambles1980 Posted October 15, 2012 Author Share Posted October 15, 2012 but id be wanting to do something similar to that too lol.. its not as much for the increase in speed / performance, its more of a pissing contest with my self lol. Link to comment Share on other sites More sharing options...
boon Posted October 16, 2012 Share Posted October 16, 2012 he he he, remember that power corrupts lol Link to comment Share on other sites More sharing options...
symonh2000 Posted October 17, 2012 Share Posted October 17, 2012 I was wondering this exact thing when I had my RXS, in fact the first picture of an exhaust port was taken by me, as I asked the same question on another forum. Most of you will probably know that a 2 stroke engine uses reflected exhaust pulses to keep the fuel charge in the cylinder (Hence why they use expansion chambers). Removing the step will change how these pulses arrive back at the cylinder. Removing the step might make an improvement, it might make matters worse. It could also be that to get the best affect from removing the step that you need an expansion type exhaust (Micron, Allspeed etc) as the original isn't a true expansion chamber type. Secondhand cylinders are easy enough to come by, maybe buy a couple and do some experimenting? The RXS is in a fairly mild state of tune, and I have heard that 17bhp is quite easy to achieve, if you are prepared to loose some low rev tractability. The book by John Robinson "2 Stroke Tuning" is a good read if you want a better idea of how 2 stroke engines work. Simon. Link to comment Share on other sites More sharing options...
shambles1980 Posted October 17, 2012 Author Share Posted October 17, 2012 lol.. but i take it you didnt do it? the way im Guessing it.. is that stupid lip thing will trap some exhaust fumes so at say 4th gear i dont be able to achieve the full amount of revs the engine is capable of. but i really dont want to make it impossible to start the bike or make low revs/mid revs useless.. i also dont want to get a new cylinder take 1mm off re build it test it then take another 1mm off and repeat until i find i can totally remove it. or that i ruined the spare head.. but Im guessing that would be the best way for me to test it.. i dont think i could get 17Bhp out of the engine. im not saying others cant. im just saying i dont think i can lol. but 1 or maybe 2 hp may be something i can manage with some tinkering, but i really had hoped some one would have known the answers lol.. i had a look at some 115 and 135 heads, and they also have these lip things but they seem to get smaller the bigger the cc atleast they look to be smaller with the pics i can see. i may have a look at the book you mentioned but i feel that it will be more directed at the actuall ports where as to me "atleast" this lip looks more like a power valve that is just stuck in the "idle" possition permenantly. and from that i have to assume it Must have some function at the lower/mid rpm otherwize why would they have power valves on other bikes if it would function properly fully open through out all the range. Link to comment Share on other sites More sharing options...
symonh2000 Posted October 17, 2012 Share Posted October 17, 2012 I didn't modify it in the end. The spare barrel I had (The one in the picture) needed a rebore, and I figured it would be cheaper to sell the RXS and buy a 125 that could be de-restricted. If you look at the inlet on the RXS it also looks quite restrictive. There is a big flat area in front of where the reed valve fits, which could possibly be sharpened up. I would spend £120 or so and send the barrel to Stan Stephens or Mick Abbey, they have been tuning 2 strokes for years, and will give you a good result, if you mess yourself chances are you will make the engine worse. The engine as standard is designed to produce a set amount of power, and be reliable and tractable. You could get more power I am sure if you are willing to sacrifice the other two. I think it is possible to get a barrel and piston from an Asian market RX-115 and fit it to the RXS, but I am pretty sure the 135 uses a longer stroke so the crank is different along with the engine cases. Link to comment Share on other sites More sharing options...
shambles1980 Posted October 17, 2012 Author Share Posted October 17, 2012 but then it would be a 115cc lol.. the inlet dosent really bother me. and i think that is probably adequate for the ammount of fule that its going to be using. and i think id get more gains from changingto fibre reed valves rather than to mess with that. infact none of the actuall ports in the cylinder worry me at all. "atleast not for now" that lip however really does look like its just there to restrict the engine. in my mind it could mess with the mixture at higer revs leaving exhaust gasses in the cylinder reducing the effectiveness of the engine. but its a 2 stroke. and although they are really simple things. they dont tollorate much, things you could get away with on a 4 stroke. a 2 stroke will probably just flat out tell you to screw your self. so maybe im a coward. maybe i actually have a little bit more common sense than i thought i had. But i wont be messing with it unless i get a definate answer from some one. preferably with a vid of it working as proof they arent just messing with me lol Link to comment Share on other sites More sharing options...
symonh2000 Posted October 17, 2012 Share Posted October 17, 2012 Indeed it would be 115cc, and there isn't any replacement for displacement. 115cc is around 15% more capacity, so all things being equal should give 15% more power and torque. I think to get the best from the exhaust port mod you would also need to modify the inlet to get more fuel in there in the first place. As long as you don't raise the top edge of the exhaust port or the angles of the transfer ports then you shouldn't loose too much torque. Removing the step in the exhaust port will not change the compression ratio as long as the top edge of the exhaust port where it meets the cylinder stays the same. Link to comment Share on other sites More sharing options...
Moderator Cynic Posted October 17, 2012 Moderator Share Posted October 17, 2012 Dont forget there is a part of the cycle where gas goes the other way. Although breifly that step may be there to smooth the flow back into the engine. Link to comment Share on other sites More sharing options...
shambles1980 Posted October 18, 2012 Author Share Posted October 18, 2012 i just cant understand the lip lol.. gasses can be compressed. if it was for a liquid i could try to explain it away.. what are the chances that its just some stupid part of mfr process that they then said "oh crap.. that wasnt ment to be there.. but what the hell it still works" having said that.. the actual port in the cylinder is smaller than the bore of the output for the exhaust. but then theres a stupid lip.. so i dont understand what the hell its there for other than restricting the escape of gasses. Link to comment Share on other sites More sharing options...
symonh2000 Posted October 18, 2012 Share Posted October 18, 2012 Did you do a port map or are you just directly measuring? As due to the curvatuure of the bore the port will be larger than it appears when measured with a vernier etc. Basically roll up some paper stick it down the bore and use a crayon/pencil to take an outline of the port. Then unravel the paper, this will give you an accurate measurement of the port area. If the port is smaller at its outlet than the port area in the cylinder it will be restrictive, if not then the step is probably there for another reason, possibly to interrupt exhaust pulses and give more midrange torque. Link to comment Share on other sites More sharing options...
Recommended Posts