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Discussion Starter #1
hi all due to my exhaust port being quite large and high i lowered my cylinder 0.5mm approx to try gain some bottom end power this means that my piston sits just above the transfer ports at bdc and also the bottom of the exhaust,will this make it run worse and undo any the gain im trying to achieve by lowering the cylinder thanks paul
 

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Not only are you changing the port heights and blocking flow from the ports, you are also changing squish and compression. Unless you are also changing the head, you might have some issues with piston to combustion chamber clearance.

Maybe a pipe, intake and/or head mod is more appropriate. Tell us about your setup and type of riding you are doing.
 

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Discussion Starter #3
hi there thanks for reply yeah i am going to modify the head to suit lowering the cylinder as yes my squish is to tight but also the squish is not starting from the edge of my bore which i have spoke to carlos in previous post,but before i do work on the head im just wondering weather there is much benifit to me lowering the cylinder and altering the port timing to gain some better low end grunt ,i race here in the uk at grasstrack which i guess is like your flat track ,i have stock cylinder 67.8mm bore ported for top end big exhaust port! so just trying to help midrange abit out of the corners cheers paul
 

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hi all due to my exhaust port being quite large and high i lowered my cylinder 0.5mm approx to try gain some bottom end power this means that my piston sits just above the transfer ports at bdc and also the bottom of the exhaust,will this make it run worse and undo any the gain im trying to achieve by lowering the cylinder thanks paul
Not ideal, however; if the cylinder is not ported or modified, lowering or raising ,y .5MM (.020") will not be really noticeableas long as you maintain the proper squish.

hi there thanks for reply yeah i am going to modify the head to suit lowering the cylinder as yes my squish is to tight but also the squish is not starting from the edge of my bore which i have spoke to carlos in previous post,but before i do work on the head im just wondering weather there is much benifit to me lowering the cylinder and altering the port timing to gain some better low end grunt ,i race here in the uk at grasstrack which i guess is like your flat track ,i have stock cylinder 67.8mm bore ported for top end big exhaust port! so just trying to help midrange abit out of the corners cheers paul
Stock cylinders are already de-tuned significantly, not much will change at .5MM movements up or down. Just maintain the proper squish as we had discussed in previous posts. Bottom line IMO, leave the cylinder alone, set the proper squish clearance. If you want TrQ; get it ported or port it, run higher compression 210PSI, High Octane Fuel like VP110 or C12.

Carlos
 

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Discussion Starter #5
thanks carlos for replying the cylinder is ported and i have already lowered it .5mm, just gotta get head squish sorted as we discussed ,yes ideally i want to bump up my compression as its 155psi at 1.5mm squish cant see how i can do it cos i want to stay on av gas and get it to 180/185 psi but i will have to have a smaller sqiush gap to obtain that so thats why i lowered the cylinder abit to try gain some torgue,kinda stuck with what igot really!i do have the cr250 head but thats gives it to, much compression but i could alter the cc of that to suit,i guess.cheers paul
 

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You can also increase compression by Decking the cylinder at Top the needed amount, and/or machining the dome accordingly.

Carlos
 

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Discussion Starter #7
yes could do that also thanks, going a bit back in this post carlos you point out that moving cylinder wont make much difference on a stock cylinder,but will it make much difference on a ported cylinder as mine is in moving it down to gain abit more torque thanks paul
 

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Talking about moving .5MM is a very small amount relative to port timing. IMO if you move up or down .5MM, ported or stock won't see much of anything as long as the squish is set correctly for either direction.

Carlos
 
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