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Old 07-22-2015, 09:57 PM
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Davezilla Davezilla is offline
Motorized Bicycle Elite Member
Join Date: Mar 2014
Location: San Antonio Texas
Posts: 2,710
Default Re: my journey begins

That exhaust port looks Nice!!
Good job... did you measure how wide it is in the cylinder and at the outlet?
That's one of the better looking ports ive seen in here for sure, just be sure to keep it 29mm or less to prevent any chances of ring snagging. Not sure if you read about the 60% rule, but it means that as long as you don't go more than 60% of the bore diameter you wont run the risk of snagging a ring. You can also raise the port roof by 1mm for more rpm,but use a spare piston and ramp it to see how it performs before raising the roof higher. Basically, ramping the piston does the same effect as raising the port roof so its a good way to experiment without going too far on a cylinder. Mine usually come out where I want them with the roof raised about 2mm but that's also really close to where it effects the low rpm torque more and more.
Ramping the piston at the transfers is the best way to get more transfer duration since raising the transfer roofs effects blowdown and will take away low rpm performance because of residual exhaust pressure can push the fresh intake charge back down the transfers if the roof is too high, this can be remedied to a certain extent by case stuffing but really good performance can be had without stuffing the case by keeping track of the blowdown and not going too excessive so ramping the piston at the transfer openings only is an easy alternative, more power can still be had by stuffing the case but as long as your not too greedy with the transfer duration you can get away with excellent performance and not have to stuff the cases to prevent the exhaust pressure from overcoming the transfer pressure. A nice wide exhaust port like what you did also helps bleed off this pressure well before the transfer ports are exposed.
You're definitely doing a good job with it so far. If you have a degree wheel and a dial indicator that'll help you set the cylinder at just the right hight for best port timing and duration when you get closer to ready to put the cylinder back onto the bottom end... I have a few posts in here showing how I made the degree wheel and mount it to the crank but we can go over that when you're ready.
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