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Old 08-07-2015, 10:33 AM
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Davezilla Davezilla is offline
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Join Date: Mar 2014
Location: San Antonio Texas
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Default Re: my journey begins

think of port timing and porting like putting a hot cam in a 4 stroke engine... it pretty much does the same thing, the hight of the exhaust and transfer roofs controls the duration and the width of the port would equate to the lift of a cam in a 4 stroke engine.
There are a different set of rules for duration and timing on a 2 stroke, but for the most part, they work the same way and the whole idea is to get the biggest bang to push that piston down and make power. The problem is that one can't just start cutting and enlarging the ports and expect more power, this tactic does work if the porter don't get too greedy, or just gets lucky somehow, but to squeeze the most power out of these (or any 2 stroke for that matter) we need the exhaust port to start to open at exactly the right time, then get the exhaust out of the cylinder before the transfers start to open. Then down on the other side we need to get the fresh air/fuel charge into the case and get as much of it in there as possible for each cycle so we want to lower the floor on the intake port so the enging can pull in the fresh sharge as soon as possible and keep pulling it in as the piston goes up. The problem here tho is that the piston has to come back down and will push this charge back into the intake and carb, and even out the carb. This is referred to as Blow back and blow back needs to be avoided or low rpm power will suffer, at higher rpm, the air/fuel charge has enough momentum to keep going the right direction as the piston returns down and closes the port so we definitely can't get too greedy with the intake duration on a piston port 2 stroke. The ideal situation here is to pull in the fresh charge and pull in enough that most of it stays in the case when the piston comes back down before the intake port can be closed (This is why reeds make a lot better low end power). The intake port also needs to be closed so the pressure inside the case builds up enough to force the charge up the transfer tunnels and into the cylinder as the piston heads toward the bottom of the cylinder.
The idea of port timing is to optimise when each port opens and closes in order to get the most fresh charge into the cylinder and get the exhaust out before the transfers start to open as well as get the intake closed as quick as possible as the piston returns down, but leaving it open as long as possible while it goes up, we do have to rely on the air/fuel charge's momentum to keep this happening and it will happen at a certain rpm range, but we still need to be conservative to a certain point so the engine will run decently out of this rpm range.
There's a lot of different details that go into doing a good port job and knowing what effects doing each part of the job does will really help in making the engine run the way you want it to whtther you want a street engine with some mild upgrades to get the best top speed without sacrificing too much acceleration or hill clim ability or you want an all out race engine that'll only perform well in a certain rpm range, but will have the most power, but the throttle becomes much more like an on/off switch. Knowing the effects of each step in the job will give you the control of how it's going to run.
It would easily take over 100 pages of explanation to explain everything, but there are some good books you can usually find online for free like the 2 stroke performance books by Graham Bell or Gordon Jennings, if you can download a copy of The 2 Stroke Tuner's Guide by Gordon Jennings, this is a really good start. There are also some good articles and discussions at that can be very helpful as well. The more you can read and learn about this, the easier it will be to design and build up one of these engines for excellent street performance
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