Author Topic: Port timing.  (Read 883 times)

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Online jmester

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Port timing.
« on: February 14, 2013, 10:05:39 pm »
When you raise or lower port timing what are to gains to the saw. I know that when you raise them you get more rpms. And if you lower them you get more torque. Can you raise the exhaust and lower the intake or vise versa. Any help would be appreciated.
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Offline 660magnum

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Re: Port timing.
« Reply #1 on: February 14, 2013, 10:10:11 pm »
Just widen the port
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Online jmester

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Re: Port timing.
« Reply #2 on: February 14, 2013, 10:21:26 pm »
I am trying to learn what happens. As a result of raising or lowering port timing weather intake,exhaust or transfers. I know that you can widen them and get more flow. Just trying to figure it out. So I can play around with some timing and porting
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Offline aclarke

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Re: Port timing.
« Reply #3 on: February 14, 2013, 10:40:28 pm »
changing the port timing will change the rpm range where the saw makes power.  Generally, the more duration the higher the peak rpm/power levels.  Timing (duration open) and port area requirements need to be met to produce a given level of power. you can change the aree, i.e widen ports, to a point to achieve the desired results, but ultimately the duration will have to be increased as the port width jeopardizes ring life after a certain point. 

Offline 3000 FPS

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Re: Port timing.
« Reply #4 on: February 14, 2013, 11:26:25 pm »
If you raise the exhaust port to get higher rpm  you cut down on the compression and you loose torque. 

 I would rather drop the cylinder alittle to increase the compression which will increase torque and widen the ports to help flow of the gases which will increase torque and muffler mod it and advance the timing by 3 to 4 degrees to increase torque.   Then I have a saw that does not lose as much rpm when in the cut because of all the torque.
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Online jmester

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Re: Port timing.
« Reply #5 on: February 15, 2013, 06:42:37 am »
That makes sense. If you mill the base to raise the compression, do you go back and make your porting numbers the same as before you milled it. Or is that the point to drop it down and get more comp and torque. I guess that is where I have to do some trail and error. Also what about the tranfers?
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Offline 660magnum

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Re: Port timing.
« Reply #6 on: February 15, 2013, 07:29:50 am »
When dropping cylinders, you must keep into context the piston skirt length and the possibility of free porting the engine.

Everything is a interrelational compromise.
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Online jmester

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Re: Port timing.
« Reply #7 on: February 15, 2013, 07:33:13 am »
So that there is no overlap in openings correct. Like the exhaust being open when the intake is open.
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Offline 660magnum

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Re: Port timing.
« Reply #8 on: February 15, 2013, 07:53:46 am »
Free porting is when the skirt of the piston rises above the bottom of the exhaust port allowing the crankcase cavity to vent to the atmosphere.

This is not considered a good trait but some engines can tolerate some mild free porting.
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Offline Al Smith

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Re: Port timing.
« Reply #9 on: February 15, 2013, 08:59:09 am »
Generally speaking if you raise the rpm range the port time area open will require some adjustment .In simple terms as the time deceases the area must increase because you can only shove a certain amount of fluid or gas  through a certain sized hole in a given amount of time with a given amount of pressure .

Another thing to keep in mind is with port loop scavenging on a two cycle engine is it starts and ends in atmospheric pressure .It's the differentials of pressures within that complete cycle that determines the flow within regarding pressure changes .--clear as mud --

 

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