Author Topic: Port timing.  (Read 1810 times)

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Offline Al Smith

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Re: Port timing.
« Reply #10 on: February 15, 2013, 09:14:37 am »
Now free porting ,kinda deep .There is a phenominan termed resonant frequency which is used on tuned exhausts but also has been attempted on intake circuits .The only example I ever heard of that worked commercialy was one model of Villers motorcycle made in England probabely 50-60 years ago .I don't think they were totally successful or else they'd still be in business .

Theoretically what you have is at one small portion of the cycle where all three ports ,intake, transfer and exhaust are open drawing fuel directly from the carb .Complete purge of the incoming  charge .Coupled with a tuned exhaust it is a two cycle tuners degree of utopia .The theoretical  push pull circuit .Problem is at least so far nobody has been able to figure it out successfully because the RPM band it would work at is extremely narrow .Now that one is a bone to gnaw on .

Offline 3000 FPS

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Re: Port timing.
« Reply #11 on: February 15, 2013, 11:31:11 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?

I do not usually drop a cylinder that much to worry about the timing changing that much and so I do not change the timing usually.   If I am working on a open port transfer cylinder then I will shape the top of the transfer so that it is pointing more towards the intake without raising the transfer itself and affecting the blowdown.   This is done so that the charge coming up from the crankcase flows more toward the intake and hopefully allowing less of it out the exhaust while it is open.
PP 505, 475, 445.

Offline Al Smith

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Re: Port timing.
« Reply #12 on: February 15, 2013, 01:58:03 pm »
It comes down to theories and personal preferences as what method is used .One method might prove to be as effective as another one .

For example I use a domed piston to alter the flow around the intake side of the piston combined with a widening of the transfer ports as well as longer blow down to more completely purge the residue of unburned products of combustion .Some prefer to use finger ports and a flat top piston to achieve the same end results .Nobody is right or wrong here in their approach if the end results work .

Some prefer to cut the entire bottom out of tunneled transfers .I prefer widening them and doing some flow work on the piston windows .

Really about the only way you'll ever get half way good at it is to do it .If it doesn't work so good find a different method .Don't start out with  a thousand dollar saw to experiment with .get a 50 dollar special .Listen ,ask questions ,ignore the idiots that poke fun at you .Armchair quarter backs that usually don't have the common sense God gave a goose anyway .Hang long enough it will come around . ;)

Offline 3000 FPS

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Re: Port timing.
« Reply #13 on: February 15, 2013, 03:10:58 pm »
It comes down to theories and personal preferences as what method is used .One method might prove to be as effective as another one .

For example I use a domed piston to alter the flow around the intake side of the piston combined with a widening of the transfer ports as well as longer blow down to more completely purge the residue of unburned products of combustion .Some prefer to use finger ports and a flat top piston to achieve the same end results .Nobody is right or wrong here in their approach if the end results work .

Some prefer to cut the entire bottom out of tunneled transfers .I prefer widening them and doing some flow work on the piston windows .

Really about the only way you'll ever get half way good at it is to do it .If it doesn't work so good find a different method .Don't start out with  a thousand dollar saw to experiment with .get a 50 dollar special .Listen ,ask questions ,ignore the idiots that poke fun at you .Armchair quarter backs that usually don't have the common sense God gave a goose anyway .Hang long enough it will come around . ;)


  I like it.  Good Karma.
PP 505, 475, 445.

Offline srcarr52

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Re: Port timing.
« Reply #14 on: February 15, 2013, 04:41:24 pm »
I do not usually drop a cylinder that much to worry about the timing changing that much and so I do not change the timing usually.   If I am working on a open port transfer cylinder then I will shape the top of the transfer so that it is pointing more towards the intake without raising the transfer itself and affecting the blowdown.   This is done so that the charge coming up from the crankcase flows more toward the intake and hopefully allowing less of it out the exhaust while it is open.

I've made a lot of gains by changing my ways on the upper transfers.  It's all about creating a good wedge of new mix to push the spent gases out the exhaust without mixing much with the incoming charge.  That wedge has to push up the intake side wall and flush out the spent gases in the chamber before coming back down the exhaust side.  Direction of the inlet, paying special attention to where either side flows intersect, and staggering the ports from exhaust to inlet side opening and entrance angle can get you some real gains.  I find when this is working well running less blowdown than normal will gain you upper end power without killing the peak torque. 

Offline jmester

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Re: Port timing.
« Reply #15 on: February 15, 2013, 05:02:45 pm »
Thanks for all the great  insight. That helps me grasp it pretty well. I think I am going to work on doing a woodsport on a 028.  Having trouble getting a good read on the tranfers. Any one know what degree they are. If you mill a base down. Should you run a gasket or not. Thanks again for sharing your knowledge.
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Offline jmester

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Re: Port timing.
« Reply #16 on: February 15, 2013, 05:03:15 pm »
It is time to play !
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Offline Al Smith

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Re: Port timing.
« Reply #17 on: February 15, 2013, 05:15:00 pm »
As a rule on an 028 being the same design as the 038 the transfer should open at about 20 degrees of rotation behind the stock opening of the exhaust port .This is an educated guess on my part because I've never done any port work on that model .Got one in the shed though to do some day .

Give you a little hint .They make a tiny thin high intensity LED pencil thin flash light that  takes AAAA batteries .It's small enough you can peer right through the spark plug hole and with no problem see when the ports start to open .Best light I've ever found .

Offline jmester

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Re: Port timing.
« Reply #18 on: February 15, 2013, 05:36:45 pm »
I think to if I were to where my glasses that might help too, they are new to me, and I forget I have them. Does 101* for exhaust sound right. Who makes the little lite you made mention of. I know stylus made some small pen lights.
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Offline Al Smith

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Re: Port timing.
« Reply #19 on: February 15, 2013, 06:50:31 pm »
101 sounds right on the money referencing to 028 to an 038 AV.If that holds true the stock squish is probabley around 45-46 thou .If that's a fact you can safely drop the cylinder 20 thou or so .

If you are hesitant pull the base gasket which is around 20 thou and swear a coating of permatex number 2 which will seal it up enough to at least test it .Without raising the exhaust it should peak out slower but with a tad more torque on a test like that .

Don't get in a rush,take it a little at a time .You can always remove metal but it's a beech trying to put it back .

Another hint : if you only widen a port some what you have not changed the port timing although by dropping the cylinder without cutting the base you have .Now if you choose to cut the base and raise it back up to stock 101 you again have not changed the dynamics of the timing except you've increased the blow down duration  .If you raise it say 4 degrees you've effectively added 4 degrees more blow down more or less .

Remember though it's not totally liniar because of the angle of rotation geometrically speaking because of the position  of the crank throw with respect to the rod and piston position .Measure often and cut slow .If you have to slide the cylinder back on a half dozen times it's no big deal .Better safe than sorry .Rome wasn't built in a day and you can't report a saw in half an hour .

 

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