To port or not to port??

[Carl Walichnowski]

I'm not sure how my fellow motor bicyclists think but for me I'm constantly thinking "What can I do now?". I'm new to the hobby but I like it very much, obviously the riding is the #1 perk involved with the hobby but to also see better performance, one MPH at a time, well that provides a lot of enjoyment also. I'm sure a majority of MBers can relate to the happiness obtained with a sense of accomplishment from improving the overall performance of their motor bikes.
Now I'm at a point where I'll be enjoying my MB as it is, once my next part arrives and is installed it will be about a month before I'm looking to buy my next performance part. This is what I have so far:
66cc Grubee Skyhawk Gta5
Sprocket Adapter
Chain Tensioner
Typical NGK Plug
Expansion Chamber
CNC 6cc Racing Head
Carb (No name clone)
So I'm asking myself the question "What's Next" once again. I've seriously been considering buying a extra cylinder & piston to port & polish it. I have no experience with doing any porting so I'd love to find some videos for beginners on porting but my search continues. (Can anyone provide a link or something?) I was just on another thread where a member said he polished his head, this may be a thing to add to my To Do list.
Questions:
1) I'd like to eventually buy a Reed Valve, would this be my next logical performance mod to buy?
2) Does porting the cylinder improve performance drastically? How many MPH would it add? Could I get away with simply porting the piston and polishing the cylinder??
3) So say I port & polish to the best of my ability or simply buy one that's already done for me and I have myself a Reed Valve, would I then need an improved CDI?
I've asked like 100 questions on this forum and I've ALWAYS received educated responses from the experienced members, to me that is awesome, I appreciate it guys!!

 

[Deleted member 12676]

just widen the ports to increase power but not change the top RPM.
don't widen the exhaust port more than 65% the bore. So .65 x 45 = 29mm
Instead of lowering the intake port you can shorten the pistons intake skirt which may even be preferable in this case because the skirt doesn't fully open the intake port when the piston is at the top of its stroke.
after raising ports you need to use a fine file at 10 degrees from the cylinder surface to bevel the sharp edge so the rings won't catch on it. One of those sandpaper-type nail files works good.

 

[Street Ryderz]

just widen the ports to increase power but not change the top RPM.
don't widen the exhaust port more than 65% the bore. So .65 x 45 = 29mm
Instead of lowering the intake port you can shorten the pistons intake skirt which may even be preferable in this case because the skirt doesn't fully open the intake port when the piston is at the top of its stroke.
after raising ports you need to use a fine file at 10 degrees from the cylinder surface to bevel the sharp edge so the rings won't catch on it. One of those sandpaper-type nail files works good.

The op's running a gt5a and there should be no skirt area blocking the port and the bore is 47.7 so you can go to 31mm width but doing this requires the flange area to also be enlarged up to 1.15 x the windows volume and then your header also needs to be much larger or you'll just over heat your engine!

 

[Deleted member 12676]

I don't know that the partial blockage of the port at TDC is important because its only as the piston is rising does it create the vacuum that creates the intake flow.
Just before and at TDC there is little flow because the piston has stopped raising.
What is mostly lacking in the intake port is an adequate width.

 

[Street Ryderz]

I don't know that the partial blockage of the port at TDC is important because its only as the piston is rising does it create the vacuum that creates the intake flow.
Just before and at TDC there is little flow because the piston has stopped raising.
What is mostly lacking in the intake port is an adequate width.

That few degrees btdc if blocked will as you say limit the duration.The current gt5a's intake is completly open as it aproches tdc.And that yeilds a little extra duration compared to one thats blocked!

 

[Deleted member 12676]

close but no cookie.
the duration is determined at the edges cross from the closed zone to the open zone.
In this case the bottom of the piston and the bottom of the port.
So if the edges cross at 60BTDC then the duration is 120 degrees.

 

[Street Ryderz]

I

close but no cookie.
the duration is determined at the edges cross from the closed zone to the open zone.
In this case the bottom of the piston and the bottom of the port.
So if the edges cross at 60BTDC then the duration is 120 degrees.

|Intake duration takes into acount for the height of the intake port or piston height on inlet side as all the calcs will show if the skirt is raised with the port height the duration incresses!

 

[Deleted member 12676]

please go to http://lambretta-images.com/archive/porttiming.php
and notice how the exhaust and transfer ports durations use the distance to the top of the port
but the intake port duration is figured using the bottom of the port.

 

[Street Ryderz]

please go to http://lambretta-images.com/archive/porttiming.php
and notice how the exhaust and transfer ports durations use the distance to the top of the port
but the intake port duration is figured using the bottom of the port.

I use lambretta's port calc regularly and where it says to input the piston height inlet side if you raise it 1mm it changes from 117.72 to 123.65

 

[gary55]

This stuff throws me off because 180 deg. taken up in a 40mm stroke is 4.5 deg. per mm. In a 38mm stroke it's 4.73. I'm guessing there is something lost in the change of movement from linear and circular at TDC and BDC, but I don't know how as each mechanical movement should have the same effect on the other.