New here! 50cc build

That opens my eyes a little bit more to porting, I've really only cleaned the ports up on mine. I wish my calipers fit down to exhaust port opening. At the exhaust flange/opening its 24x20mm on these 49mm steel sleeve cylinders I've been using. I'm just curious about porting transfer ports, I'm hesitant to mess with them with all the reading I've been doing.
That 24x20 flange area is 23.9mm equivalent dia and big enough already, the lead engineer at Aprilla told me that from the window/piston face to the exhaust flange a 1% increase in dia was except able so in the case of 23mm it would be 23.23mm with 1% not that much of a difference but that dia can flow 80 cfm.

I don't suggest any one mess with the transfers or the port timing unless they understand what the timing changes will do/effect and many people go ahead and cut the exhaust and intake thinking leaving the transfers alone they will be ok but that changes both exh and intake durations and the blow down period.

Transfers mostly rely on the cross section just after opening for their volume and in most cases are all ready very big and here's why, lets say the transfers have a width of 20mm each and just after opening as the velocity is highest have an open height of 4mm that's 9.08mm equivalent dia and combined is then 18.16mm not bad for the stock port volumes but at full open it's double that and therefore kinda big but again it's that cross section at partial open that's more important in considering the volume.

When it comes to correcting the transfers exhaust side angle this also increases the cross section of the track/feed and allows a bit of a radius to be made between the back wall and roof, the roof angle is also important and at this displacement and area 15 degrees works best.

The blow down for our engines is best around 20-22 degrees and when raising the exh timing this will effect the blow down's timing/duration so the transfer may very well need to be raised also if going with more exh timing/duration, the effect of these being mistimed is poor cylinder fill, lack of pipe aid, slow to build rpm's under load and very narrow if any real power band.

There are alot of other things that go along with all of this and I'm not going to write a book here LOL but trust Blair and Jennings published works are very informative in all of these area's and give you the formula's and explanations/examples needed to get a better understanding of it all. BTW I cheat when it comes to the math and use online calculators intended for the task.
 
Thanks @Street Ryderz I appreciate it, I was wondering if the taper in exhaust port needs to stay. I also did raise my cylinder 1.5mm to help open the transfers up some more on bdc. I'm just now getting into gordon jennings hand book, lot of good info in there. I'll try & get a degree wheel on it to get some #'s, cause I'm curious now. Sometimes I read too much into something & confuse myself occasionally & need it explained in layman terms.
 
Idk I thought that's what I downloaded. Gordon jennings handbook to 2 stroke tuning.
With Street Ryderz on our side, it really helps to clarify some of the more confusing stuff. I have been mostly a 4 stroke builder ( cars and trucks) in my life. Never really got into 2 strokes till I got into this hobby. I get most of what's in the book, but I'm not always 100%. Nice to have a guy like him around to help clarify stuff before I make a mistake that's costly.


Trying to learn all I can before I get into the rpm builds.
 
Yeah I'm glad theres guys like StreetRyderz & others on this forum that can help explain stuff. I was watching a video explaining the effects of a expansion chamber on a 2 stroke. It was a good simple video, but explained alot to me. But confused me in other parts.
 
That 24x20 flange area is 23.9mm equivalent dia and big enough already, the lead engineer at Aprilla told me that from the window/piston face to the exhaust flange a 1% increase in dia was except able so in the case of 23mm it would be 23.23mm with 1% not that much of a difference but that dia can flow 80 cfm.

I don't suggest any one mess with the transfers or the port timing unless they understand what the timing changes will do/effect and many people go ahead and cut the exhaust and intake thinking leaving the transfers alone they will be ok but that changes both exh and intake durations and the blow down period.

Transfers mostly rely on the cross section just after opening for their volume and in most cases are all ready very big and here's why, lets say the transfers have a width of 20mm each and just after opening as the velocity is highest have an open height of 4mm that's 9.08mm equivalent dia and combined is then 18.16mm not bad for the stock port volumes but at full open it's double that and therefore kinda big but again it's that cross section at partial open that's more important in considering the volume.

When it comes to correcting the transfers exhaust side angle this also increases the cross section of the track/feed and allows a bit of a radius to be made between the back wall and roof, the roof angle is also important and at this displacement and area 15 degrees works best.

The blow down for our engines is best around 20-22 degrees and when raising the exh timing this will effect the blow down's timing/duration so the transfer may very well need to be raised also if going with more exh timing/duration, the effect of these being mistimed is poor cylinder fill, lack of pipe aid, slow to build rpm's under load and very narrow if any real power band.

There are alot of other things that go along with all of this and I'm not going to write a book here LOL but trust Blair and Jennings published works are very informative in all of these area's and give you the formula's and explanations/examples needed to get a better understanding of it all. BTW I cheat when it comes to the math and use online calculators intended for the task.
Lot of info. I learned with pocket bikes to leave the transfer ports alone. Widen the base but dont touch the height. The exhaust port can be widened a bit as well as a touch of the top of the exhaust port, arched like a football.
Simple stuff. Less is always more on a small 2 stroke. The exhaust chambers are insane, the kit exhaust is stupid, somewhat tuned pipes, at least with a chamber, are great. Especially with the correct chamber size.
 
Lot of info. I learned with pocket bikes to leave the transfer ports alone. Widen the base but dont touch the height. The exhaust port can be widened a bit as well as a touch of the top of the exhaust port, arched like a football.
Simple stuff. Less is always more on a small 2 stroke. The exhaust chambers are insane, the kit exhaust is stupid, somewhat tuned pipes, at least with a chamber, are great. Especially with the correct chamber size.
I get to do alot of pb engines and go ped engines here and I can tell you that pb guy's are very stubborn and many just can't be helped LOL, I had an issue with one of the pocket bike groups admins for telling a guy to raise his transfers after he had raised the exhaust port and shrunk the blow down which dropped the power band to low and left the engine falling off way too soon.

The admin started trying to tell me that was bad advise and that these engine's (cag) could never make enough power to power wheelie or go over 50 and well that is just simply not true!

With the transfers widening the base doesn't really do anything since the highest velocity and volume is just after open when most of the port is still closed and to get any aid from the pipe the timing and blow down has to be correct or alot of the fresh charge is lost into the pipe.

The exh port timing, if you want power in the higher rpm ranges, has to be cut/raised, widened and radiused in the upper corners with a flat center section if you want the pipe to hit hard!

I've had this cag engine pocket dirt bike since 09 and it has been used to teach my kids and now my grand kids how to ride, I had my youngest son on this bike when he was 6 y/o flying through hydro fields and on single track area's at 40+ and when it needed a new cylinder about 4-5 years ago now I did a bit of correcting on it as I felt it could use a bit more for when I messed around with it like this,

https://video-yyz1-1.xx.fbcdn.net/v/t42.1790-2/18080931_1380579515342641_5228060975554363392_n.mp4?_nc_cat=111&ccb=1-5&_nc_sid=985c63&efg=eyJybHIiOjU4MSwicmxhIjo2MzksInZlbmNvZGVfdGFnIjoic3ZlX3NkIn0=&_nc_ohc=3IIq3pgYG3sAX9sFYLH&_nc_oc=AQn-YR-7n8IFnEIXZRI8QoraF8f_2Z9Aw5VXapNKrgDa-yznSLAOWzb0EnaRTQsgDr5_DTxKiIvlbTU04bxkWwO5&rl=581&vabr=323&_nc_ht=video-yyz1-1.xx&edm=ADWe-ecEAAAA&oh=8570e52b57ef9de68415c4de6c6f8e0f&oe=61B1CEBD
 
The exh port timing if you want power in the higher rpm ranges has to be cut/raised, widened and radiused in the upper corners with a flat center section if you want the pipe to hit hard!
This is a untouched 49mm cylinder, so on the exhaust port shape, you want the port to look similar to this untouched port. You want a flat roof with the upper corners radiused. So do the corners need be more square/more defined? Does the port roof need to stay the same size as port floor or is it better if the port roof is wider than the port floor? As far as port timing I've been lucky enough to raise the cylinder 1.5-2.0mm then mill the cylinder down to bring compression back up. Lol if it wasnt for you guys I probably would have not been that lucky with the cylinder. Because I knew nothing of port timing b4 this site.

20211209_062530.jpg
 
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