50CC race cylinder VS. hand porting

[proy]

hey guys im planning on playing around with a second minarelli engine i have and wanna focus more on high rpm and remaining 50cc. 

This is more of a science experiment for me. i want to use this engine as a test bed for learning to port tune and push the minarelli to its limits without risking my Etons longcase and fresh rebuild. 

my main question is if i am sticking to 50cc what makes more sense. try to find a full race 50cc cylinder or port my own. 
I have a naraku 50cc cylinder that i am curious about porting.





here is a port map i did of the naraku 50cc kit that i ended up melting the piston on. Is it possible to push this port map into a race style or "high rpm" porting. I am just beginning to read up on port timings and 2 stroke math. 

Or would it be more reasonable to just buy a full race 50cc cylinder and skip the porting on the naraku cylinder. Preferably i would like to port the naraku cylinder as a test and learning experiance on porting since its just a rusting paper weight.

[190mech]

OOHH! Lots to do here..A 40mm bore is difficult to port due to its small size,transfers will need to be raised which is impossible without a 90 degree angle grinder(likely dental). If you must try it,look at the performance layouts in the port map posts above,,got a lot of metal to remove there! 190 degree exhaust duration and 130 transfer is a good start point,transfer port bottom edges should be flush with piston crown at BDC, Combustion chamber will need to be reworked also..Getting a race style cylinder will allow you to side step many hours of work with little results..

[proy]

Mar 12, 2023 5:40:27 GMT -5 190mech said:

OOHH! Lots to do here..A 40mm bore is difficult to port due to its small size,transfers will need to be raised which is impossible without a 90 degree angle grinder(likely dental). If you must try it,look at the performance layouts in the port map posts above,,got a lot of metal to remove there! 190 degree exhaust duration and 130 transfer is a good start point,transfer port bottom edges should be flush with piston crown at BDC, Combustion chamber will need to be reworked also..Getting a race style cylinder will allow you to side step many hours of work with little results..

Thanks for the reply. i might just buy a race cylinder and focus on modifying the stock cylinders as I go along. I have no issue buying purpose built tools but the 90 degree drill will prob be a lot of dough. hmmm good to know its going to be as challenging as I thought I will look at a much longer timeframe for the project. before I start the 50cc high rpm build I'm gunna finish buttoning up my eton beamer. If anyone has advice on a 50cc race cylinder I wanna look into and research as meany as I can find before buying any actual parts. 

[repherence2]

here's some info my bruddah,

link: bigbnrg porting a cylinder.

link: pedparts blog. cylinder basic porting.

lots of info on both sites. that's where i learned about porting.


the way i see it, if you like learn, the first step is to try. i once bought a 47mm 70cc chinese cast iron bore kit from a local shop. no matter what kind of performance parts that i threw at it, the bike could not exceed 35 mph. however, acceleration was animal, up until it hit 35 mph and that was it. one day, a mechanic at one of the scoot shops that specialized in minarelli engines told me to check the port timing on the exhaust. he suspected that the timing duration was low. so i read up on the Big B NRG blog and the pedparts blog and it busted out the dremel to modify the exhaust port. i ported the exhaust according to the pedparts blog. as far as local shops here on Oahu, i was basically limited to either the YMS V8 exhaust or the Leo Vince zx-r.

in the end, i ran the ported chinese cast iron bore with a YMS V8 because all of the potholes here on the streets of Honolulu ate up the 2 Leo Vince zx-r pipes that i bought. while i was porting the chinese bore kit, i was running a Motoforce Mid-Race 70cc bore kit. i used a hammer to persuade/remove the wrist pin on the stock 40mm bore. i was newbie and did not know any better. the Motoforce piston ended up cracking and breaking the skirt. i suspect that i bent the connecting rod on the crank by persuading the wrist pin with a hammer when i removed the stock piston and wrist pin. my only option was to reinstall the ported chinese kit that i ported because the bike was a daily rider for me to get to work.

hot damn! i was impressed. the bore kit ripped. it wasn't a screamer, but it could haul almost as good as the mid-race kit with the Leo Vince zx-r. i likened it to how some of the Honda guys would run either the YMS V8, Phongeer short (PG short), or the Phongeer long (PG long) on the Elites that had Polini Corsa's here on Oahu. the PG's were more for rippin' it while the V8 was more for city grunt and less noise compared to the PG pipes.

if you do not feel like spending money on 90 degree dental style grinder, you can hand-work and file the transfers to even them out and achieve the duration/timing you need. BigBnrg has a some tables on timings for exhaust port and transfer ports. you would have to buy some small "die sinker rifflers files". most likely, you would have to modify them. use a torch to heat up and bend the shank of the appropriate file and then possibly grid down the file to shorten it. with some time and patience, it can be done.

[90GTVert]

If you're looking for a really strong high rev setup, I think you'll fare better buying a high end/race cylinder. I really like to see people learning about porting and 2Ts in general. I've done mild stuff to cylinders for years. When I held my first racing cylinder, it was very obvious that it was a different beast though. The exhaust ports especially are gigantic and I don't think you'll get that kind of port area with a single un-bridged port. I once tried to make a really large port (still not rivaling the bridged exhaust ports) and it didn't like piston rings. You can try to make gradual shapes to help the ring along, but I don't think you're going to match the results of kits designed for racing.

Aside from that, you're better off with a single ring setup if you want to rev high. It's not a total must, but it's advantageous for output. You can run just one ring in a dual ring piston, but you may experience some short circuiting from it and it's really not ideal. The designs of race kit pistons are better for racing. Go figure.

I'm not at all saying that you can't make a very strong setup, but I think a racing cylinder is the way to go if you really wanna be fast. It's not going to teach you much about porting... but you could dig into the engine cases instead.
youtu.be/XcgFI905UGU