Contemplating the pros and cons of making this:
look like this:
I thought about this too, but it looked like the ports in the malossi just run deeper in to the casting, and are plenty big to move all the volume that the actual ports on the piston could let through. In fact, I found it more necessary to enlarge the ports on the piston.
not sure about that other cylinder, but on my polini the 'open' W boost ports are much shallower, it's just a difference in design, routing it right around the rings and piston crown in stead of deeper in the cylinder.
measure the 2D transfer area at the point it meets the case. is it the same area as window inside the bore?
if yes then my understanding is there is nothing to gain, because the window is not restricting flow. but someone correct me if i'm wrong.
Very interesting. Is that a 46mm piston? Did you go wider on the piston? Or just down? Are you saying making a Polini W port deeper, wouldn't make much difference?
I have more questions :)
I hear you Joseph, but if I cut out the bridge i could make the port match bigger too.
Yes, thats the 46.5mm, I just went down on the sides of the boost, and opened and cleaned up the transfer ports in the piston too. Also widened the lower cylinder transfers on the cylinder wall, where the piston dumps in to them, to get them to match the transfer holes on the piston. If you slide the piston in there stock you'll see that it overlaps some and some of the ports in the piston will open up to bare cylinder wall.
i suppose making the boosts bigger (deeper) on a polini can't hurt, as long as the port in the piston is big enuf..
that's the 43mm/65cc piston there, my 46.5mm/73c one has a normal skirt wall, with the small window cut above the piston pin and the larger one next to it like that, but the whole skirt is not recessed/cut out like that, and there is no re-inforcement webbing from the pin to the skirt to make it like that.
i just opened up the holes in the piston, brought them up to the webbing that goes from the pin to the crown, and extended the cut outs in the cylinder skirt to match them. Mainly about 5-6mm to the left, if you're looking from the bottom of the cylinder, on one side, a bit less on the other, where like half of one of the larger windows in the piston was blocked. i just went a few mm in and ramped it over to the main port passage.
I cleaned up the transfer and boost ports in the cylinder too, but did not want to enlarge too much, as not to loose too much intake charge velocity, since this was going on a stock-mod (poor man's speed-engine) case first, and there's plenty of port there for a 13mm carb.
Also opened up the exhaust to the max so i could stick my finger all the way in it. it's kind of scary filing around the cylinder stud hole.
have more pics at home, can post some if you want.
it's torquey, got killer accel, and eats hills (and belts).
yeah, different piston design in the two kits, and of course the diameter. I focused on the upper ports that feed through the piston. the lower cutouts in the skirt i just shaved the outside of the edges down to be more earodynamic, since i had plenty of port passage outside the skirt after i matched my case, you only need the openings to be as wide as the bottoms of the main transfers in the case anyway, so hold it in each case half and see how it matches up to that more importantly.
check out the vids i posted in the 'how to make a vespa fast' thread for inspirations.
nyone else running a malossi on their vespa?
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