Modern Vespa

Can't afford to kit it but thought I might open the exhaust just a bit.

65% of the bore. I've always been fond of elliptical shapes with the widest part shifted down where the piston speeds are lower.

So 65% for stock rings? I'd have thought that was more for race quality rings.

Also, I thought the idea was to have the widest part at the top to let the pressure down fastest. Doesn't it kinda defeat the point to have the wide part lower where the pressure has largely been let off? Just trying to learn here. Thanks.

65% is safe for most rings, even stock, with the right rings and a careful shape you can go up to 70% but I wouldn't try it even with the kits.

You are working against two different factors. On the one hand you need enough area above the transfer ports to get the blowdown done (pressure drop), and enough total area to get the exhaust gas out (mass transfer). On the other hand, to be nice to the rings, you want the port narrow when the piston is moving fast and wide when it's slow.

The height of the exhaust port does more for the blowdown than the width so I'd still skew the width to the bottom and just raise the top of the port a bit more if needed.

But that's me, and last time I was on the track a 9 year old girl left me in the dust.