A lot of vespa scooters are running pipes like these: https://www.google.com/search?q=vespa+p200+exhaust&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjS-aaLkZPeAhUEyaQKHeRxBccQ_AUIDigB&biw=1536&bih=755#imgrc=7cWcGandnoRHhM:
Often the cones extend into the chamber just a bit. I was curious about building something similar for my tri-ped micro car, maybe another ped that I wanted to quite down a bit. Those sito plus style pipes seem good to around 40mph for me-I think I was actually using a gianelli.