I guess that depends on the combo your running how quiet you want it to be ect. Truthfully any engine making more than a souped up rascal scooter would benifit from at least 2 1/4 inch pipe. And with the wide offering of mufflers keep it quiet. The hot set up in the day was to run hemi style sears muzzler mufflers,quiet and flowed well,and were 2 1/2 od.