For those who are looking for best performance rather than cool (sometimes you can have both) The easiest add-ons deal with air flow (A/C, Pipes, Heads). But before you read an ad or take a recommendation you should have a good idea of what your riding style and specific comfiguration needs. The formula to calculate required flow rates for different cubic in displacements at specified RPMs at 10" is: rpm x cubic inches per cylinder x number of cylinders / 1925 = cfm For example if we want to know the ideal cfm for a Twin Cam 88 at 2500 rpm we plug into the formula this way: 2500 x 44 x 2 / 1925 = 114.29 cfm at 10" A stock Harley filter flows at 130 cfm so would be perfectly adequet if you drove no more than 2500 rpm. But lets say you wanted something for 4000 rpm you would need a rated filter of at least 200 cfm. A SE A/C flows 160 cfm which is better but in this example still restrctive. When reading ads that espouse a certain product, obstensibly proving it's assertions with cfm flow rates, it's at least necessary to know the manometer test pressure. if the manometer test pressure is diffeent you need to use a fomula to equalize the figure so you can begin making a fair comparison. First divide wanted test pressure by known test pressure. then get the square root of that result and multiply it by the rated cfm. for wxample if we have a known test pressure of 300 cfm at 18" for product X, and we want to compare it to product Y rated 250 cfm at 12" what would the air flow of product X be at 12"? To find out divide 12 by 18. The answer is 0.667. The square root of this is 0.817. This number is then mutiplied by 300 cfm which gives 245 cfm. So 300 cfm @ 18" = 245 cfm @ 12". This may all seem confusing but if you are loking for real performance from your scoot knowing what you need and how to compare whats out there will give you the result you are looking for.