I wouldn't bother. The spring that some people go to is a EPI red secondary. It has about double the closed spring pressure of the stock spring and will wear out your helix at a rapid rate. It does what it's supposed to: slows the upshift to allow for big tires. But it also kills top speed. A better aproach is to run the shallower helix from the 650 Outty. With 27" rubber, you are right on the edge of whether or not you need anything more. The secondary helix is a cheap experiment if you want to try it. Field testing has shown that with stock tires or close to stock tire sizes, the stock helix and spring has the fastest upshift.