first and foremost it is an honor to be a part of this site and the g35 family.https://g35driver.com/forums/images/smilies/bowdown.gif

a friend is selling a set of greddy coilovers off his 03 350Z and i no that they are a direct fit but i do not no much about the spring rates and how it will affect the ride quality?
what is the best advice?

 

314/342 to 560/448 is a big differance. You will get a increase in impact harshness, dependent on your local road condition's from the spring rate change. Also, going from the car's oem upper shock mounts to pillowball upper mounts will also cause a increase in noise, vibration and harshness. Not to mention that the Greddy coilover is known for having it's own ride quality issues (Talk to Adam at Z1auto)

If you want ride quality get a product that will actually delivery that. That means spring rates that are closer to oem, a company that actually does extensive on car R&D, and reusing the oem upper shock mounts isn't a bad idea. You do not have to settle for something that is like oem in ride quality, their are products that are far better.

And don't think you HAVE to have a coilover to get what you want, though I get it that your asking because of what your friend is selling.