This month Sport Compact Car mags has a great article detailing the different types of VGIs. They talk about how some have variable length, some have a butterfly to close off half of the runner and how some,like our 2.5L, have both. It is a well written article and explains the bennefits and how they work. In other words keep your IMRC and if you can optimize the lower to get the most out of your top end.
