It replaces the stock runners in the upper manifold. And no, you don't need a tune.
From ZZP's website:
The High Velocity Upper Intake Manifold Insert replaces the factory plastic runners in the upper intake manifold found in FWD Series II 3800's. Simply remove the upper intake (leaving the throttle body attached), pull out the stock runners, set the HV insert in the intake and bolt it back down! 30HP in 30 minutes!
By changing the intake tuning we are able to achieve performance numbers previously unheard of in the L36. In short we gained 9 peak WHP and 30 WHP at 6000rpm.
When coupled with ER rockers and our HV TB the performance surpasses dyno #'s of cam'd L36's not using our insert.
More torque - The HP gains of the old HV2 were quite amazing. The only issue was a loss in low end power. This meant that only highly modified setups could take full advantage of the high rpm gains. The new HV3 has slighty longer runners to give more HP and torque in all usable rpms!Easier install - The new HV3 doesn't require special length bolts, RTV or any modifying of your factory setup. It sits entirely inside of the factory upper intake. It can be installed in less than 30 minutes.More reliable - The HV3 does not require RTV, cannot be seen by anyone looking at your external engine, and it comes with a new intake gasket. Installing the new HV3 actually improves the reliability of your car by replacing a potentially worn out gasket. Because it sits entirely inside the intake, there are zero sealing issues.