I have a few sets of velocity stacks for sale that will fit 2018-> Ninja 400 and Z400.

They are made of carbon fiber and polycarbonate.
I have run a set of these on my bike for about 7,000 miles and 3 or 4 track days.
A few others on the forum are also running a set of these stacks on their bikes and race bikes.

$75 shipped for just the stacks. (this will require you to cut down your OEM rubber stacks and open the area where the stacks mount in the airbox - see videos for details)
$95 shipped for the stacks and one set of modified rubber OE velocity stacks these will mount into.
$155 shipped for the velocity stacks mounted in a modified airbox with an air filter ready to install on the bike.(sorry, used darn airbox's at the cheapest out the door currently going for $75, if you are lucky to find one for this shipped)

Shipping in the USA to the lower 48 is included in all the above prices.

Hat Fedora Headgear Wood Audio equipment

Here are the Dyno numbers for proof.

This was on a stock bike with Graves full exhaust, flashed ECU and my modified airbox (snorkels removed and an opening cut out in the lower front). No tuning was done on this run to make any different numbers with my stacks.
So with tuning, you may do even better.

*The dimpled version was abandoned as the gains are not worth the effort or change with no further improvements.
Rectangle Slope Plot Font Line

Installation and information on installing the standalone velocity stacks.

--Watch this first if you are purchasing "only" the velocity stacks and will modify your own airbox--