It looks like OBX already makes a 4-2-1 exhaust header for the HR16DE. I wonder if it will fit in the Micra?
http://www.obxracingsports.com/products.php?pk=1791#
Honestly, 1.75" primaries seem rather large for a 1.6L 4 cylinder with 109hp. According to the reference info I have, a 1.5" diameter primary would be much better suited to this engine. It would be awesome to see before and after dyno charts on this.proven designs from the runners to the collector, each header promises significant improvement over the stock system while still being fit for daily driving. All headers come with a limited lifetime warranty.
◘ Primary Tubes: 1 3/4"
◘ Collector: 1 7/8"
◘ 3/8" Flange thickness
Also, its OBX, a company not really known for its quality...