Decided to bite the bullet and get a new backbox for the NC. I’d previously been considering a Cobalt one from Moss but seeing the price of the Gravity Performance one on their eBay shop (£188 after offer from seller) I thought it must be worth a go.
I thought I’d post my thoughts on the product here as there’s not a lot of information on it from what I can see.
I’m sure in terms of workmanship the quality is probably lower than the more expensive options but my father being a fabricator/welder in his younger days said that there was nothing wrong with the backbox, just that he would have polished the welds to remove the discolouration had he been making it! I think one of the arms that it hangs from is ever so slightly bent, leading to it being off centre by a couple degrees on the car but you really have to be looking for it to notice.
The sound is honestly perfect for me. With the roof up at motorway speeds it is no more obtrusive than stock, perhaps a tiny bit more rumble but nothing much, the road noise is more of an issue anyway! At low speeds and idling the boominess and rumble is more apparent and just makes the MX-5 sound a bit more muscular but it’s definitely not an anti-social noise.
At around 3000 RPM I notice the volume starts to increase compared to the stock backbox, again not ridiculously, but noticeably. Then from that point there’s a little more growl all the way up to redline.
Overall I am really happy with it, the build quality for the price is better than expected and the sound is essentially perfect for me.
Here’s a little video to show the sound, please forgive the vertical(ness) of the video!!