I doubt a factory cat would last the life of the car, otherwise there would be no market cheap replacement cats. They have a finite service life, dependent on many factors. Many replacement cats might fail prematurely due to the original engine fault that lead to the factory cat failing not being fixed.
On my 1996 Roadster, I’ve replaced the cat 3 times since 2005. The first replacement hit a rock, which destroyed the innards; the original cat may have died because I had a failing engine (stuck scraper rings0 chucking out oil. The replacement was a longer UK 440mm cat; I had a MX5parts manifold with their poor fit adapter pipe for Roadsters. I used a cat from MX5parts; that lasted until last year, but in the last few years, the car has struggled on emissions, which Thrussington tracked down to a failing MAS
Its been said, without offering evidence, that factory catalytic converters should last 80k miles, and aftermarket, 25k miles.The Mk1 factory catalytic converter wasn’t exactly of an advanced design. That might come from US EPA requirements; you can find claims that cats (in the US) need to be warrantied for 5 years or 50k miles, other claims are cats on OBDII cars have to last for 80k miles, and on OBDI for 25k miles (better emissions control on later cars. All Mk1s sold here had neither. There are claims that aftermarket cats don’t last as long as there is a smaller volume of catalytic material available, and that you can tell that by comparing external dimensions. Others will dismiss that, presumably on the basis that 2018 aftermarket cats are taking advantage of more advanced ceramic materials not available 28 years ago to Mazda. And then you have to consider year car is now 25 years old. The engine is now a little worn, its not running as clean as the day it was new. Cat life will be diminished
I pretty sure the supplier of cats to MX5parts is a lottery; they seem to change in appearance fairly frequently, so the reviews might be of little help.
If you are looking for genuine long term collective experience of catalytic converters on MX5 Miatas, you need to look across the Atlantic. Flowmaster, Borla and Magnaflow are frequently cited there as provided the highest quality aftermarket catalytic converters. Most of the major UK suppliers don’t own up to where their components are coming from; UK, EU, US or China.
Back in the day, “Racing Beat” cats were rated above “Jackson Racing” cats. In truth, both companies used to knock up their cats in their workshops using 3rd party sourced components. I visited Racing Beat’s Anaheim shop back in 2000, and can testify everything they did was done by a small band of welding artists. JR was a bit different; long ago, Oscar Jackson sold his trademark to Moss, so who knows made the stuff for Moss. OBX? Racing Beat Europe sell cats they are assembling themselves, using Magnaflow core:
https://www.racingbeateurope.com/bmazda-mx5-1989--1998--high-flow-catalytic-converter-16-or-18-msamot-compliantb-2266-p.asp
Nice looking flanges, better than the pig iron thats tacked onto most aftermarket cats.
But whatever, I suggest that you ensure your engine is in tip top condition, before spending more on a cat; new plug leads, new OE-quality O2 sensors, maybe even investigate if a new AFM is needed.
Or just stay with the cheapest, safe in the knowledge that, unlike factory cats, the studs will never be seized.
