My 2004 Mazda RX-8 has failed its compression test. This fact did not come as a huge surprise given its age, but it's still disappointing news. Long term I will have to either get a rebuilt engine or move on to another car. In the short term I will live with this condition and I've been reading up on what to expect going forward from RX-8 Club owners' forum and sites like RX-8 Help. This post is my summary notes to future self.

Poor compression degrades power output which in turn degrades fuel economy. Beyond that, it is expected to slowly degrade my car's drivability until I reach limits of my tolerance for frustration. For example, what kicked off this investigation was the fact my car would occasionally stall instead of idle. This symptom will get worse. Increasing in frequency from "occasionally stall" to "frequently stall" and my patience will run out somewhere along that spectrum.

Low compression can also make "hot start" difficult: Unable to start an engine when it has already been warmed up and things are hot under the hood. Most commonly noticed at short stops such as when stopping off at a gas station. (... or when the engine stalls at idle...) I've noticed my starter motor cranking an extra second or two before the engine catches but so far it has always started. As compression degrades, it'll take longer and longer until at some point it just won't catch and I have to let the engine bay cool down before I can start the engine. Not a big deal if I'm sitting in a parking spot, but if I'm at a gas station I will have to push my car out of the way so other people can refuel. If this happens I will be highly motivated to act on one of my long term options.

Increased workload on the starter motor may burn it out. I could then replace it with a unit from a later "Series 2" RX-8 which runs faster. Since rotary engine compression improves with speed, a faster motor is more likely to start an engine suffering from bad compression. Some aftermarket starter motors can give me this effect as well. A faster starter is only a workaround that delays the inevitable, but it is an option if I want to spend a hundred bucks to postpone a far more expensive decision.

How long might I have before encountering such drivability issues? That is unfortunately murky. Poor compression means some of the air+fuel mixture, either before or after combustion, are escaping past one or more worn seals. The exact escape route will vary from engine to engine because every one wears out slightly differently. It is possible for certain escape routes to cause erosion that starts a destructive feedback cycle of engine wear. In which case my car's gradual descent becomes a nosedive.

Speaking of escape routes: if some of that mixture leak past side seals, they would end up in engine oil. Such an increase in oil contamination will demand frequent oil changes in order to avoid triggering bad oil problems. I've done oil changes every 3k miles for as long as I've owned this car and this is one more reason not to slack off now.

And finally, the catalytic converter might get caught up in the aftermath. Even in a healthy engine, some small amount of fuel would escape unburned out the exhaust port. The catalytic converter exists to catch those molecules and convert the nastiest bits to less-bad compounds. But if unburned fuel is leaking past worn seals, the catalytic converter may receive more than it can safely handle and fail. Usually catalytic converter failure would illuminate my Check Engine light and the car would fail regular state-mandated emissions test. In the rare case, though, it is possible for an overloaded catalytic converter to catch on fire. At some level I admire a car that decides to go out in a blaze of glory, but that's not the kind of excitement I want in my life.

For better or worse, none of these things demand immediate attention and I can still drive my car as-is for an unknown period of time. Which means I should get back to my project to-do list now that the replacement radiator has arrived.