If piston engines show low compression…
 

…are they typically replaced? Or just repaired.

Seems the term "engine failure" – regarding the Renesis – is misleading. Mazda simply chooses to replace, rather than repair, their engines showing low compression. If failing a compression test constitutes "engine failure," I have a feeling millions of reciprocating engines have had engine failure as well.

Only they're repaired. Not replaced.

The Mazda mothership does not allow lowly US dealers to do ANYTHING with rotary internals. It's a black box and if the black box fails it goes home to mama.

Yes, "engine failure" is misleading when it comes to the rotary, and causes lots of confusion.

"Engine failure" in a piston engine is typically catastrophic, and up to that point the engine generally runs fine, where as you are far beyond technical 'failure' before you get to actual 'failure' with a rotary.

For piston engines, there are also many individual components that can be replaced without doing a full engine rebuild (such as valve springs, valves, etc...), where as any one failure with a rotary does mean an entire engine rebuild.

Low compression on a piston engine typically is from piston rings being worn/shifted, or the sleeves themselves being worn away. Basically the same relationship as our apex seals to housings. Only the wear dynamics are obviously far better for the piston rings than the apex seals. And yes, low compression piston engines usually are rebuilt, technically the same as rotary engines (your core when you get a replacement from a vendor is usually rebuilt), but you can rebuild just pieces much easier.

Example, my 99 Miata's failed piston ring motor could have been rebuilt for around $700 if I wished, with the price going up dramatically the more restoration and upgrading I wanted to do in the process. I ended up opting for a $1,250 lower mileage used engine instead.


Both are really handled the same, fundamentally, just the bias between rebuild vs replace is different, and the definition of 'failure' is a bit different.

When a piston engine loses compression, the cylinders can be honed or bored, and oversized rings (pistons, too, if bored) used. Can the equivalent of honing or boring be done on a rotary? Honing a piston engine and putting in new rings is a reasonable DIY.

Piston engines can also lose compression from valves going bad, in which case all that's done is a valve job. I've been through that a few times. Just pull the head, take it to the machine shop, pop it back on when they're done.

Ken

or ignored. if you went and compression tested a random sample of piston engine cars, i think you'd have a significant percentage that scores at the low end of "good", but they still start and run, so the average person doesn't notice.

actually the dealership i was at from 04-07, was replacing piston engines right and left, it was like a 30/1 ratio to the rotaries, but somehow the rotary gets the stigma and not ford. (we had a rash of V6 6's that died, and then the Cx7's blow up too, but that's after i left)

it's kind of funny actually, we are aware of the procedure the dealerships need to go thru to get Mazda to send them an engine, but with the piston engine cars, replacing the engine IS the diagnostic procedure!

The 30:1 ratio is mostly because that's probably about the ratio of piston cars to rotary cars.

3mm seals and resurfacing?

that might be. we had a fleet of ~200 Rx8's, and we replaced 1 engine in 3 years, which is low actually.

its like a 911 engine, you're supposed to just replace the worn housings/rotors/seals.

its just too bad by the time we can get parts here they are kind of expensive, although not 911 levels.