pcimino

I have a conceptual understanding of the Apex Seal, how it works and what it does.

When the seal fails (due to pre detonation, etc), what exactly is happeing to it? Is it melting, cracking? Is the whole seal affected or just the blade edge that makes contact? Do the seals stay seated in the channel or to bits start flying around inside the combustion chamber?

If melting is the issue, I can understand why ceramic seals are more resistant. But not if cracking is involved.

tuj

Its usually the force of pre-ignition against the seals that cause them to go. Basically the fuel-air charge explodes, and must of the force is NOT the direction the rotor wants it to go (Think of a piston engine: when it knocks, the charge is exploding against the force of the piston still coming up. Thus, two forces are opposed, and that can cause things to break).

When the apex seal blows, it comes detached from the rotor edge and spins around in the housing. On older 13B's, it would often exit the peripheral exhaust (and then perhaps find its way into a turbo), but on the Renesis, the seal cannot exit the rotor, and therefore spins around, chewing up the housing.

pcimino

So pictures I've seen of the seal look like a long thin piece of metal. Why not make them wider (not thicker), so they sit deeper inside the rotor channel? Then they shouldn't go anywhere.

tuj

They do; its called 3mm seals. There's a debate as to their effectiveness, and generally they stick out of the rotor farther than a 2mm seal, so they are good for rebuilds on out-of-spec housings that have been machined.

pcimino

I was thinking they should stick in the rotor. So the housing is the same, just the channel at the rotor apex where the seal sits should be deeper. So it sticks out the same as a 2 MM seal, but maybe sits an extra 2 MM inside the rotor itself?

rotarygod

Sometimes you can get lucky and only crack a seal and not have it come out. This still allows gasses and pressure to bleed through the crack and performance suffers.

The apex seal grooves on the Renesis are actually not as deep as those from the 13B. However the 13B used a 3 piece apex seal. The 3 piece seal has a small section that sits below and another section that sits above so the total height of the seal is divided in 2. Then there is also a small triangular wedge shape that is full height that ride along one edge. The dual height seals would fold over easier during failure since there isn't much area between the bottom of the seal and the top edge. 2 piece apex seals that extend the full depth with 1 piece helped with this quite a bit and also aided in being stronger. The RX-8 uses a 2 piece seal. The 2nd piece is just the small triangluar corner piece. The total seal height is less but due to the fact that it is one larger piece it is stronger than the 13B seals. This only refers to factory installed seals as the aftermarket has much better ones.

If a seal fails on a Renesis you can just kiss that half of the motor goodbye. Due to the location of the exhaust ports, it makes it very difficult for the seals to go flying out. They just stay in and grind their way around a couple of times. The 13B engines would usually do the same thing but the seals would get expelled faster. Sometimes you'll see seal parts embedded in the faces of the rotors and the housings. Bad things happen when seals fail.

pcimino

Just seems there shuld be a way to redesign it to be more tolerant of pinging. Either a way to fail gracefully, or a brute force way to beef it up. I'm sure Mazda engineers have thought about it.

Maybe the performance penalties aren't worth it?

StealthFox

go to rx7club.com to ask that kind of stuff, this board doesnt really deal with engine work, as the car is brand new, people still have warrantys, dont have big turbos to install bad and blow up the eninge or overheat the engine, etc so you'll get better info at rx7club.

rotarygod

The seals that can fail and not do much harm are the carbon seals. They do not work well with boost, can not be detonated even once, and will only last about 20,000 miles if you are good to it but they won't kill your engine when they fail. the ones that do stand up better to detonation still aren't indestructible. They are much harder on the rotor housings and will wear your engine out very fast. They will also absolutely destroy your engine internally if they fail but they are more tolerant. The best thing to do is to just tune it right and not detonate. It's only detonation under boost that is really harmful anyways.

zoomzoom_8

so how do you know if your carbon seals are bad, any signs?

tuj

RG was talking about carbon apex seals (basically graphite) that used to be used in the NSU engines and also in some applications for racing.

You'll know if you have a bad apex seal because your engine won't run, or will run marginally at best.