kyle@insight

Hey all, new here as you could tell. I'm trying to learn as much about the FD as possible, always thirsty for knowledge. I did a search on this and didn't really find answers to my questions so please no flames.

Basically my question, as the title says is, what makes the apex seal so fragile?

I know usually all it will take to blow one is one count of knock, as opposed to a piston motor being able to take multiple counts of knock before it gives. What about the apex seal does this? Is it the material? Design? construction? What exactly is the flaw?

Why is it that more companies haven't put time and money into creating a more durable apex seal that's more resistant to knock. I know supposedly 3mm seals are more resistant, but they dont even hold up much better.

Is the design just flawed? Or can they be made to take knock similiar to how a piston motor can?

Maybe I'm just a dumb noob and dont understand yet, but I wanna learn more.

Thanks.

rynberg

Well, there's always the ceramic Ianetti apex seals. They can take a pounding (ask Artguy). Of course, they cost $1800 for a set of six......(compared to the stock 2mm set price of about $280).

kyle@insight

What kind of a pounding are we talking about? Would they 'wear out' like other apex seals over time?

Boostedtwice

wow $1800 for seals but of course they would Wear Out over time. there is a thread that has the title 2mm vs 3mm a couple of pages back i don't know how to put the link onto my post. that post can anwser a lot of your questions it also has links to other write ups too.

Rob

rx7speed811

were talking about pounding IDB.

WVRx7

The apex seal performs the same job as a piston ring.

Detonation is a (for lack of a better phrase) "shock wave". It is like a bomb going off.

When detonation happens in a piston engine the shock wave moves outward toward the cylinder bores. The shock wave must move outward and then down around the piston crown in order to hit the piston rings. Naturally, the intensity is diminished and most of the pressure is absorbed by the piston and cylinder head and those items will accept much more abuse.

In a rotary engine when detonation occurs the wave also moves outward. The absolute first thing the wave hits is the apex seals. There is nothing standing in front of the seals so they received the "brunt" of the pressure wave.

Consequently apex seals will fail more often than piston rings. Lots of detonation in a piston engine will usually put holes in the tops of the pistons or burn valves, it is just that it takes more episodes to do that damage. Apex seals are similar in size to razor blades and it, obviously, doesn't take much to damage something as thin as a razor blade.

It isn't a matter of being fragile or of a bad material, it is a design requirement of this design of engine. Neither good or bad, just a reality.

dgeesaman

iTrader: (7)

(These are my wonderings on the subject - not yet confirmed or denied - so I'll offer them to the intellectual wolves as food for thought)

Idea 1: I think another issue that makes detonation a bigger problem in rotaries is timing. The combustion chamber in a rotary is constantly getting larger->smaller->larger... etc. The reason a rotor moves around is because the chamber is ignited just after the combustion volume hits minimum and begins to expand. Therefore the explosion adds momentum to the rotor by driving the expanding chamber along. Kind of like a gas-powered wedge.

If the combustion mixture ignites too early, the combustion chamber will still be compressing, which causes the gases to resist the motion of the rotor. This increases the energy applied to the apex seals.

Idea 2: on a piston engine, the points of escape also include valves. It could be possible that the excess pressure/energy of a ping in a piston engine will force its way by the exhaust valve(s). (I imagine the intake valve is better sealed to prevent disaster by ignition in the fuel system).

On a rotary, the only points of escape are your apex seals and side seals. Both will ruin an engine, but the apex seals are made of hardened metal or ceramic to reduce wear. As a result, they will chip/crack at failure.

Experts, please revise as needed

Dave

Fd3BOOST

iTrader: (7)

Why are you guys hell bent on making this complicated.
Apex seals are apex seals.
They will break under heavy detonation. They will break weather you have $1200 3mm seals or $280 2mm stock seals.
A ping is a ping. Stock seals will hold as much as 30psi as long as the tunning is right.
Bottom line is that seals will break if the tunning sucks.
Of coarse there are alot of other ways to break them that are out of your control. Fuel slosh, loss of power to the fuel pump, a bad tank of gas etc..
so you are a fool if you pay $1200 on apex seals. They will not last any longer than stock seals.
Now if you wnat cermaic seals just because they are softer and wont completely **** up your housings and rotors when they break thats another story.

dgeesaman

iTrader: (7)

I think you missed the point. The point was a discussion of WHY apex seals are the weak point in the engine. Kind of an engineering comparison of happens during detonation in a piston engine vs. rotary engine.

If you don't care or don't think it's relevant, then pass over it.

Dave

dgeesaman

iTrader: (7)

OK, but how is this condition any different than normally ignited combustion? The apex seals are just as exposed to the pressure wave in that case. Or is there a difference in the shock wave for a detonated combustion vs. ignited combustion?

Dave

zyounker

Bad tuning breaks apex seal via detonation..


Also lack of lubrication from the oil metering pump can break them too..


Carbon buildup can also break them, this is why you need to give the engine some revs every now and then.

-Zach

Resource

You've got it completely backasswards pal. Ceramics will completely destroy your engine when they break.

SPOautos

Resource is right, they will absolutely trash everything they touch on the way out. The only seals that are easy on the engine when they bust are hurley 2mm seals and they SUCK. You can snap a Hurley seal with your thunb they are so weak.

stock oem seals get brittle after about 50K miles and due to that they wont take detonation at all. The point of the ceramic seals is I believe they dont get brittle. It should be just a strong at 100K miles as it was at 1 miles. But if you detonate hard enough to break it (which isnt hard) then it will be toast.....and expensive toast at that lol. You blow a ceramic and its going to cost you $1800 for the seals, and another $1500 or so in housings and rotor + your turbo....ON TOP OF YOUR STANDARD REBUILD COST

STEPHEN

WVRx7

Detonation is several times stronger than regular combustion.

Normally aspirated rotary engines prove that apex seals are very reliable at atmospheric combustion. How many first and second gens last forever and only die when they lose coolant seals. Hell, if you can get a 1st gen started and just never turn it off you can drive across country.


The fragility is just exposed at high boost levels combined with detonation. Again, it is not a design flaw or problem, just a reality.

kyle@insight

Ok, I think I definatly got some of my questions answered here. Highly appreciate that. Coming from piston motors before, knock during tuning was never a huge concern like it is with these cars so it's kind of a new concept to me. WvRX7, great comparison, that's exactly what I needed.

Couple follow up questions tho. I see that the ceramic apex seals don't wear out over time, is this the case with any other apex seals? Are they worth the extra cash? Is there anything you can do to prevent the factory apex seals from wearing out on a fresh rebuild?

zyounker

The stock iron seals would take 500k+ miles before wearing down probably (thats a guess) but it i have seen 300K on a stock engine before.. It would take FOREVER to wear one down..


Much before then the chrome on the housings will flake and lower compression and you will develop coolant leaks between the housings..

dgeesaman

iTrader: (7)

Is the embrittlement of apex seals caused by boosting / overdriving on a cold engine?

Technically speaking, what process is making them brittle?

Dave

BLKTOPTRVL

If you are right about the valve in the piston engine reliving pressure, it would be interesting to see if a rotary could be built with a small relief valve - built into the housing - that would open under preset pressure, working like the wastegate on the turbo.

911GT2

That would vent out any boost from the turbo when it got into the compression chamber too wouldn't it?

I don't know what kind of pressure we're talking about here during detonation, but you figure 10psi gets into the engine, and then is compressed by the rotor further during its compression stage, so it would get up there...

I think a valve would just vent any pressure created by the turbos unless the pressure during detonation was considerably higher.

dgeesaman

iTrader: (7)

I wonder if there is a valve that would stay open until the moment of ignition, then shut during normal firing. But this idea has surely been considered - I think Mazda isn't stupid - it either wouldn't work or cost wwaaayyy too much.

Dave

stik6shift00

I never knew this about apex seals before...thanks alot guys!
Cant you buy something that prevents knock...(I cant think of it....)

Kento

In "normally ignited combustion", the burning of the intake charge in the combustion chamber starts at the spark plug (or plugs, in the case of the rotary) and travels outward from that point. Detonation is literally that; a spontaneous ignition of the intake charge (rather than a controlled burn) in the combustion chamber. Kind of like the difference between lighting gunpowder by itself, or igniting it in a firecracker. You can imagine the difference in combustion pressure from there.

dgeesaman

iTrader: (7)

I was just reading about that topic. For anyone interested, this link shows that the burn in a piston engine is dramatically faster than normal ignition:
http://www.stanford.edu/~bmoses/knock.html

and this link mentions double the peak pressure for detonation vs. normal ignition.
http://www.aera.org/Members/EngineTe...997/engine.htm

Until reading about the stoichiometric a/f ratio and how temperature increases strongly with the leanness of the mixture, I had always been a bit foggy about the difference between detonation damage vs. overheating damage. Turns out (as many of you know), detonation is the most severe symptom of lean conditions, and the most instantly destructive. Lean a/f does the same basic thing, except the excess heat can damage seals other than the apexes (side seals) as well.

I'm still going to do some reading to try and determine what makes the detonation so much more destructive compared to a piston engine.

Dave

Kento

If you think the burn (known as "flame speed") is fast in a regular piston engine, just imagine how quick it is with an intake charge two to three times as dense in the combustion chamber (due to the turbo forcing that charge in there). This is why ignition timing is so critical with forced induction engines (and why compression ratios need to be dialed back compared to NA engines); the slightest bit too far advanced, and your engine is toast in the blink of an eye.

Interesting fact: when you get to the point of pre-ignition (which, obviously, is worse than just detonation), the temps are so high during the combustion process of pre-ignition that it actually starts using the combustion chamber metals for "fuel" to burn. This is what causes pistons to get "eaten away" like one of the photos in the first story.