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The Misinformation of the Rotary Engine PT.2 – yOu’Ll BloW YOuR ApeX sEalS

FORWARD:  In this series of posts, I will attempt to de-mystify the rotary engine, from my perspective.  I’ve been an enthusiast of the rotary platforms for around 25 years now.  I do not consider myself an expert in engine building, tuning, or anything like that; so this series will be more focused on how the spread of misinformation through word of mouth, the internet, and ultimately social medial has skewed the general understanding of the rotary engine.  This is the second in this series, please see PT.1 here: The Misinformation Of The Rotary Engine PT.1 – The Overarching Cause Of The Misunderstanding

Yes, this was my last bullet point in my first article, but really, we can’t talk about rotary engines and misunderstandings without talking about APEX SEALS!  So, let’s start off by saying:  Apex seals do not suddenly explode.  Over the years, this particular part of the engine has gotten a reputation that insinuates that they are an inherent engineering/design flaw of these engines.  I’d argue that is completely false.  They are, however, a ‘weak link’ so to speak, so when something does go wrong within the engine, the apex seal is generally the item which ‘fails’.  Think about this; if you have a drag car, with more power than was intended, and are launching the car, there’s a good chance you will break an axel.  Is that axel a design flaw?  I don’t think anyone would argue that it is in that situation.  It’s just the next part in line that would need upgrading, right?  I believe we can look at apex seals in a similar way.  So, if it’s that simple, why do the apex seals get such a stigma about them, when an axel doesn’t?  Well, I’ll lay out several reasons I believe this happened.  Spoiler alert:  it is rarely the seal’s fault an engine will fail.  It’s more often than not, a symptom of another cause.

First, there are several ‘seals’ within the rotary engine.  Apex, Side, Corner, Coolant, etc.  but often when someone has had a failure of any of these seals, they simply state: “I blew a seal.”  This is then interpreted by the people not given the full explanation of the failure (or that have a lack of understanding of the failure) to mean “I blew an APEX seal.”  If you’ve read my first article in this series (and understood my point, through my poor writing) you can see that game of telephone starting.  And here we have our foundation for which the apex seal meme is built.  Of course, there is more to it.

Next, we need to understand that the apex seal is a wear item.  The same way any other part, in any other engine, that experiences metal to metal contact is.  This bullet point is really more aimed at the MSP engine, as apex seal wear was not really a phenomenon in the preceding engine designs.  I’ll get more into this topic when I do an article on the RX-8 as a whole, but I’ll quickly point to two reasons here for the sake of my argument.  One, early MSP engines did not have sufficient lubrication from the OMP.  Two, the apex seals themselves were shorter, and therefore once they started to wear, the seal spring did not have enough force to press the seal against the housing.  Both lead to lower compression, and “blown apex seals.”  Related to this, are super hard apex seals.  This was far more common in the past as a way to deal with detonation (which I’ll discuss here a bit later).  In this instance, if the seal isn’t lubricated properly, it will lead to the seal eating away at the chrome of the housing.  Again, we have lower compression and a “blown apex seal.”  Yes, in the first example the seal itself does fail, but in the second, nothing happens to the seal at all.  It almost does its job too well.  The point here is, poor lubrication leads to a failure associated with the apex seal.  A symptom, not the cause.

Now, the big one…detonation, specifically predetonation.  This is the main cause of “blown apex seals.”  Detonation can destroy any engine, but yes, the rotary is far more likely to completely fail during any knock event.  It’s not just the apex seal which will fail in these instances though.  Corner seals crack (especially when someone is excessively bouncing off 2-step); side seals stick; rotors dent…. heck, if the event is bad enough, rotors will contact the housings/plates and ruin them, along with the bearings/oil control rings/stat gears/e-shaft.  We could ‘blame’ any of these parts for the failure, but that would be passing the buck.  Here again, the apex seal failure is an effect, not the cause.  The cause in this instance would most likely be poor tuning, or bad gas.  Back when ECU, ignition, fuel systems, fuels weren’t as sophisticated as they are now, we had several ways of dealing with detonation.  We had the afore mentioned super strong apex seal, or 3mm apex seals, or even ‘knock elimination’ devices (they were basically a dummy spark plug for the trailing plug).  Admittedly, these “band-aids” were in place to “combat” a perceived apex seal issue, but again we were pushing these engines past their engineered design.  Yes, a failure was bound to happen, and as we’ve discussed earlier, the apex seal would be the weak-link in the engine.  So, I’ll concede here that the seal does in deed need to be looked at when deciding how to build a rotary engine for a particular goal.  Even after knowledge, tuning technology, and fuels progressed, this Achilles’s heal has remained.

One of the trends today when choosing a seal, is to have a seal that is soft enough to be very gentile on housings, but also to be strong enough so when there is a detonation event, the seal will simply bend.  OEM seals have always been the go-to seal for those looking to keep housings reusable, but the OEM will tend to shatter when things go bad.  This can cause shards of seal to be sent into turbos or even embed into plates/housings and exacerbating the cost of an engine rebuild.  So, a seal that just bends will allow for an engine to be rebuilt and simply need new apex seals (and probably seal springs).   I’d like to discuss a situation in which a company, that builds such a seal, came under some heavy scrutiny about a year and a half ago:  i-Rotary.  A brief history; the i-Rotary seal was developed by Dr. Francesco Iannetti, who has over 3 decades of rotary seal research, development, and testing under his belt.  His seals were basically what Mazda used in their competition engines.  He knows a thing or two.  Anyhow, he came out with a new seal right around 4 years ago.  Then, during covid times, there was seemingly a growing issue with these seals.  They were constantly ‘bending.’  This got the community in an uproar, and people were coming forward left and right with similar issues, blasting all over the FB groups and other social media platforms.  However, over on the forums…. yes, the rx7club forum is still active, and still has a lot of dedicated people who continue to push and develop this platform…. a common theme was found with the vast majority of the cases looked at:  spark plug choice.  The long and short is; it is believed that the plugs, that have historically been the advised, were found to be too hot of a plug for the big boost/sustained high rpm builds that have become the norm.  The plugs were retaining so much heat that they were warping the housing around the trailing park plug hole and exhaust port [REW engines].  Thus, causing the seal to ‘warp’ and the engine to lose compression.  Now, it should be noted, that there was also a correlation to Haltech switching to the MSP software, and there was some evidence put forth that this may have been the cause.   It’s a debate which still has no definitive answer, but I bring this situation up because the vocal majority were blaming apex seals for the issues.  I would even see people who I know had no experience with i-Rotary telling people not to use them.  However, again, if one of these situations are the true source of the failures…. people were once again on the bandwagon of blaming apex seals, not the root cause.  [A note just so the full picture can be seen:  The cause of these issues could also have been poor seals out of the box.  Possibly from a manufacturing defect, or something along those lines.  However, i-Rotary never came out with an official statement that confirmed that to be the case.  So, without an official statement, we can only speculate.  Also, there were several other ‘warped apex seal’ cases that came forth during this time period which did not use i-Rotary seals, but did fit into ether one or both of the scenarios discussed above.]

I’ll leave you with this; a recommendation to anyone looking to build a turbo rotary for a fun, weekend warrior, type build.  Today we have seals for virtually any type of engine build.  I am a huge proponent for the gentler on housings seals which would tend to bend if something were to be off on the tune.  Yes, these seals tend to be a bit more expensive, but if something were to happen, the cost of the seal would be a drop in the bucket over buying new housings, rotors, plates, possibly a turbo, etc.  Let me go back to my broken axel analogy.  If a weekend warrior type builds a car for the drag strip, and spends a ton of money on engine, suspension, drivetrain, sticky tires, but leaves the axel as the weak point.  Then goes to the strip and gets a pass or two down, but on the third, breaks the axel.  Well, I believe that’s a win over that axel being beefier and transferring that energy into the differential, or the transmission.  Heck, I actually know people who take multiple axels to the track and if one breaks, they are able to change it out, and make more passes the same day.  Obviously, I’m not saying that replacing apex seals in a rotary is as easy, and certainly not as cheap, as replacing an axel.  But the end result is similar, you can save your really expensive bits by having a (relatively) cheap ‘weak point.’   Let me reiterate, this obviously does not apply to high end racing or people looking to get every last ounce out of a car.  We are talking about more of the weekend warrior-types…. and all this is just my perspective.

As I said in my first article in this series, I am no rotary apologist, and I do acknowledge that the apex seal is a weak link to the rotary engine (even more so with the RX-8…but we’ll get to that topic one day).  But I do believe the game of telephone, which I also discussed in that article, has turned the apex seal into the bad guy, the scapegoat, the reason to avoid a rotary engine like the plague.  I just don’t think that’s justified.  I’ve seen far too many comments like: “oh, that engine has over 80,000 miles, better change those apex seals” or “your apex seals are about to be toast.”  These are generally from people who have no experience with a rotary engine at all, but these comments dissuade others from getting into the platform.  Think about this, the naturally aspirated SA/FB and FC engines were as close to bullet proof as you could get from Mazda.  It’s not unheard of for one of them to go over 200K miles.  Heck, when I owned my first FC, I had virtually no mechanical aptitude, and would constantly run it low on oil and coolant.  Apex seals were never an issue…. coolant seals though…. they were an issue.  Look, I’m not saying we need to start a Save the Apex Seal movement or anything.  The subject isn’t that important.  It’s simply something that I’m passionate about, and I hope someone who is thinking about getting a rotary car can read this and get a better understanding of what they are getting into.  Cause facebook, youtube, tic tok, etc. aren’t helping people to understand.  They are just parroting old misunderstood, misinformed talking points.

Goober Goalie

Just an old guy who's into rotaries and all things 90s Japanese. Been involved in the rotary community for over 20 years now, and have dabbled in other platforms over that timeframe as well. I'm so old, I took a modified FC RX-7 to see the original FnF in theaters. I'm looking forward to sharing my experiences and learning some new things from this community.