In theory, Wankel-style rotary internal combustion engines have many advantages: they ditch the cumbersome crankcase and piston design, replacing it with a simple, single-chamber design and a thick, plectrum-shaped chunk of metal that spins around inside that chamber to create virtual combustion chambers. This saves weight and maximizes performance-to-weight. Unfortunately, these types of engines are also known for burning a lot of oil and endless seal troubles, especially with early rotary Mazda engines that easily died.
Yet even 1980 versions were not without issues, a case in point is the Mazda 1st gen RX7 with a 12A rotary engine that the [Baked Beans Garage] over at YouTube got their paws on. Starting with unsuccessful attempts to make the car start, the next step was to roll the car into the morgue garage for a full teardown of the clearly deceased engine.
About 35 minutes into the video, we get to the teardown of the engine, with its parts contrasted with those of a newer revision rotary engines alongside illustrations of their functioning, making it as much an autopsy as a detailed introduction to these rotary engines. Technically, they also aren’t the original DKM-style Wankel engines, but a KKM-style engine, as designed by [Hanns-Dieter Paschke]. [Wankel] didn’t like the eccentric KKM design, as he thought it’d put too much stress on the apex seals, but ultimately the more economical KKM design was further developed.
During the autopsy of the 12A revision Mazda engine, it becomes clear that it was likely overheating that killed the engine over the course of years of abuse, along with ‘chatter’ marks of the apex seals destroying the inner chrome coating. This would have compromised compression and with it any chance of the engine running, not unlike a piston engine with badly scored cylinder walls after ingesting some metal chunks.
While the Mazda 12B and subsequent designs addressed many of the issues with the early rotary engines, its use was limited to some sports models, ending in 2012 with the RX-8. The currently produced Mazda MX-30 does use a rotary engine again in its plug-in hybrid version, but it’s only as a range extender engine that drives a generator. Looking at the internals of those Mazda rotary engines, it’s easy to see how complex they are to keep running, but you cannot help but feel a little bit of sadness that these small-but-powerful engines didn’t make much more of a splash.
They’re not any more unreliable versus anything else if you follow one simple trick:
Don’t run OEM 2-piece apex seals, premix your fuel correctly (80-ish:1 synthetic two stroke and fuel), run the mechanical cam advance up one or two clicks, don’t run factory timing (do about two degrees less leading advance at peak than Haynes spec, but a little more initial), and run colder plugs than Mazda spec (the spec was written for 80s Japanese gasoline).
Congratulations, you’ve just eliminated varnish starvation at low RPM, eliminated hot spots and have stopped the seals from chipping… and now you’ve got a ~250k mile streeter 12A that’s basically immortal… The greybeards figured a lot of this out by accident in early scene, but data loggers confirmed it later.
that’s really useful, thank you! what does it do to mileage? I’ve been meaning to grab one of these engines for a ultralight plane.
Regarding what it does to mileage, surprisingly little. What really ruins mileage is throttle pumping loss, mixture tuning, VE losses from poor metering and cruise AFR. The real lever for rotary miles-per-gallon is cruise AFR + ignition curve. If you keep the mixture safe and run cruise at ~15.2-15.5:1 AND your leading timing is slightly less at peak but slightly more at cruise… MPG actually goes up. The stock map is weirdly wasteful. In testing:
Bone stock 12A, typically 17-21 MPG US
The above “immortal recipe” + cruise AFR tune = 19-24 MPG US
The “magic” is the seal longevity recipe above, which actually reduces cruise EGT, thus less residual heat -> better volumetric efficiency next cycle -> less throttle to maintain same power -> less fuel. The irony is that the stuff that makes the apex seals “live forever” is mostly the same stuff that makes the engine use less fuel at steady state.
With ultralights the duty cycle is quite different (you’re operating at fixed power for long durations, and thus a fairly constant temp region) so your mixture and timing stability matter more than throttle response. Where the road formula overlaps w/ ultralight benefits is this:
1) lower EGT for same torque
2) OMP enrichment + premix helps solve the worst part of aero use: hot idle/hot restart on the ground (think taxi choke -> varnish wipeout). This is where many flight-adapted rotaries die… ground idle/hot soak, not mid-air.
3) the longevity curve tends to produce the same power for less throttle -> less fuel.
In other words, the very tuning profile that extends apex life also optimizes the exact regime aircraft engines sit in 90% of the time. That said, you must tune the ignition map “conservatively dumb” at the top, because unlike a car you can’t back off instantly. Street greybeards typically shape the timing curve like this:
1) more timing between 2200-3500
2) less timing by ~7000
But aircraft guys usually fly in the 4500-6000 plateau, so tune accordingly. Advance plateau -> smooth ceiling -> no dip at the cruise band. In other words… flat, not peaky.
I put 240k on my 83 Rx-7 LE over 13 years or so. Just did the recommended servicing. Never had to add extra oil in between regular oil changes. The catalytic converter rusted out a few times and the plastic trim inside was getting brittle (no garage).
It was a blast to drive.
Is the coolant corrosion caused by galvanic corrosion?
I was hopping you would comment on the the success in the 24 hours La Mar race which they won out right.
Only for the rules to be changed so it couldn’t happen again.
it had a massive weight advantage in the rules and Le Mans was the only race it won, because it was quite slow compared to the competition, and the rules wasn’t changed because of it. The rule had already been made much before it the following year all engines other than a 3.5L “F1” engine would be banned
That’s not even remotely true. It won not because of a massive weight advantage, but because the R26B had absurd reliability at sustained 9,000+ rpm. It could run at full stint pace without backing off, in stark contrast to Mercedes/Jaguar/Porsche, which had to throttle down to survive the distance. Mazda didn’t. It ran full chat the entire time. It won because it was the most consistently fast car that DIDN’T BREAK. While some of the other cars had higher single-lap peak speed potential, the 787B repeated it LAP AFTER LAP… which is monumentally important for endurance races. The following statement is historically and categorically true, verifiable by historians… with verifiable provenance: the 787B won the 1991 Le Mans. It beat everyone outright. No other overall winner in 100+ years of Le Mans has ever had a rotary or anything at 10k rpm without reciprocating pistons. It was obscenely reliable and could run as fast as it could go as long as they wanted, which is why it won.
My father built some helicopters and gyrocopters using the 12A (Rotorway Scorpions and Air Command gyros). The weight, smooth running, and optimal RPM band were well suited to his use case. However, they lacked the torque of a piston ICE; he always had to be “ahead” of the throttle to keep the rotor RPMs up as recovering a low rotor RPM was not instantaneous.
An idea that will not die.
https://www.liquidpiston.com/
I love the fact that he does all his mechanic-ing barefoot.
I do much of mine in shorts and T-shirt while barefoot – time and a place to get the safety gear, always conscious of the risks, but being uncomfortable all the time around those times isn’t worth it (unless of course you are working for somebody else as keeping your job probably is worth it) – maybe I’ll drop a pretty harmless spanner on my toe or something (never happened yet), but all it’s going to do is bruise maybe at worst fracture the bone – sure a steel toe cap would save you, heck regular footwear might well spread the impact out enough, but the risks are low.
My 1986 RX-7 N/A ‘died’ at 276k miles, side seals had started giving way, but was still running. Turbo allows these to run free at the cost of longevity without taking on some modifications.
N/A engines last way longer than most people probably realize, in the RX-8’s case they should have all been 6-port motors.