Running Rotaries Reliably
This is my first post on how to buy the right rotary car, how to refresh it to help it be more reliable, when to rebuild it, how to rebuild it, and how to avoid being yet another rotary horror story. And NO, you do not need to LS swap the FD to make it reliable.
1 - Why does the rotary engine have such a bad reputation?
To answer this question, let me first start with my own experience: I’ve driven 100k miles on turbo powered rotaries from 1995 to today. That first car I bought in 1995 was a 1993 Silver FD from an insurance auction- it was one of the original engine fire cars that caused Mazda to do the fuel line recall.
I’ve NEVER blown an Apex seal. I’ve never caused an engine to need a rebuild. And I didn’t even treat my cars very nicely when I was 22 years old. Change the oil? Sure when the low oil light came on. Change the coolant? Never, and so on. So, why is that?
Why do so many rotary engine owners have horror stories? Here’s my take:
Bad tune. I used to wonder what “bad tune” meant. After working with various rotary owners and seeing failed rotary engines, I’m pretty sure bad tune just means this: The tune / turbo setup allowed the car to lean out. And on a rotary, leaning out is death. I’ve leaned out my 2JZ motors at 18 PSI of boost at 90 MPH more times than I can count. They just lean pop. That’s it.
If my Coco Crisp Supra with a “bad tune” was a rotary engine, I would have blown that engine 50 times by now. It really does have a bad tune, 2 years ago the FPR vacuum line cracked causing it to lean out, last year the 25 year old Greddy T88 wastegate diaphragm failed from age, just last week we found out the second fuel pump feed hose split in the gas tank causing low fuel pressure. I’ve caused this poor 2JZ powered car to lean out so many times, but it still runs like a champ.
I’ve been fortunate to live in the DFW area and I’ve taken my FDs to Chris at Rotary Performance since 1995. Chris has taken great care of me. He’s given me tunes that run extremely rich and are as safe as any tune can be for a turbo powered rotary engine. I’ve run the Power FC and lately I’m running a Link both tuned by Chris, and my car is always PIG rich in boost compared to a piston motor with a tune from master technician / rotary shop owner Mr. Chris.
And running this rich am I giving up some WHP? Absolutely. But I’d rather give up 20 WHP and get to enjoy my build 20k miles than blow an apex seal with an aggressive tune. Unless you’re daily driving your 30 year old FD (I do know some guys that do), 20k miles is basically forever.
So, what’s a safe AFR? My cars run around 10.8 AFR under boost. You won’t blow an apex seal out of boost. You’re going to pop one when you’re in boost at full throttle blowing and going. And my cars run 10.8. If a piston engine tuner tuned my car they’d never make it so rich that it was 10.8. My Supra would barely even run at 10.8 AFR. I had a Subaru WRX with a bad turbo which caused the AFR to be around 10.8, I couldn’t even get that car over 40 MPH it was so rich.
Pilot error - NOT USING A WIDEBAND and/or Not knowing what the numbers mean. I didn’t have a wideband on any of my rotary cars until a few years ago. I was lucky that most of the miles I drove were when the cars were relatively young form 1995 to 2001 so everything was fresh and working perfectly, and I ran stock ECU. My mod list was: K&N air filter. I bet my stock ECU actually ran pretty lean compared to Chris’s tunes. But, I haven’t yet hooked up a wideband to one of my FDs that still run the stock ECU, so I don’t really know.
What AFR is good? I want to see 10.8 AFR under boost. Most people are familiar with 14.7 AFR. Here’s what AI just told me:
14.7:1 is the stoichiometric air-fuel ratio (AFR) for gasoline, meaning 14.7 parts air to 1 part fuel by mass. It represents the chemically ideal point for complete combustion, where all fuel is burned using all available oxygen, maximizing efficiency and enabling three-way catalytic converters to operate optimally.
14.7 AFR is death for a rotary under boost. Absolute death. Piston engine tuners shouldn’t go anywhere near a rotary tune.
Horror stories from my friends and how to avoid being like them:
I have a friend that blew two motors past 2 years.
First motor - Rebuilt by a local shop that sold the car to him and built the motor years previously. The $19 throttle body coolant hose from the rear iron to the back of the throttle body burst. He saw the low coolant light and heard the blaring alarm and pulled over. Too late, engine was running rough and needed a rebuild.
So what’s the way to avoid repeating his error? Whenever I refresh / restore any of my cars, I replace EVERY SINGLE COOLANT hose. EVER SINGLE ONE. There are 15 on USDM LHD cars and 14 on RHD cars. Almost no one else does this. I actually replace about $1250 worth of gaskets, O-rings, fuel lines, oil pressure sensor, fire recall fuel hose between the primary/secondary rail, fuel pulsation damper that causes engine fires from age or > 60k miles, literally every wear item under the hood. For $1,250 I won’t leak oil, I won’t lose all my coolant, it won’t burst into flames, and I won’t leak anywhere. Those parts at MSRP are around $2,500. Message me for a list of the parts and where the best price is for them.
I can drive my car from my shop in Allen, TX to Michigan and I won’t blow a coolant house on any of my FDs (I own 6 right now). And I replace the water pump radiator fill cap and the AST pressure circle cap. When these caps are 30 years old, the rubber is hard, it has a permanent indention, and it’s going to leak eventually. $60 on ebay and problem avoided. And if you have the old 2 part thermostat housing, replacing the $4 O-ring prevents another guaranteed leak. Even my 29k mile cream puff always garaged inside automatic to manual single owner car leaked out the coolant when I parked it after driving just 10 miles to a car show. Chris sold me the two radiator caps and problem solved.
The second blown motor was from a bad tune from about 1-2 years ago. The shop that rebuilt and tuned his motor is another Texas shop that is no longer in business (good news for rotary owners everywhere). They screwed up the tune so badly that it leaned out so hard that the gas detonated on its own before the spark plug even fired. He has 2-3 HUGE dents on each rotor well ahead of the compression pocket. There is NO DOUBT whatsoever that his car leaned out and destroyed his motor.
Another shop quoted new rotors and rotor housings at full MSRP and the total estimate: $13k. That’s pretty much the going rate for a rebuild where the shop has to pull your motor, strip all the accessories, tear the motor down, professionally clean all the motor parts, replace the stationary gear bearings and rotor bearings if the motor has any mileage on it, add new rotor seals, add new internal O-ring / sealing kit, they should clean up all the accessories before re-installing them, they should replace all of the $1,250 parts that I do, they have to re-install the engine. This is a roughly 60 hour job. At todays labor rates say $155-185 / hour + parts, do the math.
I have a friend I sold an FD RX7 to in 2022. I installed a brand new Mazda crate motor, drove the car 3500 miles before selling to him. He blew it up within 1k miles. Why? He changed the exhaust to a 4” Tomei Midpipe and Catback which was extremely loud and higher flowing than what I had it tuned with. We also changed the stock intercooler to a SMIC without re-tuning it. He over-boosted it on the twins by going 8k RPM in 4th gear (something like 130 MPH). DO NOT MAKE ANY CHANGES WITHOUT RE-TUNING THE CAR. What’s the issue?
The stock twins employ an internal wastegate. EVERY internal wastegate is insufficient to be able to control boost at some point (Even Turbo source admits this may happen on the page where they sell their excellent IWG turbo kit). With a wide open exhaust (even when the wastegate is ported the small amount that is possible), the car will over-boost the longer you hold it in gear. My own stock twins car with open exhaust would probably pop the engine if I drove it as hard as he did. Since I don’t want to get arrested, I don’t drive it that hard so I don’t blow my engines this way.
The external wastegate on even my medium sized turbo HKS Special Setup on my Dom car is 46mm. The one on my Coco Crisp T88 is 60mm or so. Internal wastegates are great, for you stock turbo powered car made by Ford. They cannot bleed off enough exhaust to prevent over-boosting unless you run a restrictor somewhere in the exhaust (or have a hi flow cat or full cat to act as that restrictor).
He didn’t have a wideband, he didn’t check the wideband, he has no idea what the AFR was when he broke an Apex seal which spun around in pieces and destroyed one of his rotor housings. His car has been down 3 years (fewer rotary shops, fewer rotary mechanics, higher percentage or very old rotary powered cars that need rebuilds). We got it running mid 2025 finally, and it’s almost done. If he had a wideband and knew what #’s to watch for, he might have seen how stupid lean it was getting and he could have backed off before epically beating some beater Lexus at 140 MPH.
TIPS FOR SUCCESS: If I could ask every rotary owner to do just a few things to help us rotary owners not have such a bad reputation it would be this:
Heat kills rotaries:
When you get your rotary powered car or if you’ve owned it for awhile, repolace EVERY SINGLE coolant hose, replace the two radiator caps, replace the 30 year old AST tank, replace the 15-20 year old first generation Pettit Racing AST tank that corrodes and is thin at the cap. Pettit makes an awesome billet swirl pot AST tank - install that. One of my OEM original Mazda plastic AST tanks was fine, then I installed it on my Dom single turbo and it leaked just a tiny bit from a crack in the 30 year old plastic. That could have cost me my $10k motor.
Message me for a list of the 15 part numbers. Hit the “Contact us” button at the bottom.
Boost kills rotaries:
Get a wideband AFR, watch the AFR reading at least every other pull. Know what’s a safe AFR, and back off the throttle if you’re not < 11.0 AFR.
There are 15 reasons why your car might be leaning out. All you need to know is that it’s leaning out and then back off the throttle, drive it home without boost, and get it checked out. A 30 year old fuel injector might be clogged (I flow test them and often see 20% blockage), the FPR vacuum line might have a crack in it so the fuel pressure doesn’t go up as boost goes up. Your fuel filter might be clogged, you might have a split line in your feed line in the gas tank like my Supra did. Any of these reasons cause death to a rotary. Easy to avoid, just watch your AFR and back off the throttle when necessary.
Stock ECU kills rotaries / get a Power FC tuned by Rotary Performance or comparable
Everyone wants to open up their exhaust, they want a free flowing intake, they want a bigger stock mount intercooler or V mount or FMIC. If you do more than 2-3 mods on the stock ECU that’s already running lean, you will lean out your car and eventually blow an apex seal. Just adding a midpipe on a stock ECU car will eventually blow an Apex seal. I know this because I sold a car on Bring a Trailer for someone. They ran stock ECU and a midpipe, and they blew the engine from leaning out.
So what’s the fix? Every FD except the poor folks in CA that can’t change their ECU out under penalty of death should run a Power FC that is setup with a fuel table that is far more rich than stock. AND the Power FC should be set to turn the cooling fans on at 91C and off at 89C like Chris programs my cars to do.
The stock ECU turns the fans on at something like 105C coolant temp. So the stock ECU turns the fans on after you have already started to actively rapidly destroy the coolant seals inside your engine. 105C coolant temp while running the car is death to the coolant sales. Mazda set the fan turn on temperature too high. Fix it.
More cooling tips: Heat kills rotaries:
For advanced users, change your FD fans from 3 speed to single speed. This way if the A/C demand switch is on, or if the ECU sees hi temp and turns them fans on, they run at high speed. Message me for instructions on how to do this (I’ll try to post it later when I have time). I do this to ALL of my FDs.
Replace the ridiculously small single row OEM radiator with a Koyo or similar (ideally N flow) radiator. It’s kind of a pain because the N flow is larger and pushes the intercooler up if you’ve upgraded beyond the stock intercoler, but it’s worth it. The single radiator is about half the width of the end caps. It’s just sad to look at. Heat kills rotaries, and Mazda put a radiator in that would barely cool a Mazda MPV minivan.
Replace the front bumper with a 99 front bumper. My friend has a car setup just like mine and his even has less power. I run considerably lower coolant temp than he does on the same day. We think the reason is I have a 99 spec OEM Mazda front bumper which has a giant air inlet compared to the 93-95 style front bumper. Airflow is king. It looks cool and it flows more air.
Be sure to have your undertray installed. Chris saw a car I took to him 4 years ago and the undertray was off since I was still messing with it. He told me to reinstall it as it helps with cooling. I ran a car this summer and got about a 5C coolant temperature reduction after adding the stock undertray. I had it off since I’d just installed the engine and didn’t want to fully button everything up yet.
Get a Power FC with OLED commander and get the commander mount from Banzai Racing. WATCH THE COOLANT TEMPERATURES. If you’re running over 91C even in Texas during hot summer days on the highway, something is wrong. It might be as simple as an air bubble where you need to bleed the rear throttle body coolant line. I did that on a car running 95C past summer and it dropped to 89C. Just a coolant air bubble.
Never believe the stock coolant gauge. When the car is cold, it’s at 7 O-clock. When the car is getting warmed up say 60C, it’s at 8 O-clock. When the car is happily running 89C it’s at 8:30 (somewhat less than 9 O-clock). When the car is overheating and actively melting your coolant seals, it’s at 9 O-clock. When your engine is a little campfire at 105-110C and you might as well just rebuild it, it’s still at 9 O-clock. No one ever told me this / just figured this out last year.
The stock coolant gauge is not linear, it’s misleading, and it will lead to rotary engine death. Just get the Power FC and read the actual digital value (in Celsius). Slow down / turn the heater on full blast if its 95C or higher.
Note that when you park the car, it heat soaks. So when you start the car 25-30 minutes later it might read insanely high like 115C. That will cool down within 1-2 minutes. This is alarming but not a cause for alarm. It’s just how things are.
This might sound like a lot, but it’s really pretty simple if you read through it a few times and make a few changes to your car. And I do work on a few friends’ cars a year to help cover the costs of my little hobby, so if you want “not quite a real shop”, but “way better than a shade tree mechanic” message me and see if we can become friends since I only work on my friends’ rotaries. Happy Rotoring!