Revved Modification Map: Power Thresholds For Tuning Mazda’s 13B-REW

The 13B-REW arrived on American shores in 1993 under the hood of the newest Mazda RX-7. This sleek and compact sports car was not alone, however. Alongside a fleet of new, expensive, overengineered Japanese sports cars, the Mazda RX-7, known among fans as the FD3S, aimed at establishing itself among the German and American offerings accepted as the dominant machines on the market. Turbochargers and compact size worked in the 13B-REW’s favor, but a reputation as an unreliable motor spread quickly, and that widespread assumption still exists. Though a turbocharged rotary is a demanding motor with a slew of idiosyncrasies, if understood and tuned properly, it is a piece of art.

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Part modern art, part weapon, the 13B-REW has divided opinion since its inception over twenty years ago.

Calling it anything other than high-maintenance would be euphemistic. Of course, a talented machinist and clever tuner can make a rotary last a long time, but in reality, many 13B-REWs meet their demise early on because they’re either run too lean, use the incorrect fuel, or don’t get enough oil. If an owner can avoid these problems, they’ll find that a harmonious existence can occur between themselves and their motor. Therefore, looking for gains in power requires constant vigilance when it comes to timing, boost pressure, fuel flow, and the like.

Stock Turbos: 300 Spiffy Stallions

As the FD3S was a typically-overcomplicated sports car of the ’90s, its stock HT12 turbochargers deliver power sequentially, meaning one turbocharger spools up first, covering the lower end of the rev range. As the tachometer traverses the 4,700 rpm mark, the second turbocharger engages with an odd gap in power, and being larger, it carries the remainder of the revs. The objective was a wide, responsive powerband without much lag, and for street purposes the stockers are often adequate. Nevertheless, they’re derided because the system includes a vacuum and pressure chamber, four control solenoids, and yards of hoses — all of which are subject to cooking in the FD’s engine bay.

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Pulled free from its chassis, the 13B-REW’s compact size is apparent.

Retaining the stock turbos is the least expensive means to increasing power — to a point. With the addition of a few crucial modifications, Rob Golden of Pineapple Racing notes “a set in good condition should push enough air for 350 whp at 15 pounds of boost, which is the maximum the stock turbos will take before they run out of steam. That may not seem like much power by today’s standards, but since the FD3S is a featherweight, those ponies feel more like Clydesdales.”

Getting a rotary to breathe is essential since these engines thrive on high revs, and with turbos, a reduction of backpressure is always a good thing. Downpipes, a full turbo-back exhaust, a high-flow cat and a decent conical filter will start things off well. Aside from the improved spool time and overall power, a downpipe can help the engine run cooler.

Stock or mildly modified, the factory turbos make the engine bay into a labyrinth.

With increased airflow, the fuel rate must be increased to compensate. Programmable ECUs like the A’pexi Power FC monitors temperature, knock, boost, injector duty cycle, and ignition timing. It can even improve throttle response at low revs. The less sophisticated options include modified engine management computers from brands like Mostly Mazda or Pettit Racing. However, some of these cannot manage the increased flow rate from larger injectors. Additionally, the MAP sensor controlled by the stock ECU will not register increased manifold pressure from upgraded intercoolers or intake systems. One can get by with minimal mods and a mildly upgraded computer, but they’re rolling the dice.

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Sitting on the left edge of the center console, this A’pexi Power FC looks totally inconspicuous.

A new fuel computer allows the user to bypass the stock 10 pounds boost limit, moving up to 13 pounds. With a front-mounted intercooler and larger injectors, 15 pounds is possible — but not before the fuel pump is modified, and the offerings from Bosch or Tomei are some of the more popular within the FD community. Typically, some minor cooling modifications like silicon hoses and a water injection system find their way into the upgrade list at this point, and round out a well-sorted, responsive and reliable 13B-REW capable of covering the quarter-mile in a little over 12 seconds. The first steps are detailed, but they set the groundwork for bigger turbos and more power, and from here on out, few supportive modifications are needed.

Hybrid Twins Team For 400 Horsepower

For a factory appearance and a similar powerband, the BNR Stage 3 turbos are an attractive option. Though they don’t inherently make more power than the stock twins, they are more efficient. Whereas the stock turbos will begin to run out of steam at 15 psi, the BNRs are capable of 20 psi before dwindling off, thanks to greater efficiency and larger inducer and exducer specs. They can produce 430 horsepower comfortably with only a modest increase in turbo lag. They also come with an external wastegate, which allows boost pressure to be adjusted mechanically. This reduces strain on the boost controller, which becomes more effective as a result. Their installation only requires an upgraded the MAP sensor, as the stock piece maxes out at 16.5 pounds of boost.

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RE Amemiya’s famous touge monster FD uses hybrid turbos to give it incredible response and a progressive power delivery for those narrow, bumpy backroads.

Ultimately, the twin turbochargers available for this car are somewhat limited. The flow rates will make for an impressive, responsive and tractable motor, but for those who want to reduce some of the mess that the sequential setup brings, dissipate heat and make additional power, a bigger, single turbocharger is the way to go.

Going Single For 500 Horsepower

Going single requires a good deal of money, but it cleans up the rat’s nest of vacuum lines, and allows customers to work on their own cars with far fewer headaches. Is that peace of mind and ease of access worth the added cost? With modern turbo technology and the ease with which power levels can be transcended now, it’s a definite yes. At this point, the intent of the car needs to be considered.

As most street-driven cars will only see 7,500 to 8,500 rpm, choosing the turbo with the best-matched hot side is imperative. The GT35 offers a great compromise between response and top-end power, seeing as much as 10 pounds of boost by 3,500 rpm, which doesn’t put it too far behind the response of the stock twins. Additionally, there’s no odd transition between turbos with a single turbo, and the pull towards the top of the rev range is infectious.

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A single turbo reduces clutter and heat issues.

Seeing as the modifications needed to fully exploit the stock twins already support the single turbocharger, the biggest expense is the turbocharger itself. Of course, a new manifold is needed, but there are a few fundamental changes to the motor which should be considered, increasing oil pressure being the first on the list. This can be achieved through a dry sump system, or using an upgraded oil pump and changing to the larger front cover-to-housing O-ring available through Mazda.

As power levels increase, keeping the motor stable is a major concern. Josh Cook from Atkins Rotary advises adding more dowels and a serious stud kit to keep the motor intact at higher rotational speeds and with greater pressure loads. For the same reason, Jesus Padilla of Saint Cloud, Florida-based Kilo Racing offers a two-piece eccentric shaft to accommodate for whatever engine flex is present. However, the rotary engine is a piece of machinery which demands the touch of the virtuoso artist to realize maximum results, and different shops voice different opinions on the matter.

Two-piece eccentric shafts and stud kits are recommended for engine rigidity at higher power levels.

One of the more informative conversations an RX-7 enthusiast can have is with Pineapple Racing’s Rob Golden. In Golden’s words, “Keep it simple,” when it comes to modifying the rotary engine. In his opinion, adding more dowels should be done only when a machinist skilled in working with minute tolerances is on hand to install the right parts correctly.

He espouses the use of oversized studs on the compression side only for extreme builds above 700 horsepower. This is because “these studs keep the rotor housing from expanding under immense pressure loads, which in turn causes the dowel pin to expand and damage the cast-iron plates.” Additionally, “a one-piece dowel pin distributes load evenly across the engine, and prevents the edges from kicking out.”

In addition, he does not advocate the 3 mm apex seals which many believe are the answer to high horsepower builds. In some tuners’ eyes, the larger seals help make up for imperfections in a warped rotor, but ultimately, this is a quick fix that does not help in the long run, as inconsistent pressure across the seal will cause them to crack. Golden suggests finding a skilled machinist — and there aren’t many in the country — who can hone the apexes with superlative precision. “Find someone who’s comfortable working in millionths,” instructs the exacting Golden.

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Rather than opting for oversized apex seals, the best bet is making sure the apex seal groove is perfectly straight.

The internals of the 13B-REW are strong enough to endure 700 horsepower, so tuning focuses predominantly on the maintenance of very specific tolerances, fluid pressures and viscosity, and air-to-fuel mixture. Throw in plenty of boost and the problem grows more complex. While the old weak points of the rotary have been elaborated on numerous times, the reality is that the minimalist design requires each piece to work in perfect harmony, and that is no simple task.

Swapping To The 20B Three-Rotor In Search Of 700 Horsepower

For those who want to keep it in the family, but want more displacement or a touch of exotica, one alternative to the 13B-REW is the three-rotor motor by the name of 20B.

Stepping on the pedal is like grabbing ahold of the horizon and quickly, effortlessly, pulling it towards you. -Rob Golden, Pineapple Racing

Though Americans never got a chance to experience the thrust of a twin-turbocharged, 2.0-liter rotary, the Japanese market did through the Mazda JC Cosmo, a car that was as pricey as it was powerful.

With 300 horsepower and 297 lb-ft of torque, the 20B was one of the most powerful motors to come out of Japan during the 1990s. With another rotor and the added displacement, the thrust and response of the engine does belittle the 13B-REW somewhat. As Golden mentions, “stepping on the pedal is like grabbing ahold of the horizon and quickly, effortlessly, pulling it towards you.” For added curb appeal, the exotic shriek it makes is otherworldly and will turn heads wherever it goes. Yet, the swap isn’t terribly common these days.

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Even when fitted with a huge turbo, the visibly-larger 20B doesn’t take up much space.

This comes down to economics: the 20B swap is only for the well-heeled. Golden warns, “By the time you purchase a good, late series core — roughly $5,500 — and all the accessories, it is easy to spend $20,000 doing the install yourself. Also, they don’t drop into any of the chassis over here, so you have to pay someone to do the fabrication work or buy an expensive adapter kit.” Of course, the wow factor is there, as is the performance with modified versions cranking out 700 to 800 horsepower in ‘street’ trim. Though there’s no better way to pay tribute to the rotary than swapping one of the most exclusive versions into an FD3S, more and more owners are turning down a different path, and committing a cardinal sin in the process.

Size Is Sacrilegious, LS Swap For Big Power

Some rotary aficionados may agree with that sentiment, maintaining that less is more. But without using any tired adages, there’s something to be said about the value of displacement and the driveability that comes with it. However, the real reason behind the animosity towards this particular engine swap has less to do with the size of the motor, and more with the make. For those who don’t mind committing automotive blasphemy, there’s always the acclaimed Chevy LS.

The LS is light, plentiful, reasonably affordable and exceptionally reliable. Available in different variants from the Impala to the Corvette, an American can easily procure an LS, and why not? With an incredibly flat torque curve, a brutish, imposing exhaust note and instantaneous response, there’s plenty to like. Plus, its aluminum composition means it only weights 50 pounds more than the 13B-REW, so it won’t spoil the FD’s sublime weight distribution.

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LS engines look at home in the narrow confines of the FD’s engine bay.

Engines within the family offer anywhere from 345 to 430 horsepower right out of the box, and that’s excluding the big-block 7.0-liter LS7. The motors from the F-body (Camaro/Trans Am) are preferable because they’re cheaper and the oil pan’s sump is located at the back of the motor, which matches the Mazda’s layout. Ideally, the post-1999 engines are the best, since the piston design improved that year.

The popularity of this swap has encouraged several shops to design comprehensive, hassle-free swap kits. Hinson Supercar’s kit costs $1,700, though some ancillary parts will add to the total. This kit eliminates bumpsteer and offers a custom steel driveshaft, transmission brace, torque arm, and an aluminum subframe. In total, expect to spend somewhere in the range of $10,000 to 12,000. Essentially, the price it costs to bring a 13B-REW to similar power levels isn’t far off, and the Chevy requires less vigilance, as well as having a larger aftermarket so with forced induction mixed in four-digit power is not out of the realm.

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On-demand torque has made the LS a popular swap among drifters.

For those of us who want to keep it in the family, swear an allegiance to the rotary, or just enjoy being a little different, there are plenty of reasons to stick with the 13B-REW. Smooth-revving, capable of enormous power and defined by its distinctive bee-in-a-tin-can shriek, there aren’t many powerplants that can provide the thrills this engine can. Fireballs galore are at one’s disposal if those narrow parameters are respected and contrary to popular opinion, the 13B-REW is not hewn from tinfoil, ready to burst at the slightest provocation. No, it’s an oddity with a stout cult following that offers more any 1.3-liter ever has, and has earned some notoriety for its quirky combination of grunt, compact size, and ability to spit massive flames at the drop of a hat.

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One of the benefits of running rich.

 

About the author

Tommy Parry

Tommy Parry has been racing and writing about racing cars for the past seven years. As an automotive enthusiast from a young age, he worked jobs revolving around cars throughout high school, and tried his hand on the race track on his 20th birthday. After winning his first outdoor kart race, Tommy began working as an apprentice mechanic to amateur racers in the Bay Area to sharpen his mechanical understanding. He has worked as a track day instructor and automotive writer since 2012, and continues to race karts, formula cars, sedans, and rally cars in the San Francisco region.
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