Building a Reliable, Monster-Power Turbo RX-7

Growing up on dirt bikes, NASA Mid-Atlantic racer Carson Stone quickly acquired a sixth sense for finding available grip in loose conditions. This made his teenage transition from two to four wheels much simpler. Drag racing held his interest briefly, but then an open-eyed friend pushed him to try something new — something he wasn’t that fascinated by, really.

His friend Drew Hall sent him an ad for a 1993 Mazda RX-7, and the price was too low to ignore. It came loaded with a bunch of period-correct parts: a T67 turbo, a noisy twin-disc, and an Apex’i Power FC piggyback — everything a budding tuner could want back in 2005.

“I knew nothing about rotaries, but I dove into YouTube to learn. I sent the car off to a rotary shop in Kannapolis called RotorSports Racing. Bryan Smith, shop owner and former RX-7 racer, fixed the fuel leak and transmission seal, then street-tuned it. He gave me some pointers on keeping the motor happy, and then added an air-fuel gauge. If you know how sensitive to detonation these cars are, you know how vital an AFR gauge is,” he elaborated.

Creeping Confidence

Within a month of owning the car, an apex seal broke and forced him to replace the motor before he could get on track.

Stone returned to RotorSports for a street-ported 13B-REW engine and stayed for an education. “The project depended on Smith’s help, which he provided because he enjoyed seeing someone young and interested in rotaries. I wanted to know why it went wrong and how to avoid the problem going forward,” Stone said.

His friends wouldn’t let him leave the car sitting in his garage, though. Enter Mr. Hall and his push toward tracking the car. Being an optimist, Stone agreed to a track day at VIR. He even added a set of CXRacing coilovers and Wilwood big brakes.

Sadly, the motor wasn’t ready. Above 4,500 rpm, it choked. “I was topping out at 80 mph with some weird hesitation, but I was already on track, so I drove as fast as I could. That was fun enough on a set of Toyo R888Rs, even if it rolled too much, because a bad day on the golf course is better than a good day at work, I say.”

Afterward, he learned the fuel filter had been bypassed by the former owner, hence the clogged injectors. Smith did not catch the fuel filter bypass as Stone pulled the engine himself.

Bulletproofing, Part One

With the fuel filter replaced, he trailered the RX-7 to Dominion Raceway in Richmond. Going alone, he had to be sure he could make any minor fixes, but Smith had given him reason to relax. “This was the first time I could turn laps at a higher pace, and, suddenly, I was catching everyone else. Finally, I was able to practice car control since I could spin the wheels without much effort,” he laughed.

Thanks to his motocrossing and avid sim racing, he was able to enjoy the car near its limit of adhesion faster than most. By sharing Garmin data with friends in comparable machinery, he was able to put in commendable times with only a few days under his belt.

“The only issue was a failing wastegate. I’d used a push-to-connect fitting on the wastegate reference line, which melted after a few laps,” he explained. “The spring was set to 14 pounds, and if I went WOT, I’d see 19 pounds on the AFR, so my foot became the wastegate. It still ran lean, but I got away with it because I ran race gas exclusively,” he said.

Researching A/N lines, fiberglass wrap, and all the other heat-related solutions brought him peace of mind and exposed the baselessness of some of the fears surrounding the rotary engine. It’s a reasonably stout engine, provided the owner follow three rules:

1. Get your premix right.
2. Avoid detonation at all costs.
3. If it goes lean, pull off.

When Stone felt confident enough to move into HPDE3, he slapped on his first set of Hoosier slicks and trailered it down to Daytona, where sustained high revs and high temps didn’t faze the motor. “I got it up to the 140s on the banking, and nothing broke all weekend. That was my green light,” he recalled.

Now that the finicky rotary had proven its resilience, Stone decided to step into sanctioned competition. With Ryan Oakes helping, he stripped the car and installed a mild steel DOM cage, bringing weight down to 2,680 pounds with fuel and driver. With that, Stone strapped it down and drove south to Carolina Motorsports Park to attend comp school.

Spirits were high at the start, but they faded when the oil feed for the turbo loosened and started a campfire under the hood during the first race of the weekend. “I didn’t want to pull the fire suppression, nor did I want to a park on the grass, and most of all, I didn’t want to end the race for everyone else, so I rolled into the pit-out lane and sped through the pits, which pissed some people off. Thankfully, the only thing that melted was the wiring to the fuel connectors.”

A Time to Be Bold

Still, one blaze is enough to give one pause, since the factory engine harness was cooked, he laid out a big plan. A Haltech Elite 1500 ECU would ensure his investment was better protected, and a new turbo manifold would save him from having to repair cracks, as he had been doing with the old Greddy manifold.

“I got a new Reactive Fab twin-scroll twin-gate manifold, liquid-cooled Turbosmart Hypergates, a Garrett G30-660 turbo, a PD16, an IC7 digital dash, and a complete Radium fuel system,” Stone said. “Most of all, I wanted to bring temps down however I could, so I set it up to run ethanol.

“The complication from flex fuel I didn’t anticipate was how it cramped my schedule,” he explained. “Rotaries are awful on ethanol, so I started leaving the classroom early to get my fuel calculations right. Getting the premix right requires time and a few beakers — it’s science. You don’t just fill up and go.”

The race motor’s compression was down, though, and he decided it was time for a full internal overhaul. “Inspection revealed the rotor faces were dented from detonation and the housings were scarred. I said, ‘Let’s go for new irons, new housings, a lightened and balanced rotating assembly, and a big half-bridge.’”

The “bridge” comes from the side seals on the rotor/corner seals being ported more extensively, leaving a narrow bridge between the primary and the secondary “eyebrow” port for the corner seal to ride on so it doesn’t fall out. If a street-porting job can be compared to porting the heads on a piston engine, bridge-porting brings the same sort of improvements as a big camshaft does. “That’s when you get the brap sound,” he added.

Once the motor was back from RotorSports, he installed the Chris Drum-designed front aero, as well as a set of RaceLouvers to the hood to help the v-mount and heat extraction.

The unfortunate trade-off in torque and top-end that the bridge-port brings rendered the Garrett turbo a bit too small. Excessive EGTs and a clipped powerband pushed him to seek out a turbocharger better suited to the application, but there was no clear answer for that online. “You can’t rely so much on forums for answers to these problems because those guys are mostly street racers. When you’re generating a lot more heat, as you do on the track, everything changes,” he explained.

Fine Tuning, Finally

After analyzing some flow numbers and turning to RotorSports and RaceOnly, he decided the larger G40-1150 variant of the Garrett turbo would bump power to just a tick under 500 horsepower, with a much broader power band and real midrange torque. “You have got to be easy on the skinny pedal now. The torque hits hard at 3,500 rpm.”

There is enough grunt to break third gear, actually. There’s a big price that comes with adding another 30 percent more power, and that wasn’t limited to the strength of driveline components, either — something he would learn about later.

Stone then went from a 17” wheel with 245-section fronts and 175-section rears to 18-inch Volk TE37s wrapped in Michelin SM8 slicks measuring 26 x 65 x 18. “The hard part with wheels and tires is that nobody makes the right offset for the FD3S,” he noted.

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Moving to a square setup enhanced stability under braking — a welcome improvement after deleting the ABS in his cramped engine bay — but added to a feeling of nervousness compounded by the additional power.

With the transmission repaired, he took version 3.0 of the car to VIR to compete in the Hyperfest HFGT class alongside Larry Chen. After crossing the line in third, he decided he’d skip waiting for the scales and go offer ride-alongs instead — a commendable way to forfeit a trophy. What better way to celebrate a reliable build than by putting it through another few sessions without failure?

Reliable is a relative term. Further running revealed that the catch can worked, but since the new turbo moves so much air, it was over-pressurizing the crankcase. After leaving left turns, smoke would funnel from the tailpipe.

“I learned this was an oiling issue, and since I couldn’t afford a dry sump, I went with a baffled sump from an Australian company called Billet Pro. It’s the shiniest piece on the car, and nobody will ever see it, sadly.”

It is the cherry on the sundae: the mark of a long-running project finally — well, mostly — finished.

Hard Road Glory

A limited aftermarket, little track-specific information on the forums, turbo problems, and a treacherously hot-running engine make the RX-7 enthusiast’s track life trying at times.

“Why do I persist in tracking it? I don’t like taking the easy road. I believe that whoever builds the fastest car should get the bragging rights. To me, I’m happiest when I’m racing on my wits and having to figure things out myself,” he professed.

“When it’s dialed, it brings me to tears because of all the time I’ve dedicated to it and the fact that I’ve never given up. Now, I take my time — I don’t want to cause a fire or an accident because I’m rushing. I’ve said it before — before it was true — when I was hoping I’d figured it all out, but it’s true now: all I need is a little fine-tuning,” he said with a sigh of relief.

“I want to get into the mid-1:50s at VIR. The power’s there,” he stated.

Finally, the motor is functioning well enough for Stone to begin studying the rest of the chassis. With the promise this finicky platform has already shown, it’s certain he’ll set new personal bests in no time.

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