Chris’ DCT-Swapped Miata: Grasping the Big Picture

While Chris Watson was tracking an RX-8, his friend, a talented driver and builder, was outpacing Chris in their championship-winning TTE Miata by multiple seconds. “I just got tired of paying more to go slower than him,” he admitted. This prompted Chris to buy a lightly-modified 1999 Miata with the intention of turning it into a Spec Miata, but due to reasons of space and another race car entering the picture, he decided to turn the Miata into a wilder kind of machine.

I originally decided to turbo the car when I realized it didn’t have enough power to accelerate on the highway with my bike on the hitch rack. I was already running an MS3 PNP and a friend was selling a DIY turbo kit so I took him up on that.

Building Upon a Proven Base

Working as an engineer with a well-known IMSA team, he made good use of all his contacts and left few stones unturned when developing this well-rounded track car. Building upon the established Supermiata S1 formula, he used the proven parts that’ve made turbo Miatas with relatively little power capable of remarkable times around high-speed tracks.Like the S1, this uses the standard Xida coilovers and square-245 setup to achieve its impressive level of mechanical grip.

It’s nice to be connected; the aero package uses a few parts plucked from retired race cars. The front splitter is made from a hand-formed PVC foam core wrapped in carbon fiber (custom, but informed by industry knowledge), and the rear wing is a prototype wind tunnel section for one of his team’s old Pirelli Challenge race cars, with custom mounts to fit the Miata.

Some of the driveline and the footwork is sourced from odd donors. The differential is a Getrag unit from a Cadillac CTS modified to use Mazdaspeed Miata bushings. The whole car is on Mercedes-Benz spherical joints, the front hubs are from the durable BMW E30, and the rear are custom billet items built to accommodate the 600-horsepower axles needed to handle high power.

The first iterations of the turbocharged motor ran on MegaSquirt for about 5 years. At first, his turbocharger of choice was an TD04-20T with a 0.45 A/R. While this was about as snappy as anyone would want, the small turbine raised EGTs and the engine retained too much heat, plus the setup was limited to about 300 horsepower at the wheels.

The small-displacement engine needed a small turbo for the sort of response and powerband needed to work well with a manual transmission, and while the TD-04’s negligible transient lag and rev range matched the manual transmission and its ratios, the Mazda factory transmissions never really worked. “After a lap or two, they never felt right. I once ran a factory five-speed with low miles at 230 lb-ft and it broke after two hours on track. The ones that lasted, I ended up replacing them every season, too.”

These drivetrain limitations pushed him into looking for an alternative, which was compounded by another desire: to find a better computer. Though Megasquirt’s support was great, Chris ultimately ran into its limits and desired an ECU with more accessible customization. The third reason to change was that he’d stepped up into a new realm of time trials and need more grunt. “Power goals and transmission concerns determined the next iteration. I needed about 450 wheel horsepower to be competitive in Gridlife Street Mod.”

Appreciating the Whole Picture

The turbo that would make that power with a larger A/R was acceptable response was the BorgWarner 7163 EFR. “I picked an 0.83 A/R to free up airflow and gain power, and I’ve run it up to about 400 WHP on track. My previous intercooler would heat soak, but it now has a 900-HP rated Garrett intercooler in it, which has got it to the point that there’s no appreciable power loss over a session.”

Still, the problem with the bigger turbo was that its transient lag would not complement an h-patten synchronized gearbox well. He was convinced he’d be making a significant loss with the delay between gears, which would impact acceleration considerably and make life a little easier on track.

He had to find a faster-shifting solution to make good use of the new turbo. One option was the BMW 7-speed DCT, and the other option was a sequential. “The sequentials are lighter, but considerably more expensive, and I’m not sure they’re more durable than the OEM DCT unit. They’d also put me in another class which I was not interested in joining. The DCT is roughly a hundred pounds heavier than the manual transmission and auxiliaries, but at least the weight is low and in the center of the car.”

The challenge is that modern semi-automatic control is complicated, and the only people who really have the resources to figure it out are the OEMs. Chris’ recommendation is to use the solution that enables you to retain the stock GCU, and in his case, it was the MaxxECU.

“The MaxxECU was already proven to work with the factory BMW DCT GCU and offered native DBW support and good configurability” he explained.

“Other offerings try to directly control the solenoid valves in the transmission. I didn’t even attempt to do that – it still uses the BMW controller. For the price point, support, and interface (with the OEM TCU, which had all the OEM protections in it), as well as the internal configurability of the ECU, I found the MaxxECU hard to beat.”

“Being able to shift instantly with the DCT, despite the additional weight, would allow me to lap about a second faster over a 90-second lap than an h-pattern version of the car, according to my simulations. Plus, there’s the control aspect — I never have to take my hands off the wheel, and I can also left-foot brake.”

Making Good Use of More

There’s a bit of response lag at 4,500 revs, but the seamless power and tighter gear spacing makes the latest iteration of the motor far more tractable than the previous one. It’s also making more power everywhere over 4,000 rpm compared to the old TD04.

“It can still produce more torque than the car can put to the ground in those corners. So I use TC as an anti-lag in hairpins when I don’t want to drop a gear. In those corners, I’ll get harder on the throttle a little sooner and have the TC retard the timing to limit power to the wheels while spooling the turbo, and when the grip is available, the turbo is spooled up and ready to take advantage of it.

The TC slip targets are speed and yaw-rate dependent, so it stabilizes the car and is also on-dash adjustable to suit the tire and track condition. Not intrusive; feels like magic. The only thing I have to be careful about is not to use it too frequently because it will burn up the exhaust. This is a good example of the configurability offered by the MaxxECU. I can make TC dependent on basically anything including pulling it back if EGTs rise too much.”

Lingering Little Fixes

There’s some splitter damage which limits the car’s front end currently. Both the aero balance and the mechanical balance is about what I want it to be. However, because the front end is slightly heavier, it puts a little more heat in the front tires, so as the car heats up, it pushes a bit later in the session.

The tires are really small on the Miata, and many street tires are liable to overheat, but the CR-S compound does really well despite the car’s 2900+ lb competition weight. “With other tires, I only get one flyer per session on the first hot lap. With these, laps two, three, and sometimes four are opportunities to set a fast lap. And with one cool down lap, I can go for a fast lap late in the session.”

In terms of engine heat, he’s managed it well enough to run full sessions without a dropoff in performance. As mentioned before, the larger intercooler plays a part in this, as does its PWR/C&R Radiator (highly recommended for any serious turbo car), Improved Racing transmission and oil coolers, and a Mocal power steering cooler.

“I’ve been informed I might be able to improve the response with more intake cam advance which I haven’t tried to tune on this turbo yet,” he added.

Although he welded the turbine to the manifold to eliminate any chances of studs backing out and gaps forming, it’s not really up to race-spec quite yet. “I’ve got this cast iron manifold hot enough to deform it and crack it, so I’m now looking into ways to better support the turbo and reduce EGTs. I still need to make a cold air intake for the car. The turbo ingests hot engine bay air at low flow rates, which affects the spool up behavior later in the session, but not peak power.”

Over the winter, Chris will keep himself busy installing a dry sump and refreshing the motor to make up for poor wet sump performance. “Once we get a little more cooling air to the brakes, we’re gonna turn the wick up. The turbo can flow over 500 horsepower, but I want the car to remain reliable and driveable, so I’ll gradually turn the power up and solve any issues on the way.”

Though the basic foundation of the car was proven, Chris was convinced that there was a better way to solve the typical turbo Miata problems and paved his own path. Of course, he had some wise and experienced folks helping him along the way, but his consideration of the whole car as an ecosystem helped him make the decisions that have given it greater all-around performance and a service history that any owner of a turbo Miata would envy. The race-proven EFR turbo and the OEM-controlled DCT have performed flawlessly. In fact, there’s been negligible maintenance needed in the last ten months – he hasn’t even had to change the transmission fluid over 2000 miles of track usage.

“I’d like to thank my parents for putting me on a path to be able to do this, my wife Lola for her unwavering encouragement and support, and my good friends Will and Erik at E3Machine for helping me realize the car’s potential.”