The idea of transplanting a lighter powerplant into the venerable E46 has been discussed over the years, but it was only relatively recently that NASA NorCal racer Scott Smith was able to integrate a turbocharged four-cylinder N20 engine into his ST4 car. The technical challenges were intimidating, but Smith persevered to create a competitive car at the 2023 NASA Championships.

NASA NorCal racer Scott Smith took third at the 2023 NASA Championships in his N20-powered E46 chassis.

Meanwhile, Southern California’s Brett Strom of Strom Motorsports had been toying with a similar idea, but issues with finding the requisite amount of power from a smaller engine needed to run in the faster classes convinced him, momentarily, that an E46 with a six-cylinder was the way forward.

That is, until he discovered a new four-cylinder with plenty of tuning potential.

Brett Strom in his Orange County, Calif., shop tuning a new “B46” racecar.

Though Strom has stood atop the podium at the 24 Hours of Dubai and the 12 Hours of Spa, he’s spent most of his time and effort supporting club classes over the last two decades. “Some of my favorite racing memories are from multi-car battles in Spec E46 and Spec E30,” he said.

As a builder, he had to accept the fact that these aforementioned classes didn’t provide the outright speed many of his clients wanted. For years, he built these clients’ no-expense-spared E92 M3s and F82 M4s, which, while fast enough to satisfy a discerning customer, required exhausting schedules and extensive R&D to make them right and reliable.

“There’s more to bolting on parts when you want to build a car that fast. It takes a lot of development — especially with newer cars, which have electronics that are doing their best to stop you. The factory programs are able to build hundreds of the same car incorporating what they have developed on the prototypes and from real-world running, but when you build a one-off club car, you take on that burden with just one chassis. I can say this: It’s not fun.”

Taking his experience from pro racing, Strom started supporting factory racecars at the club level and soon had his clients in factory BMWs, Porsches, and Audi TCR cars. After a few years of that, it became apparent that competing in club classes with heavy, complicated factory cars was not cost effective.

Then came his eureka moment. After running GT4-worthy lap times in his old S54-powered E36, he saw an opportunity to build the car with all the speed and sophistication and at half the cost. What if he could take his know-how from the pro ranks and integrate factory-quality and features into a lighter, simpler BMW chassis?

The trusted, well-developed E46 has served many club racers well.

Over subsequent late-night discussions with his engineer Nick Karnes, they posed the question: if you were to build the perfect BMW production club racecar, what components would you use and what decisions would you make to go as quickly as possible while keeping running costs reasonable?

They established the criteria and hung a scribbled sheet of objectives on their shop wall. This is what they came up with:

  1. Lightweight, fun to drive and easy on tires.
  2. Easily available and common parts: nothing handmade. Every part can be ordered from a catalog.
  3. Lower running costs compared with professional cars running similar speeds.
  4. High-end, feature-rich motorsport electronics for reliability and future adaptability.
  5. Fit and finish of a factory racecar.

So the quest for the ideal chassis began. The E36 and E92 were considered, but it was clear the E36 was too old and the E92 was too big. They saw the E46 330ci as the happy medium. Its cheap, small chassis and aftermarket support would create the foundation upon which they could build an easily reproduced racecar.

Breathing New Life 

Then it came time to decide on an engine. They considered the S54, but they’re scarce and expensive these days. “Because we were adamant about not building motors for these cars, because it’s just another time-intensive production process that adds to our liability, we wanted to find a cheaper engine that was already race-ready. Preferably, something in-house,” Strom said.

After going through BMW’s catalog, the S54’s rightful successor became clear. The availability, cost, weight, packaging, and power potential of the four-cylinder B48D earned it the pick.

BMW performed a complete redesign of its engine family around 2015 and went to a modular design that shares many components across multiple engines. Essentially, the B48 is a B58 with two cylinders lopped off. It uses a closed-deck construction, a twin-scroll turbocharger, direct injection, variable valve lift, and variable valve timing.

Thankfully, the engine had many of its issues sorted over its multi-year run. Strom opted for the latest variant with forged rods, a forged crank, huge bearings, and variable volumetric oiling. If he could have the same sort of stoutness and power that the S54 offers at half the running cost, he believed his creation, which he’d dubbed the B46, would be a hit.

The preferred version of the B48, sourced from a 2020 328i costs between $3,800 and $5,500.


The engine block is based on an old iron diesel design, so it wasn’t difficult to source a stout European-spec gearbox that bolted up directly. The GS6-53 six-speed gearbox, as used in the E90d is ubiquitous and inexpensive. Some cost as little as $400 shipped from Europe.

Compared to the Audi TCR cars he’d run in ST2, the B46 is far easier to run.

It’s also much easier to operate than the 8HP ZF Strom tried in his test car at the 2022 NASA Championships. The ZF8’s shifting was clunky and the downshifts occasionally would snatch the driven wheels. Even with that handicap, Strom led much of the ST2 race. He was also fighting high engine temps due to a failing engine fan. The car also led the following day’s 6.5-hour Seaside Cup enduro, when a chance for glory faded after he was meatballed due to the fuel cell breather spilling fuel on the track. If he weren’t dogged by mechanical gremlins, the car could’ve been a dominant force that weekend.

The ZF helped demonstrate the package’s potential, but Strom shelved it in favor of the reliable, economical GS6-53 as the B46’s standard transmission. For those willing to spend more, the optional Holinger SG3 sequential is available.


“I want it to be easy for people to get in and drive. If the parts are designed well and we can maintain a high standard of quality control, we can put something together for the customer at a reasonable price and give them the satisfaction that their car is up to par. My engineer Nick has worked crazy hours to design all the integration and production parts, which are laser-cut, CNC’d, and 3D-printed for consistency — and not one of our 139 parts is handmade! All this makes it easy for us to stock parts and simple for the owners to order replacement parts,” Strom added.

“We certainly didn’t want to make all the parts for this car ourselves. It was important that we choose something with a wide aftermarket so we could rely on our technical partners like Brembo and APEX for parts.” — Brett Strom

This streamlined build process and quality control should make regular, low-stress racing a reality, but that peace of mind and easy operation comes at a price. The B46’s turnkey costs begin at $152,000. Still, that’s a reasonable cost for an Super Touring 2 car, and the ease with which owners can jump from class to class with a simple power stick exchange means minimal headache and maximal seat time.


Weighing roughly 2,800 pounds with driver, the B46 doesn’t need an exceptionally powerful engine to be fast, but it has more than enough to help it get to the front of the ST2 field. In standard ST2 trim, its 330 horsepower is more than adequate, and because of the turbo torque, it moves like something with more than its on-paper stats suggest. “Because the horsepower curve is flat, you don’t need to shift much. For instance, at Willow Springs, you only need to shift twice a lap because you are always making sufficient horsepower at any RPM,” Strom said.

For those willing to spend more, Strom can replace the standard turbo innards with the hybrid gear from IPOS, which improves power and response. With the revised turbo setup, peak torque is available from 3,200 rpm and the horsepower curve is flat from 4,800 to 7,200 rpm. “The aftermarket turbo allows the B48 to breathe like a true race engine. With the boost turned up, it’s currently able to produce 360 horsepower,” Strom noted.

“I’m confident this engine can make 400 pound-feet of torque reliably with our modifications,” said Brett Strom.

As impressive as all that is, straightline speed is only a secondary concern. The salient change a smaller, lighter engine makes is to weight distribution. “The spread over the front axle is reduced, and so front tires wear better, and the entry speed the car’s capable of just has to be experienced,” he said.

Modernized Motor

The modern engine is meant to make quality seat time a priority. Fewer issues in tech, a greater sense of security with its programmed fail-safes, and a wide range of adjustable parameters to help make the motor as tractable as possible are just a few of its appealing traits.

The MoTec M142 ECU gives Strom complete control over the engine and its drivability, which is paramount for getting a car to best fit into its power/weight classing requirements. Its motorsports features are numerous: advanced knock control, direct injection, boost control, antilag, motorsports-grade traction control, throttle sensitivity adjustment, automated rev matching, an intriguing torque modeling system, and all the various fail-safes to preserve the motor. The system will even shut the motor down if the wrong fuel is detected, or if oil pressure drops too low. What’s more, the system displays engine faults to the driver in real time and color codes them in yellow or red so the driver knows whether to finish the race or pull off track immediately.

The modernized electronics go the extra mile with Dual ECUMaster ADU7 driver displays, a CAN bus-based STROM steering wheel, a solid state power module, and even an ECUMaster smart battery disconnect that can disconnect the battery if it detects a cable short.

For all its complexity, the B48 engine makes tech inspections much simpler. Strom has encrypted the part of the car systems that controls its power levels, and is working with NASA tech officials, so as the number of cars grows, they will be able to verify power levels by simply looking at a data stick — no long hours on the dyno necessary. Less time arguing with officials means more time in the seat.

The B46 is a car built for the data-obsessed.

If all goes to plan, Strom’s turnkey creation will be versatile enough to shuffle between ST2, ST3, and ST4 with a quick swap of the power stick. Fast enough with full power to run with the Corvettes and six-cylinder cars, but nimble enough to work well in ST4. Again, the aim is to get drivers into competition as easily as possible so they can dedicate all of their energy to driving.

Strom has high hopes for the project. If all goes to plan, he’d love to see a B46-specific class and upward of 500 cars built, in which case, he would support drivers at the regional races with a backup supply of parts.

The foundation has been laid and the formula is convincing, but it’s still debatable whether a medium-sized shop like Strom’s would be able to produce that number of cars. It’s a Herculean task, without a doubt. If that’s possible, will enough well-heeled amateurs be able to justify the up-front cost? If the B46’s proposed pace, reliability, and support are proven, it’s entirely possible.

Images courtesy of Brett Becker, , Brett Strom and


  1. Does he have a more detailed write-up? Getting valvetronic working with an aftermarket ECU seems like a question with very few answers…

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