In Part 1 of this series, we repaired a heavily damaged E90 chassis and turned it back into a solid foundation. With the chassis straight and the major body repairs behind us, it was finally time to start building the car.

One of the biggest differences between a sprint car and an endurance car is how you approach reliability. Instead of asking whether a part will survive a 30-minute race, you’re asking whether it can survive six, fourteen, or even twenty-four hours of continuous abuse without becoming the reason the car ends up back in the paddock.

Our endurance team has more than 20 combined endurance races under its belt, and one lesson has been reinforced over and over again: race weekends are not the place to discover weaknesses.

Every endurance race exposes the same things. Heat finds the weak links. Small issues become big problems. Parts that survive a sprint race don’t always survive hours of continuous abuse.

That mindset shaped nearly every decision we made during this phase of the build.

An Engine With Racing History

Sometimes the right parts have been sitting in your shop all along.

Years ago we purchased an N52 from an old IMSA Continental Tire Sports Car Challenge ST-class BMW 128i campaigned by Crucial Motorsports. It was intended to be a spare, and after spending nearly five years sitting on an engine stand in the corner of the shop, this project finally gave it a purpose.

There was never much debate about which engine this car would use.

The N52 is the engine used in Spec E9x, it’s the same platform we’ve raced in Gridlife, and we’ve spent years learning its strengths, weaknesses, and maintenance requirements. When you’re building a new race car, there’s tremendous value in starting with something you already know inside and out.

Refreshing the N52

Although the engine came from a successful race program, we treated it like any other used race engine.

After building enough race cars, you learn which jobs are worth doing while everything is apart. Replacing common failure items with the engine on a stand takes a fraction of the time compared to doing them in the chassis.

Before the engine ever went into the car, it received a thorough refresh including but not limited to:

• Complete engine gasket and seal kit
• Front crankshaft seal
• Rear main seal
• Valve cover & gasket
• Oil filter housing gasket
• Electric water pump
• Thermostat
• All coolant hoses
• Belt tensioner
• Spark plugs
• Mishimoto silicone intake boot
• Underdrive power steering pulley
• Aftermarket oil cooler adapter

The goal wasn’t to make more horsepower. It was to eliminate as many preventable failures as possible before the car ever reached the racetrack.

One thing we’ve learned with the N52 is that it usually tells you very early whether it’s going to survive track use. We’ve seen engines fail during their very first session because casting debris and magnesium flakes clogged the oil pickup or restricted oil passages. On the other hand, once an engine survives that initial track time and receives a fresh oil change, we’ve generally found it to be an extremely durable platform.

When we removed the oil pan from this engine, everything looked exceptionally clean. It already had a fabricated oil baffle from its previous life, which gave us additional confidence that it had been properly prepared and maintained.

Endurance Lesson #1: Reliability isn’t created on a race weekend. It’s built months earlier while the engine is still on the stand.

Continuing Development

Since the oil pan was already off, it was the perfect opportunity to continue development of one of our own products.

Our original steel oil pan baffle has accumulated years of successful track testing and served as the foundation for our new aluminum version. As factory steel oil pans have become harder to find, more racers are switching to aluminum pans. Rather than ask customers to hunt for increasingly scarce parts, we wanted to offer the same level of oil control for the pans they’re actually using today.

This car became the perfect opportunity to continue validating that design before it reaches more customers.

Condor Aluminum N52 Oil Pan Baffle

Why an Automatic?

We covered the decision to run a ZF 6HP automatic in Part 1, but the more interesting question is why we never seriously considered a manual transmission.

The answer comes from experience.

Over the years, our endurance team campaigned an E34 with a six-speed manual. As more guest drivers spent time behind the wheel, the transmission gradually became one of the biggest maintenance items on the car. Between missed shifts, clutch wear, and the cumulative abuse of endurance racing, we replaced seven or eight transmissions during that program.

When this project started, there really wasn’t a debate.

It was always going to be an automatic.

The transmission itself came from Facebook Marketplace and, before installation, received a complete service using a Bimmerworld overhaul kit that included a new transmission pan with integrated filter, fresh fluid, and the common sealing components that are easiest to replace while the transmission is out of the car.

The ZF 8HP has deservedly become the popular choice for BMW swaps, but relatively few people have documented long-term endurance experience with the factory 6HP. Since it’s legal in Spec E9X and originally came in these cars, we wanted to see just how capable it could be.

Designing Around Heat

If endurance racing has taught us anything, it’s that heat management should never be an afterthought. We didn’t build the cooling system around what we hoped would happen. We built it around what we’ve already experienced.

We’ve seen N52 oil temperatures climb well beyond 300°F without an oil cooler. We’ve watched coolant temperatures exceed 245°F, forcing the engine into reduced-power mode during sprint races. Adding an automatic transmission only increases the thermal load, since both the engine and the transmission now require dedicated cooling.

That meant maximizing airflow and cooling capacity from the beginning.

For engine cooling, we chose a CSF N54 aluminum radiator. The N54 radiator is thicker than the factory N52 unit while giving up some overall height. That combination worked perfectly for this project because it created the space we needed beneath the radiator for a dedicated engine oil cooler while still providing cooling capacity designed for a turbocharged engine.

CSF 2 Row High Performance Aluminum Radiator - E8X / E9X / Z4

Ahead of the radiator, we mounted a Mishimoto transmission cooler using our Condor oil cooler brackets. We chose the Mishimoto cooler because it provided the largest core we could package in front of the radiator while still maintaining good airflow through the rest of the cooling stack.

Mishimoto Transmission Cooler

Our friends ar Guru Autowerks supplied an AN adapter for the 6HP transmission, making it easy to plumb the cooler with AN hose and fittings. If you’re planning a 6HP or 8HP swap, they’re a great source for transmission-specific components and conversion parts.

6HP AN Cooler Adapter 6HP19 21 26 28 - BMW Gas N54 N55 N62

Below the radiator sits an aftermarket oil cooler originally designed for a second-generation Mazda RX-7. Rotary-powered RX-7s produce extremely high oil temperatures, making this cooler an excellent fit for an endurance application. It made efficient use of the available space while providing significantly more cooling capacity than many of the universal options we considered.

To finish the system, we added a 1.5-quart Accusump. We already had one on the shelf from an older project, and when you’re building an endurance car, an extra layer of protection against oil pressure loss is easy insurance.

Endurance Lesson #2: Build the cooling system for the worst conditions you’ll ever see—not the average lap.

 Parts We Never Question

Some decisions required research.

Others didn’t.

Every race car we build runs Condor engine mounts. We designed them because we wanted mounts capable of surviving years of racing abuse without the compression and loss of support we’ve repeatedly seen from factory rubber mounts.

Since the engine was already hanging from the hoist, installing a fresh set before lowering the drivetrain into the chassis was simply part of the build.

 Looking Ahead

By the end of this phase, the E90 had gone from a repaired shell to a complete rolling drivetrain with the major cooling components in place. It was finally starting to look like a race car again.

Next comes one of the more involved parts of the build: converting a factory manual chassis to run a ZF 6HP automatic. That means wiring the transmission into the chassis, integrating paddle shifters, installing the suspension and brakes, and working through the inevitable surprises that come with bringing a race car to life for the first time.

After more than 20 endurance races, we’ve learned one final lesson that shapes every build we do:

The fastest car doesn’t always win. The car that stays on track the longest usually does.