Rebuilding A BMW M20 Engine For Spec E30 Racing: The Introduction

As far back as I can remember, my fascination with cars came down to two things. The first was visual – what kind of impact the lines, curves, and wedges of the machine had on my heart and my pre-teen loins. The second, was the motor. A product of the ’80s supercar poster era, followed by the sport compact era, and a lover of the DTM days of the ’90s, I fell in love with the scream of high-revving motors.

There is the intoxicating rush that comes with naturally-aspirated motors building power as the revs increase. My ears tingled at the psycho howl of the Ferrari 333 SP V12, Formula 1 cars of the late ’90s/early 2000s, and the gorgeous sound of high-revving V8s found in Indy Car. Scream a high-pitched scream and haul ass — that’s what cars and racing were about.

Now I’m lucky enough to have a racing car, and recently, my thoughts have turned to building a great motor for it, humble as a Spec E30 might be.

The stuff of dreams, Lambos adorned my walls as a kid.

Spec racing is designed to be a low-cost alternative to what is generally a very expensive endeavor. However, while the theory behind spec racing is sound, the reality is that — to be competitive — you will likely end up spending a fair amount to compete at the razor’s edge where the podium finishers are.

We start off with old cars that are cheap to buy and service. We spend money on safety, suspension, and brakes, and go racing with our major consumables being tires, brakes, and the odd fender or two. Oftentimes, the last thing to get any attention is the drivetrain, as it can be enormously costly to rebuild an engine or transmission. While all the cars must run stock motors, tired engines have less power, tractability, and reliability than a properly built fresh motor.

While my lovely little SpecE30 seems to do ok as a mid-pack car, I’ve decided to take on the rather monumental task of building a fresh race motor for next season. Now, I’ve spent plenty of time refreshing cooling components, installing cams, and performing general maintenance on motors, but to completely overhaul a used motor is a new endeavor.

Never one to back down from a task once I set my mind to it, I recently purchased a used stock BMW M20 to break down and rebuild. I’m looking forward to taking you on the journey with me and hope through my foibles and accomplishments I can show you that, with the right tools and resources, you too could rebuild a motor in your garage or backyard without breaking the bank.

Picked this lump out of a beater E30 with unknown mileage. All I know was the car was “running when parked!”

The BMW M20 used in the 325i is a 2.5-liter, single-overhead cam, water-cooled six-cylinder motor with all six cylinders in-line.

Inline-six motors are known for their smoothness and torque, and have been the heart of some of the most-winning racing cars.  From early Aston Martins and Jaguar E Types in the ’60s, to the more contemporary and popular versions found in the world-beating Toyota Supra MK 4 turbos, and Nissan Skyline GTRs preceding the current model all used the inline-six.

The Jaguar E Type — light, fast, and sounding amazing with that straight-six snarl.

Oh baby! The heart of a 3.0 CSL race car, this bitchin’ straight-six won numerous championships in the ’70s.

BMW’s 2.5-liter M20 is one its most popular engines and is plentiful; being used in the 325i and 525i of the late ’80s. In new condition, the engine makes 168 hp and 164 lb-ft of torque at 5800 rpm and 4300 rpm, respectively. With a rather low compression ratio of less than 9 to 1, the motors are robust and only require regular maintenance. The biggest difference I’ve seen between fresh motors and tired ones on track, is the the mid-range power delivery.  Fresh motors rev faster and pull harder through the mid-range, a very important attribute when accelerating out of turns.

The goal of this build is to refresh components that are worn out through decades of use. New bearings and piston rings will accompany a refreshing of the cylinder bores. We aren’t allowed to make any modifications, so it’s going to be a totally stock build.  It might not be a high-revving v10 out of an F1 car, but it’s my race motor, and I’m stoked to finally dig in to build one.

About the author

Kasra Ajir

Having competed, raced, or trained with SCORE, SCCA, NASA, BMW CCA, and more, Ajir is well seasoned in the various series that pollinate the grassroots and professional auto racing world.
Read My Articles

The Art of Driving delivered to your inbox.

Build your own custom newsletter with the content you love from Turnology, directly to your inbox, absolutely FREE!

Free WordPress Themes
Turnology NEWSLETTER - SIGN UP FREE!

We will safeguard your e-mail and only send content you request.

TURNology - Subscribe to our Newsletter

We'll send you raw engine tech articles, news, features, and videos every week from TURNology.

TURNology - Subscribe to our Newsletter

Turnology NEWSLETTER - SIGN UP FREE!

We will safeguard your e-mail and only send content you request.

TURNology - Subscribe to our Newsletter

Thank you for your subscription.

Subscribe to more FREE Online Magazines!

We think you might like...



Engine Tech

Hot Rods & Muscle Cars

Corvette Enthusiasts

TURNology - Subscribe to our Newsletter

Thank you for your subscription.

Subscribe to more FREE Online Magazines!

We think you might like...

  • Engine Tech
  • Hot Rods & Muscle Cars
  • Corvette Enthusiasts

TURNology - Subscribe to our Newsletter

Thank you for your subscription.

Thank you for your subscription.

TURNology - Subscribe to our Newsletter

Thank you for your subscription.

Thank you for your subscription.

Loading