A screaming straight-six with triple carbs isn’t an uncommon sight in Street Machine, but you probably haven’t seen one quite like this before. Despite the angled block, this isn’t a Chrysler slant, and nor is it from Ford or GM. Nope, this is as European as they come.

First published in the February 2024 issue of Street Machine

“The engine is a BMW M20B25, which is a small six”, says Danno ‘Grooty’ Groot, the South Australian responsible for this raging beast. “It’s a street/circuit build for my partner Jo’s 1985 BMW E30 323i race car. We’ve chased everything in the engine for every last horsepower, aiming for two horsepower per cubic inch.”

Starting with a mint 1988 325i iron block, it features a factory forged crank from an M52B28 2.8-litre that copped some isotropic polishing courtesy of Bullet Race Engineering. “That’s for improved oil control, and we wanted to gain between 5hp and 7hp just from doing that,” explains Danno. “It’s now 2.9 litres [175ci] with a perfect 84mm stroke and 85mm bore, which means there are no harmonic issues, so we’ll spin it to 8200rpm. With the clutch and flywheel weighing 5.5kg as a set, it’ll rev like a motorbike.”

Danno decked out the little six with 138mm H-beam rods and custom-forged Ross pistons for a spicy 12.2:1 comp. And since stock E30s sumps aren’t the greatest, Danno and Jo made a custom high-volume unit with baffles and a crank scraper.

Up top, the single-cam, 12-valve head has been gone through by Tony Knight. The alloy 883 casting has been kitted out with Supertech valves, titanium retainers, and a solid-roller cam conversion.

“It’s got a huge cam,” Danno laughs. “It’s 309-degree with a 106-degree lobe separation, so I can’t wait to hear this thing when it’s in the car!”

The most eye-catching feature is undoubtedly the triple 48mm Weber carburettors, which replace the stock long-runner intake manifold and ancient Bosch Motronic 1.3 EFI system.

“Webers make more power than EFI,” says Danno. “I would love individual throttlebodies and coil packs, but this isn’t a no-budget build, as we have other cars to do.”

Danno custom-made the headers as six-into-one 1¾-inch primaries, because even though the stock BMW exhaust manifold flows just fine, this engine is pushing way past what the German engineers ever considered possible for their workhorse six-pot.

The stock timing cover went in the bin for a trimmed-down Nuke Performance adjustable cam pulley, and the stock electronic ignition plug lead cap was replaced with a recurved distributor from an earlier E21 3-Series BMW set-up.

“We’ve squeezed every little bit out of this engine that we can, like the 50mm velocity stacks, and the intake has had 12 hours of port work,” Danno explains. “We’ve done a bunch of oil gallery mods and polished inside the block to improve oil shedding inside the motor for improved power. The expected output is around 340 to 350hp.”

“It has been built in-house at Grooty’s Race & Resto,” he adds. “I didn’t do it all myself; I had some excellent help along the way, and there were some big setbacks.”

To follow the installation and start-up, check out Grooty’s Race & Resto on YouTube.

HIT FOR SIX

Introduced by BMW in 1977 to sit under its M30 ‘Big Six’, the M20 inline-six was produced in 2.0L, 2.3L, 2.5L and 2.7L capacities but retained the basic iron block and alloy single-cam head architecture. They made between 88kW and 125kW and could be ordered with either a carburettor or Bosch K-Jetronic, L-Jetronic or Motronic injection. The M20 finished production in 1993.

Grooty’s Race & Resto
Adelaide