BMW B38

The B38 is BMW’s 1.5-litre turbocharged three-cylinder, the smallest engine in the modular
B-family. In the United States it reached exactly one car, and it is not the one people expect:
the i8, a carbon-fibre plug-in hybrid sports car with scissor doors.

What the B38 is

A 1.5-litre turbocharged inline-three with direct injection and Valvetronic. It is one third
of BMW’s modular family by design: roughly 500 cc per cylinder, three cylinders here, four in
the B48, six in the B58, sharing bore
spacing and much of the hardware.

The 2014 i8 brochure in this archive describes how it
worked in that car: the electric motor “gets its energy from the high-powered 7.1 kW
lithium-ion battery contained in the Drive Module. Working together with the TwinPower Turbo
3-cylinder combustion engine in the rear, for a combined total system output of 357 hp and 420
lb-ft of torque, the BMW i8 is launched from full stop to 60 mph in a breathtaking 4.2
seconds.”

Note where the engine sits: in the rear. The i8 drove its rear wheels with the
three-cylinder and its front wheels with an electric motor, which is an unusual arrangement and
the reason the car works at all.

Where it is fitted

Four specification pages. The 231 hp figure is the combustion engine’s
own rating from certification data; the 357 hp in the brochure is the system total with the
electric motor contributing. Both numbers are correct and they answer different questions.

Later certification records for the 2019 and 2020 i8 Roadster carry the B38 factory code
directly, which is how we know the engine family rather than inferring it from displacement.

231 hp from 1.5 litres

That is 154 horsepower per litre from a three-cylinder engine, and against everything else
in our records it sits third — behind only the M division’s
S58 at 181 and the B48 at 156 in its
most highly tuned form. It is ahead of the S63 and
S68 V8s, and well ahead of the B58 six
at 127. For an engine of half the B58’s size, in a car sold on efficiency, that is the
surprising number on this page.

The i8 brochure sets out the rest of the arrangement: pure electric running “up to 75 mph”,
after which “if the eDrive button isn’t pressed, the combustion engine launches into action at
37 mph”, and an electronically limited 155 mph. The three-cylinder was never meant to do the
job alone.

Where else the B38 went

In most of the world it is an ordinary engine: the 118i, 218i, X1 18i and the front-wheel-drive
MINI range all use it, and it is common. None of those reached the United States, which is why
an engine that is unremarkable in Europe appears here in one exotic. Our records cover
United States certification only, so we can name the i8 with a source and cannot do the same for
the rest.

What we do not publish

No reliability rating. The NHTSA complaint database records faults against a
vehicle, not an engine, and with four United States model years of a low-volume car
there is no population to draw a conclusion from. Anything you read about B38 durability comes
from markets where it was sold in numbers, and we have no way to check it against a source we
hold.

Common questions

Which BMWs have the B38?

In the United States, one: the i8, 2014 to 2020. Elsewhere the 118i, 218i, X1 18i and much of
the MINI range use it.

How much horsepower does the B38 have?

231 hp from the combustion engine itself in the i8. The car’s combined system output with
its electric motor is 357 hp, which is the figure the brochure quotes.

Does BMW really make a three-cylinder engine?

Yes, and it has since 2013. It is a deliberate part of the modular family rather than an
economy measure bolted on: the same 500 cc cylinder module appears in the four-cylinder
B48 and the six-cylinder B58.

Is the i8 engine the same as a MINI engine?

The same family. MINI’s three-cylinder cars use B38 variants, tuned differently and mounted
transversely rather than behind the cabin.

Sources

Quotations from the 2014 BMW i8 brochure in this
archive. Output, model years and the factory engine code from EPA certification and fuel economy
data, with the source named on every specification page.