IGNITION
MERCEDES-AMGA45 S ENGINE
‘AMGHAS
GONE TOGREAT
LENGTHS TO
KEEP BOTH THE
ENGINE AND
TURBO COOL’
You’d think 415bhp would require a big-capacityengine. So how will the A45 S achieve it with just2 litres?
Powerpoint
A
MG’S NEW M139FOUR-CYLINDERENGINE
has been designed from scratch, and shares
nothingwith the previous M133installed in
the originalA45 and its derivatives, or the modified
M260found in the A35. As before, the unit is hand-
builtin the same Affalterbach facility that produces
AMG’s V8 engines, but it now incorporates a suite of
new technology designed not only to generate the
mad208bhp-per-litre specific outputin peak ‘S’ form,
but do it reliably.
The clean-sheet design starts with a more rigid
aluminiumblock,with comparable toughness to that
of a diesel engine.This is needed as the M139has an
immensely high load rating due to the physicality of
producingso muchpower withinso little capacity. The
headers are of a non-symmetrical design that reduces
back-pressure in the valvetrain, whiledown in the
bowels the crank has been trimmed of 1.2kg, reducing
inertiawhilenot compromisingon strength.
The turbocharger is a new twin-scroll unit with
a peak boost pressure of 30 psi. Managingthat
boost is a new electronically controlled wastegate
that streamlines the turbocharger’s breathingwith
more precisionthana traditionalmechanical set-up.
The fuel-injection system is by direct and indirect
injection, the latter only applyingunderheavy loads to
essentially‘fuel dump’ into the cylinders more quickly
thanthe direct injection system would otherwise be
able to do on its own.
All of this performance generates a considerable
amountof heat, so AMGhas gone to great lengths
to keep boththe engineand turbo– and the air
they breathe – cool. The enginehas been rotated
180 degrees, for instance, placingthe exhaust and
turbocharger on the bulkhead side of the enginebay.
With the intake at the front of the car, cold air can now
enter more effectively withoutthe need to navigate a
hot exhaust and turbocharger.
In additionto the usualintercooler and radiator is a
front-mounted air-to-airchargecooler, whilethe block
gets its own electric water pumpand the headers a
more traditionalmechanical one. In the high-output
S models,even the air conditioningsystem is used
to cool the enginein extreme circumstances, such
as whenrefuellingafter a high-speed stint on the
autobahn. Clever stuff.
Whatwill the enginefeel like whenused in anger?
AMGsays that it has designed the M139to ‘feel
sporty’, and one look at the unit’s power and torque
curves supports this assertion.In contrast to the flat,
cliff-like torque curve of its predecessor, the M139, like
the A35’s M260, instead has a more tailored curve that
rises more progressively with the revs, augmented by a
slightbumpat around4000rpm.
As four-cylinderengines go, AMG’s new arrival may
deliver on power and tech, but the real test will lie in
whether it’s an enjoyable engineto use. The previous
M133, althoughimpressively powerful, lacked a sense
of occasion worthy of its outputs. Let’s hopethe M139
lives up to its maker’s promises.