In 2015, the engine in this car was the most powerful four-cylinder in series production anywhere in the world. 355 hp from 1,991 cc. 178 hp per litre. 26.1 psi of boost, which is Mercedes' own figure. A closed-deck block, a forged crank, forged pistons, a bore coating sprayed on with a twin-wire arc. Every one of them assembled by hand at Kölleda by a single pair of hands, with the builder's signature on the plate. 0-60 in 4.8 seconds in something that also does the school run.
And when you start it in a car park, it barks once, drops to a fast flat idle, and then goes quiet. Someone walking past looks at the wheels, not at the car.
That is the letdown. Not that it is silent — it is not. It is that you bought a hand-built AMG engine and the sound is the part of it that got left in the factory.
Why the M133 sounds like this
Three reasons, in order of how much each one costs you.
The turbo is a silencer, and it is doing its job. Every exhaust note begins as pressure pulses leaving the cylinders. On a naturally aspirated engine those pulses travel the whole length of the system and arrive at the tailpipe intact — that is the sound. On the M133, the manifold feeds a twin-scroll turbocharger, and the turbine wheel exists to convert pulse energy into rotation. By the time gas exits the turbine it is no longer a series of pulses. It is a smooth, continuous flow. You cannot put the sharpness back downstream, because the energy that made it was spent upstream. This is true of every turbo car, and it is why a 355 hp four can be quieter than a 240 hp naturally aspirated one.
Four cylinders fire in pairs. Firing order 1-3-4-2, two combustion events per crank revolution. That produces a dominant second-order frequency — an even, busy buzz instead of the uneven rumble of a cross-plane V8. It is not a fault; it is the arithmetic of the layout. But it puts the M133's natural voice in the midrange, and the midrange is what your ear reads as rasp rather than depth.
We measured the customer's clip for this article, and that is exactly where the energy is. The 100–800 Hz band sits about 2 dB below the overall level. Everything below 100 Hz is 11 dB down. Everything above 2 kHz is 11 dB down. The entire voice of this car lives in one middle band. That single measurement explains why an AMG four sounds aggressive instead of rich, and it also tells you what not to do: a big-bore system that tries to drag the note down will only flatten it.
The rear section is sized for a crossover. The X156 GLA is the raised, family-facing version of the same MFA platform that also builds the A-Class hatchback. The 45 version gets the engine, the chassis, the wide arches and the big brakes — and it also keeps the body's acoustic package and a large rear silencer. Contemporaneous road tests of the 2015 car singled out the optional AMG sport exhaust as the thing that made it come alive off-throttle. Read that sentence back: the car needed an option to sound like an AMG.
How we'd voice this engine
The first thing to accept is that you are not going to make it sound like a V8. You cannot, and everyone who has tried has produced something that sounds like a recording of the wrong car. The goal on a four-cylinder AMG is different: make the midrange honest, then give it a top end that rewards using the 6,700 rpm limiter.
What that means in practice, on our systems: we keep the pipe diameter modest, because the usual instinct on a performance car — go bigger — is the wrong instinct here. We use a crossover where the layout allows it, to even out the two banks of a four. We valve it, so the loud version is available and not compulsory. And we spend time on tailpipe length, which is the cheapest and most underrated variable in exhaust design — a few centimetres moves the point where the note lands in the cabin.
On material, this car gets titanium. We argued the same way about the Golf 7 GTI, and the case is stronger here. A four's energy already sits in the metallic upper-mid band; titanium — thinner wall, higher stiffness, less mass — keeps more of that harmonic and gives back less of the low-frequency drone that a heavier system tends to add. Our titanium 2.0T Mercedes catback for the C300 W205 is the same idea on a different engine. On a V8 we go the other way and reach for stainless. The material should suit the engine, not fight it.
What the clip actually shows
One clip, eighteen seconds, filmed in a garage with a phone held about a metre from the tailpipe. You see the rear quarter of a white GLA, the LED tail light lit and washing red light across the bodywork, and the exhaust doing its thing while the car is revved. One frame catches a flame at the pipe on the overrun — unburnt fuel lighting off in hot gas. That is a glimpse of what the car does on a hard lift-off after a rev. It is not a promise that yours will do it, because we do not know the full spec of the customer's car beyond the valved titanium rear section.
Two things the numbers say. First, the recording peaks at 0.0 dB and clips repeatedly, which means the microphone hit its ceiling — that is a car that is genuinely loud a metre behind it, not merely audible. Second, that clipped energy is concentrated in the middle band rather than the bottom, which is exactly what you would predict from the firing order of a four and is why this clip sounds like an attack rather than a rumble.
And one thing it does not say. There is no load in this clip. The car is stationary, revved in a room with a hard floor and a hard ceiling, which is the most flattering environment an exhaust can be recorded in — the reflections bounce the note right back at the phone. It is the only footage we have, and we would rather tell you what it proves and what it does not than pretend an eighteen-second garage clip is a sound test. If someone shows you one and calls it a review, they are selling you a room.
Two ways to build this car
Route one: a valved titanium catback. This is what the customer car runs and it is where we would start you. It is legal almost everywhere, it keeps the factory catalyst and both oxygen sensors untouched, and it gives you the character on demand — closed for the morning school run, open when you actually want to hear a hand-built engine. Since the car's energy lives in the midrange, a correctly sized valved titanium rear section changes the note more than it changes the volume, which is usually the thing owners actually wanted.
On fitment, one honest note: we do not currently list an X156 GLA 45 kit. The nearest things we build are on the same architecture. The A200 W176 catback is the A-Class hatchback of the same MFA generation as this GLA. The CLA 250 catback sits on the M270 — the ordinary engine AMG took apart to build the M133. And the A35 W177 catback is what the same formula looks like twelve years later. If you send us your model year and whether your car has the AMG exhaust option, we can tell you exactly what fits.
Route two: catback plus a 200-cell downpipe. On a turbo car the real restriction is at the turbo outlet, and we have said so plainly elsewhere — on the W212 E550 we tell owners the downpipe is the first upgrade, not the last. We are not contradicting that here; we are saying it applies differently. The E550 owner's complaint was a choked top end on a car whose character was already good. This owner's complaint is character, and a downpipe does not fix character. It adds volume and a harder edge. The pair of firing events per revolution is still there. And it drags you into check-engine lights, a calibration, and emissions rules that vary country by country. If your GLA is a daily, stop at the catback. If it is a toy and you are prepared to tune it, the downpipe is a real gain and worth doing on top.
Either way, the Mercedes-Benz range we build covers the rest of the line-up, and the titanium collection is where the lightweight systems live.
The things we should tell you
Titanium costs roughly double a stainless system of the same design, and it will heat-tint. If you want a mirror finish that stays a mirror finish, buy stainless and be honest with yourself about why.
Titanium is a thin-wall material and it is less forgiving of a hard strike than stainless. Normal road use is fine. A kerb at speed is a different event.
The valve will wear. It lives in the hottest part of the car and it sees carbon every day it is driven. A car that only ever does short, low-load trips is the worst case for a valve, because it never gets hot enough to burn the deposits off. Ask us about replacement actuators rather than replacing a whole system.
A catback on this car will not make it fast. On a 2.0 turbo the restriction is at the turbo and the catalyst, not the rear box. Buy it for the sound, the weight and the character, and treat any response improvement as a bonus. Anyone quoting you 20 hp from a rear section on a GLA 45 is quoting a dyno run they did before the baseline.
The GLA 45 is a 2014–2015 car now, so the engine is over a decade old. Check the turbo actuator, the wastegate and the ignition system before you spend money on noise. A tired engine makes a tired sound, and no exhaust fixes that.
And cold start with the valve open is loud. In a terraced street at six in the morning, that is the part your neighbours will remember.
A question for the comments
Here is the one we keep coming back to.
The M133 is one of the genuinely remarkable engines of its era. 178 hp per litre, forged internals, hand-assembled by one person. Nothing about it is a compromise. Its problem was never engineering — it was that Mercedes put it in a small crossover whose buyers mostly wanted a small Mercedes. So the noise got tuned for the crossover buyer, and the character got left to the A45 hatchback crowd. It is a defensible decision. It is also the sort of decision that turns a great engine into a quiet one.
So: if a rear section and a valve can give this engine its voice back, why is the factory setting still "quiet enough to hand back at the end of the lease"? Would you rather buy it loud from the start and pay for the privilege, or do you prefer that the choice stays yours to make in your own garage?
Questions we get asked about this car
Will a catback void my warranty? On a 2014–2015 car the factory exhaust warranty is long expired. If you are covered by an aftermarket extended warranty, a rear section is usually the least contentious thing you can change and a downpipe is not. Ask before you buy rather than after.
Do I need a tune with a catback? No. A rear section does not change what the sensors read. A downpipe does, and that one needs a calibration.
Why titanium and not stainless on this car? Because the engine's energy sits high. A four-cylinder's voice is in the metallic upper-mid band, and titanium preserves more of it while adding less low-frequency drone. On a V8 we recommend stainless, because there you want the weight of the note rather than the edge of it.
How loud is it with the valve closed? Slightly louder than factory at idle and effectively standard at a cruise. The whole point of a valve is that the loud version is a decision rather than a condition of owning the car.
Will it drone at motorway speed? It should not, and it is the first thing we check. A four-cylinder turbo is easier to keep quiet at a cruise than a V8, because there is less low-frequency energy to fight in the first place. If you have a drone at 70 mph it is usually a badly sized system or a downpipe, not the material.
Do you make a GLA 45 kit? Not for the X156 yet — send us your model year and we will tell you what we can do. In the meantime the Mercedes-Benz collection and the valved catback range show how we approach the rest of the line-up.