Inline-6 vs. V6

E-class enthusiasts like to debate the merits of the early M104 inline-6 engine and the later M112 V6 engines. Both are excellent engines and one is not clearly superior to the other. Let me briefly explain the pros and cons of each motor.

M104 Inline-6

This engine is dervied from the tried-and-true M103 inline-6 that has powered many Mercedes for hundreds of thousands of miles since the late ’80s. The M104 shares its block with the M103, but gets a more sophisticated DOHC cylinder head.

Pros

Configuration

Inline-6 engines like the M104 have perfect primary and secondary balance and require no balance shaft. No special tricks are required for this engine to run without vibration. This engine revs happily and smoothly all the way from idle to its 6500 RPM redline.

24 Valve, Double Overhead Cam

The M104 has one intake and one exhaust camshaft that operate two intake valves and two exhaust valves for a total of 24 valves. Four valves per cylinder are optimal for high RPM power output. This engine was designed to run wide open all day long on the Autobahn.

This engine sounds emits a wonderful turbine-like exhaust note as it revs — something the V6 cannot match.

Easy maintenance

All the spark plugs and ignition coils are in a line on the top of the engine. Amateur home mechanics can change all the spark plugs in under 45 minutes. Maintenance on this engine could not be easier.

Cons

Head Gasket

As detailed on the common problems page, this engine is prone to head gasket failure. Cost to repair is about $1100-$2000 every 100,000 miles or so. Read the common problems page for more details.

Availability

This engine is not available in the E-class after 1997. Although there’s nothing wrong with the 1996 & 1997 models, you can’t have this engine if you’re shooting for a newer year.

M112 V6

This aluminum-alloy engine from Mercedes is an engineering marvel. It produces more torque than its predecessor the M104, while emitting fewer pollutants and returning better fuel economy. It won a spot on the Ward’s Automative Ten Best engines for four years in a row — 1998, 1999, 2000 and in 2001 — more than any other Mercedes engine. For more details about this engine, click here.

Pros

More Power

Although its maximum power output is identical to the inline-6 M104, the V6 M112 produces 85% of its maximum torque at just 2000 RPM. The torque curve is completely flat above 3000 RPM.

Lower Emissions and Better Fuel Economy

Two spark plugs per cylinder that fire in sequence ensure a complete burn, which helps lower emissions and provide better gas mileage. The new 3-valve increases exhaust temperature which helps reduce emissions and increase efficiency.

Lightweight & Compact

The lightweight block means that less power is wasted moving the engine around. The more compact design allows engineers to improve safety.in frontal collisions.

100,000 Mile Tune-Up Interval

The Platinum spark plugs are designed to go 100,000 miles between replacement. The days of 15,000 or 30,000 mile tune-ups are long gone.

Cons

90-degree V6: Not Naturally Smooth

Unlike the inline-6, this V6 is not inherently balanced. The optimum ‘vee’ angle between the cylinders of a V6 is 60 degrees. The best angle for a V8 is 90 degrees. This engine uses a 90 degree angle to allow for modular construction on the same assembly as the M113 V8 engine — an obvious cost cutting decision.

To compensate for the vibration, Mercedes installed in the engine block a counter-rotating balance shaft. The balance shaft zigs when the engine zags, canceling all of the first order vibration and most of the second order vibration.

The balance shaft and sophisticated engine mounts eliminate almost all of the vibration. The result is a very smooth running engine. But it’s not true smoothness like the inline-6.

The rotational inertia of this balance shaft makes the M112 less eager to rev than the inline-6 M104.

The exhaust note of the V6 sounds rough and unrefined in comparison to the inline-6.

Tune-Ups Difficult

Although the tune-up interval is 100,000 miles, you could do a tune-up on the inline-6 four or five times in the amount of time it takes to replace the spark plugs on the M112. The spark plugs are difficult to access on the V6, particularly the rear cylinder on each side. And since there are two spark plugs per cylinder, you have 12 of them to change. (It could be worse. The E430 has 8×2=16 spark plugs!) A tune-up at the dealer costs just shy of $1,000.

Harmonic Balancer

As documented on the common problems page, early harmonic balancer pulleys are prone to failure. When they fail, they cause about $3,000 of havoc. Mercedes issued a recall on this part. The new-and-improved harmonic balancer still can fall apart — just not as often. Plan to replace this every 60,000 miles. Replacement cost (parts & labor) is about $200.