Audi 4.0T vs Mercedes M278: The Ultimate Hot-V Twin-Turbo V8 Comparison

Published on: Sun Feb 15 2026

Audi 4.0T vs Mercedes M278: The Hot-V Twin-Turbo Showdown

You’re shopping for a used luxury sedan with a hot-V twin-turbo V8. You’ve narrowed it down to two options:

1. Audi RS6 / RS7 / S6 / S7 / S8 with the 4.0T V8 (EA824) (420-560hp)
2. Mercedes S550 / E550 / CLS550 / GL550 with the M278 V8 (402-449hp)

Both are hot-V twin-turbo V8s. Both are Alusil aluminum blocks. Both have catastrophic failure modes that can turn a $20,000 used car into a $30,000 repair nightmare.

This guide breaks down exactly which engine is more reliable, which is cheaper to own, and which you should buy.

1. The Engines: Architecture Comparison

Verdict: Architecturally, these engines are nearly identical. Both use hot-V turbo layouts and Alusil bores. The differences are in the details.

2. The Major Failure Modes

Audi 4.0T: Turbo Oil Screen Clogging

The 4.0T’s signature failure is turbo oil screen clogging. Audi placed a small mesh screen in the oil module to protect the turbos from debris. This screen clogs with sludge, starving the turbos of oil.

• Symptom: Screeching noise, loss of power, check engine light, rough running.
• Frequency: MODERATE. Peak failure window is 40,000 - 80,000 miles on early C7-era 4.0T engines (2013-2017) if the recall has not been completed.
• Repair Cost: $5,000 - $10,000 (turbo replacement).

[!WARNING] Audi issued a recall (TSB 2044640) covering 2013-2017 S6, S7, S8, A8, and 2014-2017 RS7. If the recall has NOT been completed, the car is a ticking time bomb.

Mercedes M278: Cylinder Scoring

The M278’s signature failure is Alusil cylinder scoring. The bore surface degrades, allowing metal-to-metal contact.

• Symptom: Rough idle, misfires (cylinders 5/1), blue smoke, low compression.
• Frequency: MODERATE. Techs report seeing scoring on “nearly all M278/M157” by 100k+ miles if you test carefully, but not all engines show symptoms.
• Repair Cost: $10,000 - $20,000 (long block replacement).

[!WARNING] Mercedes does NOT cover cylinder scoring under warranty. They consider it a “maintenance issue.”

Verdict: The Audi 4.0T’s turbo oil screen failure is more preventable (recall completion). The M278’s cylinder scoring is less preventable and more expensive.

3. Secondary Failure Modes

Audi 4.0T

• PCV / Oil Separator Failures: Valley-mounted PCV fails, causing whistling noise and idle instability. Repair cost: $1,000 - $2,000.
• Carbon Buildup: Direct injection causes carbon buildup. Repair cost: $600 - $1,200.
• Timing Chain Tensioners: Less common than N63 or M278, but failures occur. Repair cost: $2,000 - $4,000.
• DSG / ZF Transmission Mechatronic: S-Tronic DSG or ZF 8-speed mechatronic unit can fail. Repair cost: $2,500 - $5,000.

Mercedes M278

• Timing Chain Tensioners: Early M278s have tensioners that bleed down at cold start, causing a rattle. Repair cost: $1,500 - $6,000.
• Turbo Coolant Lines: Plastic coolant lines crack and leak. Repair cost: $800 - $1,800.
• Oil-Coolant Mixing: Oil cooler seals fail, mixing oil and coolant. Repair cost: $1,500 - $3,000 (or $10,000 - $20,000 if bearings are damaged).
• Carbon Buildup: Direct injection causes carbon buildup. Repair cost: $600 - $1,200.

Verdict: Both engines have expensive secondary failures. The Audi 4.0T has more PCV and transmission issues. The M278 has more timing chain tensioner and coolant line issues.

4. Cost Comparison: 100,000-Mile Ownership

Here is a realistic budget for owning each engine from 50,000 to 150,000 miles:

Audi 4.0T

Total (50k - 150k miles): $12,200

Annual Average: $1,220/year

[!CAUTION] If the oil screen recall is NOT done and turbos fail, add $5,000 - $10,000 to the total.

Mercedes M278

Total (50k - 150k miles): $11,400

Annual Average: $1,140/year

[!CAUTION] If the M278 develops cylinder scoring, add $10,000 - $20,000 to the total.

Verdict: The Mercedes M278 is slightly cheaper to own ($1,140/year vs $1,220/year) if you avoid cylinder scoring. The Audi 4.0T has higher transmission costs but lower timing chain costs.

5. Frequency of Catastrophic Failure

Audi 4.0T

• Turbo Oil Screen Clogging: MODERATE (40-80k miles on early C7-era engines if recall not done).
• PCV Failures: MODERATE (60-100k miles).
• Timing Chain Tensioners: LOW (less common than N63 or M278).

Mercedes M278

• Cylinder Scoring: MODERATE (techs report seeing it on “nearly all M278/M157” by 100k+ if you test carefully, but not all show symptoms).
• Timing Chain Tensioners: MODERATE (early M278s, fixed on 2015+ models).
• Oil-Coolant Mixing: LOW (high-mileage issue, not common).

Verdict: Both engines have similar frequency of catastrophic failures. The Audi 4.0T’s turbo failure is more preventable (recall completion). The M278’s cylinder scoring is less preventable.

6. Preventability

Audi 4.0T

• Turbo Oil Screen Clogging: PREVENTABLE. Verify recall completion (TSB 2044640) or proactive screen service.
• PCV Failures: PARTIALLY PREVENTABLE. Short oil change intervals help.
• Carbon Buildup: PREVENTABLE. Walnut blasting every 50,000 miles.

Mercedes M278

• Cylinder Scoring: PARTIALLY PREVENTABLE. Short oil change intervals (5,000 miles) and proper warm-up habits reduce risk.
• Timing Chain Tensioners: PREVENTABLE. Buy a 2015+ model with updated tensioners.
• Turbo Coolant Lines: PREVENTABLE. Replace proactively at 80,000 miles.

Verdict: The Audi 4.0T is more preventable with recall completion. The Mercedes M278 is more preventable with proper maintenance.

7. Which Should You Buy?

Buy the Audi 4.0T IF:

1. You are buying a 2016+ C7.5 RS7 or S6/S7 (revised oiling system).
2. You can afford a $1,500/year maintenance budget.
3. You have a trusted Audi specialist nearby.
4. You get oil screen recall verification (TSB 2044640) and clean PCV (no whistling).

Buy the Mercedes M278 IF:

1. You are buying a 2015+ model (updated tensioners).
2. You can afford a $1,500/year maintenance budget.
3. You have a trusted Mercedes specialist nearby.
4. You get a compression test (all cylinders 150+ PSI) and no cold-start rattle.

8. The Verdict

If you want the safest long-term bet: Audi 4.0T (2016+) with recall completion.

Why?

• Lower catastrophic failure risk (turbo oil screen is preventable with recall).
• Fewer timing chain tensioner issues than M278.
• More robust long-term if recall is done.

BUT: The Audi 4.0T has higher transmission costs (DSG/ZF mechatronic). The M278 has higher timing chain tensioner costs.

The Smart Buy: Audi 4.0T (2016+) with oil screen recall verification and clean PCV.

Related Guides:

• Audi 4.0T V8 Reliability
• Mercedes M278 V8 Reliability
• BMW N63 vs Mercedes M278 Comparison
• Audi RS7 Common Problems
• Mercedes S550 M278 Reliability
• Turbo Failure Guide
• Cylinder Scoring Guide
• PCV Failure
• Timing Chain Tensioner Failure