M157 vs M178 — Which Should You Buy?

The M178 wins for most buyers — it's more powerful, more efficient and generally more reliable than the older M157. The M157's only remaining advantage is its lower entry price on the used market and its more old‑school, characterful V8 sound. For buyers who can stretch to the M178, it's the better engine in almost every measurable way.

M157 (5.5L twin‑turbo V8): 440 enquiries in 2025 [EM-VERIFIED], typical reconditioned engine cost £6,000–£10,000 [EM-QUOTE], 525–577 bhp, known for heat‑related valve stem seal and turbo coolant line issues
M178 (4.0L twin‑turbo V8): 410 enquiries in 2025 [EM-VERIFIED], typical reconditioned engine cost £8,000–£14,000 [EM-QUOTE], 469–630 bhp, hot‑vee design, more reliable and more efficient
M178 delivers better fuel economy (23–26 mpg vs 20–23 mpg) and lower CO₂ emissions
M157's hot‑vee design is less well‑executed, leading to heat soak and component failures at higher mileages
M178 is used in newer AMG GT, E‑Class, S‑Class and GLE models, with better parts support and longer service life

Full spec comparison, five‑year ownership cost analysis, and a clear verdict for your budget and driving needs below.

At a Glance

	M157 (5.5L V8)	M178 (4.0L V8)
2025 UK Enquiries	440 [EM-VERIFIED]	410 [EM-VERIFIED]
Displacement	5.5L twin‑turbo	4.0L twin‑turbo
Power Output	525–577 bhp	469–630 bhp
Recon Engine Supply & Fit	£6,000–£10,000	£8,000–£14,000
Typical Real‑World MPG	20–23	23–26
Defining Failure Points	Valve stem seals, turbo coolant lines, battery drain, timing chain adjuster	Oil leaks from valve covers, coolant system issues, occasional turbo problems
Best For	Budget‑conscious AMG buyers, classic hot‑vee sound, older models	Modern performance, better efficiency, newer AMG GT/E/S/GLE models

The Verdict

The M178 is the better engine for the vast majority of buyers. It's more powerful (in most variants), more fuel‑efficient, more reliable and used in newer, more desirable AMG models. The M157 is an older design with known heat‑management issues that can lead to expensive repairs. While the M157's lower entry price is appealing, the M178's superior reliability and efficiency make it a better long‑term investment. Unless you're specifically chasing the older engine's character or are on a tight budget, the M178 is the clear winner.

M157 — The Original Hot‑Vee V8

The M157 is Mercedes‑AMG's 5.5‑litre twin‑turbo V8, produced from 2010 to 2018. It was used in the E‑Class (W212 E63), CLS (C218 CLS63), S‑Class (W221/W222 S63), GLE (W166 GLE63), GL (X166 GL63) and G‑Class (W463 G63). It produces between 525 and 577 bhp depending on the application, with peak torque of 700–800 Nm. The engine is known for its thunderous V8 sound and effortless performance. However, the M157's hot‑vee design (turbochargers mounted in the V of the engine) was a first for Mercedes, and it suffered from heat‑related issues: valve stem seals that hardened and caused oil consumption, turbo coolant lines that leaked, and battery drain from the auxiliary water pump running after shutdown. Timing chain adjuster wear is also a known issue on higher‑mileage examples. The replacement cost of £6,000–£10,000 for a reconditioned unit [EM‑QUOTE] is significant, but the M157 remains a popular choice for buyers wanting AMG performance on a tighter budget. See our M157 Knowledge Centre and M157 timing chain adjuster failure page for more detail.

M178 — The Evolved AMG V8

The M178 is Mercedes‑AMG's 4.0‑litre twin‑turbo V8, introduced in 2014 and still in production today. It's a significant evolution of the hot‑vee concept, with better thermal management, improved turbocharger response and reduced internal friction. The M178 is used in the AMG GT (C190/R190), E‑Class (W213 E63), S‑Class (W222/W223 S63), GLE (V167 GLE63) and G‑Class (W463 G63). Power outputs range from 469 bhp (AMG GT) to 630 bhp (AMG GT Black Series and E63 S). The M178 is more fuel‑efficient (23–26 mpg vs 20–23 mpg) and has significantly lower CO₂ emissions than the M157. Known issues are fewer and less severe: oil leaks from valve covers, occasional coolant system problems and, on high‑mileage examples, turbocharger wear. The M178 is widely regarded as one of the best modern V8 engines, combining brutal performance with surprising refinement. Its replacement cost is higher (£8,000–£14,000 [EM‑QUOTE]) but the engine is more robust and likely to last longer with fewer major repairs. Our M178 Knowledge Centre has full specs and fitment details.

Cost & Ownership Comparison

The M178 is cheaper to run than the M157 over a typical five‑year period, despite its higher purchase price (and therefore higher depreciation). The M178's better fuel economy saves around £500–£800 per year in fuel costs, and its superior reliability means fewer unexpected repair bills. The M157's lower entry price is tempting, but the risk of valve stem seal failures (costing £3,000–£6,000), turbo coolant line repairs and timing chain work can quickly erode any upfront saving. Over 50,000 miles, the M178 typically costs its owner £2,000–£4,000 less to run than the M157, making it the more financially sensible choice in the long run. For those on a tight budget, the M157 can still be a good option if you find a well‑maintained example with documented service history, but budget for the known failure points.

Metric (5 yrs, 50k miles)	M157 (5.5L)	M178 (4.0L)
Fuel cost	£8,500–£9,500	£7,200–£8,000
Servicing (scheduled)	£1,800–£2,500	£1,500–£2,200
Tax (VED)	£1,200–£1,500	£1,000–£1,300
Insurance (estimated)	£4,500–£6,500	£4,800–£7,000
Total estimated	£16,000–£20,000	£14,500–£18,500

These figures are [ESTIMATED] based on UK market data and our proprietary enquiry patterns. Actual costs vary by driving style, location and individual vehicle condition.

Which Should You Buy?

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Best Value & Lower Entry Price

M157 — older AMG models are significantly cheaper to buy. Just budget for known failure points and factor in higher running costs.

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Modern Performance & Efficiency

M178 — more powerful, more efficient and more reliable. Used in newer, more desirable AMG models like the AMG GT and W213 E63.

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Simpler Maintenance & Lower Risk

M178 — fewer known catastrophic failure modes, better thermal management and more modern engineering. Lower probability of a major repair.

🎵

Old‑School Character & Sound

M157 — many enthusiasts prefer the deeper, more old‑school V8 sound of the 5.5‑litre. If that's your priority, the M157 has charm the M178 can't match.

Whichever you choose, our repair vs replace calculator can help you decide what to do if you face a major engine failure.

Other AMG & V8 Comparisons

If you're still weighing up your options, these comparisons cover related AMG and V8 engines.

M157 vs M178 →

You're here — 5.5L vs 4.0L V8.

C63 vs E63 →

AMG saloon comparison.

Diesel vs Petrol Running Costs →

Which fuel type suits your mileage?

M276 vs M278 →

V6 vs V8 petrol comparison.

For more detailed engine‑specific information, visit our M157 Knowledge Centre or M178 Knowledge Centre.

Frequently Asked Questions

Which AMG V8 is more reliable, M157 or M178?

The M178 is more reliable. It has better thermal management, fewer known failure modes, and a more modern design. The M157 suffers from heat‑related valve stem seal failures and turbo coolant line issues that are expensive to fix.

Which is cheaper to run, M157 or M178?

The M178 is cheaper to run — it uses less fuel (23–26 mpg vs 20–23 mpg) and has lower servicing costs. The M157's lower purchase price is offset by higher running costs and repair bills.

Which sounds better, M157 or M178?

Many enthusiasts prefer the M157's deeper, more traditional V8 rumble. The M178 is more refined but can sound less characterful. It's a matter of personal taste.

Can I fit an M178 into an older AMG model?

No — the engine mounts, ECU, gearbox and cooling systems are completely different. An engine swap between generations is not practical or cost‑effective.

Which has better aftermarket tuning potential?

Both engines have strong tuning potential, but the M178's more advanced engine management and turbocharger design make it easier to extract reliable power. Up to 700 bhp is achievable with a remap and hardware upgrades.

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850 Combined EnquiriesM157 and M178 analysed from real 2025 UK data.

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A Real VerdictWe name a winner for most buyers, and explain when the M157 still makes sense.

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15+ Vetted Mercedes SpecialistsExperienced with both AMG V8 engines.

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100% IndependentNo sponsors, no manufacturer influence.