BMW 8 Series G15Coupé2018–2026
Every engine fitted to the 8 Series Coupé G15 — from the 3.0-litre B58 straight-six in the 840i, to the 4.4-litre twin-turbo V8 in the M850i xDrive, and the fire-breathing S63 V8 in the M8 and M8 Competition.
Real UK costs, honest failure data, and the 2024 model year updates that keep BMW's flagship grand tourer relevant.
Backed by 120+ real UK F06/F13 enquiries in 2025 [BMW-VERIFIED].
Engine Database
Every engine fitted to the G15 Coupé — from the entry-level 840i to the range-topping M8 Competition, including the V8-powered M850i and the Alpina B8.
Overview
The BMW 8 Series G15 is the two-door coupé variant of the second-generation 8 Series, unveiled at the 24 Hours of Le Mans on 15 June 2018. It marks the return of the 8 Series nameplate after 19 years and serves as the successor to the BMW 6 Series (F06/F12/F13). Built on BMW's CLAR platform and manufactured in Dingolfing, Germany, the G15 is powered by a range of turbocharged six-cylinder petrol, six-cylinder diesel, and V8 petrol engines, all mated to an 8-speed ZF automatic transmission.
The range includes the B58-powered 840i, the N63 V8-powered M850i xDrive, the B57 diesel-powered 840d xDrive, and the S63 V8-powered M8 and M8 Competition. Production is expected to continue through to 2026.
Key Facts
Market Intelligence
The 8 Series G15 shares its engine line-up with the G14 and G16 — B58 enquiries totalled 540 and N63/S63 enquiries totalled 440/410 in 2025 [EM-VERIFIED], with the M850i xDrive attracting the highest proportion of performance-oriented enquiries.
04Best & Worst Engines
Best Diesel Overall
B57D30B (840d xDrive, 2018–2026)
“The 3.0-litre straight-six diesel produces 340 PS and 700 Nm — effortless, refined, and with 11 hp electric boost from the 48V mild-hybrid system. The B57 is chain-driven and proven across the BMW range. In the G15 coupé, it's the diesel you buy if you want six-cylinder refinement without V8 running costs.”
Who it's for: buyers who want effortless torque and 40+ mpg potential.
Best Petrol Overall
B58B30C (840i, 2019–2026)
“The B58 is BMW's modular straight-six masterpiece — 340 PS, 450 Nm, silky smooth, and chain-driven with a robust bottom end. The B58 is widely regarded as one of BMW's most reliable modern engines. In the G15 coupé, it's the petrol you can buy without anxiety.”
Who it's for: buyers who want six-cylinder performance without the V8's running costs. (Model-wide: B58B30 is the benchmark across the BMW range — the G15 offers the best expression of it in a grand tourer.)
Highest Risk
N63B44T3 (M850i xDrive, 2019–2026)
“The hot-vee V8 suffers from heat-related failures — valve stem seals, turbo coolant lines, and battery drain are common. The N63 has a well-earned reputation for high ownership costs. The M850i is fast, but it's expensive to keep fast.”
Who it's for: buyers with deep pockets who want V8 performance and are prepared for the maintenance bills.
Most Expensive Failure
S63B44T4 (M8 / M8 Competition)
“The S63 is the high-performance version of the N63 — up to 625 PS, twin-turbo, and even more complex. Valve stem seal failure, turbocharger wear, and the M-specific cooling system all add to the ownership cost. A full engine replacement sits at £9,000–£16,000.”
Who it's for: M8 buyers who budget for high-end maintenance and have a specialist they trust.
Best Value
B58B30C (840i, 2019–2026)
“The 840i offers 340 PS, 450 Nm, and near-40 mpg potential — it's the most efficient petrol in the range and the cheapest to replace. The B58 is robust, the chain is accessible, and the G15's weight doesn't overwhelm it.”
Who it's for: buyers who want the best balance of performance, economy, and reliability.
Most Overlooked
B57D30B (840d xDrive, 2018–2026)
“The 840d is often overshadowed by the V8 M850i and the M8, but the B57 straight-six diesel is the most efficient engine in the range — 340 PS, 700 Nm, and 44+ mpg potential. It's the grand tourer's grand tourer — effortless torque and real-world economy.”
Who it's for: buyers who want diesel refinement without the V8's running costs.
Best Diesel Overall
B57D30B
(840d xDrive, 2018–2026)
“The 3.0-litre straight-six diesel produces 340 PS and 700 Nm — effortless, refined, and with 11 hp electric boost from the 48V mild-hybrid system. The B57 is chain-driven and proven across the BMW range. In the G15 coupé, it's the diesel you buy if you want six-cylinder refinement without V8 running costs.”
Who it's for: buyers who want effortless torque and 40+ mpg potential.
Best Petrol Overall
B58B30C
(840i, 2019–2026)
“The B58 is BMW's modular straight-six masterpiece — 340 PS, 450 Nm, silky smooth, and chain-driven with a robust bottom end. The B58 is widely regarded as one of BMW's most reliable modern engines. In the G15 coupé, it's the petrol you can buy without anxiety.”
Who it's for: buyers who want six-cylinder performance without the V8's running costs. (Model-wide: B58B30 is the benchmark across the BMW range — the G15 offers the best expression of it in a grand tourer.)
Highest Risk
N63B44T3
(M850i xDrive, 2019–2026)
“The hot-vee V8 suffers from heat-related failures — valve stem seals, turbo coolant lines, and battery drain are common. The N63 has a well-earned reputation for high ownership costs. The M850i is fast, but it's expensive to keep fast.”
Who it's for: buyers with deep pockets who want V8 performance and are prepared for the maintenance bills.
Most Expensive Failure
S63B44T4
(M8 / M8 Competition)
“The S63 is the high-performance version of the N63 — up to 625 PS, twin-turbo, and even more complex. Valve stem seal failure, turbocharger wear, and the M-specific cooling system all add to the ownership cost. A full engine replacement sits at £9,000–£16,000.”
Who it's for: M8 buyers who budget for high-end maintenance and have a specialist they trust.
Best Value
B58B30C
(840i, 2019–2026)
“The 840i offers 340 PS, 450 Nm, and near-40 mpg potential — it's the most efficient petrol in the range and the cheapest to replace. The B58 is robust, the chain is accessible, and the G15's weight doesn't overwhelm it.”
Who it's for: buyers who want the best balance of performance, economy, and reliability.
Most Overlooked
B57D30B (840d xDrive, 2018–2026)
“The 840d is often overshadowed by the V8 M850i and the M8, but the B57 straight-six diesel is the most efficient engine in the range — 340 PS, 700 Nm, and 44+ mpg potential. It's the grand tourer's grand tourer — effortless torque and real-world economy.”
Who it's for: buyers who want diesel refinement without the V8's running costs.
Engine
Typical Mileage
Common Major Failure
Repair Cost (Specialist)
Replacement Cost (Recon)
Ownership Verdict
Best Petrol -340 PS, proven, dependable
Highest Risk -530 PS, high ownership costsPerformance Choice -600 PS, M-division costs
Most Expensive -625 PS, ultimate M costsBest Diesel -340 PS, efficient, dependableLuxury Choice -620 PS, Alpina refinementEngineTypical MileageCommon Major FailureRepair Cost (Specialist)Replacement Cost (Recon)Ownership VerdictBest Petrol -340 PS, proven, dependableHighest Risk -530 PS, high ownership costsPerformance Choice -600 PS, M-division costsMost Expensive -625 PS, ultimate M costsBest Diesel -340 PS, efficient, dependableLuxury Choice -620 PS, Alpina refinement
The G15 rule
A £5,000–£8,000 B57 recon against a £25,000–£65,000 G15 sits at 8–32% of the car's value -well within our 70% BMW threshold. Even the most expensive engine replacement (M8 Competition S63 at £10,000–£16,000) is only 15–64% of a well-kept G15's value. The G15 is a premium grand tourer with strong retained values -replacement is almost always financially viable. Extended warranty cover is still recommended, especially for V8 models and the 48V mild-hybrid system on the 840d.
N63
Valve Stem Seal Failure (M850i xDrive)
Affected models:
G15 M850i xDrive (2019–2026, N63B44T3).
Typical failure mileage:
50,000–100,000 miles.
Root cause:
The N63's hot-vee design (turbochargers located in the V of the engine) creates significant heat -valve stem seals can harden and fail, leading to oil consumption, blue smoke on startup, and eventually, catalytic converter damage. This is the defining issue of the N63 across all BMW models.
Tiered Cost Table
Preventative (Repair)
Dealer
Specialist
£4,000–£7,000
£3,000–£6,000
Catastrophic (Replacement)
Dealer
Specialist
£12,000–£16,000
£7,000–£12,000
Our Recommendation
On any N63-powered G15, have the valve stem seals inspected at 50,000 miles -a £3,000–£6,000 preventative repair is dramatically cheaper than the £7,000–£12,000 engine replacement that follows if ignored. This is the defining ownership cost of the M850i.
S63
Valve Stem Seal & Turbocharger Wear (M8 / M8 Competition)
Affected models:
G15 M8, M8 Competition (2020–2025, S63B44T4).
Typical failure mileage:
50,000–100,000 miles.
Root cause:
The S63 shares the N63's hot-vee architecture but with higher outputs -valve stem seal failure is common, and the turbochargers can suffer from wear due to the increased boost pressures. The M-specific cooling system also requires specialist maintenance.
Tiered Cost Table
Preventative (Repair)
Dealer
Specialist
£5,000–£8,000
£4,000–£7,000
Catastrophic (Replacement)
Dealer
Specialist
£14,000–£20,000
£9,000–£16,000
Our Recommendation
On any S63-powered M8, have the valve stem seals and turbochargers inspected at 50,000 miles -a £4,000–£7,000 preventative service is dramatically cheaper than the £9,000–£16,000 engine replacement that follows if ignored. The M8 is rewarding but requires serious maintenance commitment.
B58
Cooling System Maintenance (840i)
Affected models:
G15 840i, 840i xDrive (2019–2026, B58B30C).
Typical failure mileage:
60,000–100,000 miles.
Root cause:
The B58's cooling system uses plastic components that can become brittle with age and heat cycling -the expansion tank and water pump are the most common failure points. The B58 is otherwise one of BMW's most reliable engines, but cooling system maintenance is essential.
Tiered Cost Table
Preventative (Repair)
Dealer
Specialist
£600–£1,200
£400–£800
Catastrophic (Replacement)
Dealer
Specialist
£1,500–£3,000
£1,000–£2,000
Our Recommendation
On any B58-powered G15 over 60,000 miles, have the cooling system inspected at every service -a £400–£800 preventative repair is far cheaper than the £1,000–£2,000 cooling system overhaul or engine replacement that follows an overheat event. The B58 is otherwise a robust engine.
48V
Mild-Hybrid System Integration (840d xDrive)
Affected models:
G15 840d xDrive (2018–2026, B57D30B with 48V MHEV).
Typical failure mileage:
Unknown -new technology.
Root cause:
The 48V mild-hybrid system (integrated starter-generator, 48V battery) provides 11 hp of electric boost and smoother stop-start operation. Early issues may include battery degradation, software integration problems, or starter-generator failures.
Tiered Cost Table
Preventative (Repair)
Dealer
Specialist
£500–£1,500
£300–£1,000
Catastrophic (Replacement)
Dealer
Specialist
£2,000–£4,000
£1,500–£3,000
Our Recommendation
As a relatively new system, long-term reliability data is limited. Ensure the 48V system is covered under warranty and have it inspected at major services. The mild-hybrid system provides efficiency benefits and smoother stop-start operation but adds complexity.
VariantEngine Code
Used (Supply)
Reconditioned (Supply)
Rebuilt (Supply)
Labour Hours
Diesel Variants
Petrol Variants
Scope note: All variants listed are G15 Coupé (two-door grand tourer) only. G14 Convertible and G16 Gran Coupé share most of these engines but are covered on their own Generation Pages. The 840i uses the B58B30C (340 PS). The M850i xDrive uses the N63B44T3 (530 PS). The M8 uses the S63B44T4 (600 PS) and the M8 Competition produces 625 PS. The 840d xDrive uses the B57D30B with 48V mild-hybrid (340 PS). The Alpina B8 uses a tuned N63 V8 producing 620 PS. The 840i and 840d are available with rear-wheel drive, while the M850i xDrive, M8, and M8 Competition are all-wheel drive (xDrive) only.
Most Requested Engines (2025)
Most Requested Variants
Average Replacement Cost
£4,000–£10,000 supply only
[BMW-QUOTE]
Most Common Failures
Live FeedBMW 840i G15
B58
Manchester
Cooling system check4
enquiries
Updated MonthlyBMW M850i xDrive G15
N63
LondonOil consumption check3
enquiries
Updated MonthlyBMW M8 Competition G15
S63
BirminghamTurbocharger inspection2
enquiries
Updated MonthlyMost Requested Engines (2025)
Most Requested Variants
Average Replacement Cost
£4,000–£10,000 supply only
[BMW-QUOTE]
Most Common Failures
Most Requested Engines (2025)
Most Requested Variants
Average Replacement Cost
£4,000–£10,000 supply only
[BMW-QUOTE]
Most Common Failures
Diesel Variants
Petrol Variants
Scope note: All variants listed are G15 Coupé (two-door grand tourer) only. G14 Convertible and G16 Gran Coupé share most of these engines but are covered on their own Generation Pages. The 840i uses the B58B30C (340 PS). The M850i xDrive uses the N63B44T3 (530 PS). The M8 uses the S63B44T4 (600 PS) and the M8 Competition produces 625 PS. The 840d xDrive uses the B57D30B with 48V mild-hybrid (340 PS). The Alpina B8 uses a tuned N63 V8 producing 620 PS. The 840i and 840d are available with rear-wheel drive, while the M850i xDrive, M8, and M8 Competition are all-wheel drive (xDrive) only.
Most Requested Engines (2025)
Most Requested Variants
Average Replacement Cost
£4,000–£10,000 supply only
[BMW-QUOTE]
Most Common Failures
Most Requested Engines (2025)
Most Requested Variants
Average Replacement Cost
£4,000–£10,000 supply only
[BMW-QUOTE]
Most Common Failures
Most Requested Engines (2025)
Most Requested Variants
Average Replacement Cost
£4,000–£10,000 supply only
[BMW-QUOTE]
Most Common Failures
Live FeedBMW 840i G15
B58
ManchesterCooling system check4
enquiries
BMW M850i xDrive G15
N63
LondonOil consumption check3
enquiries
BMW M8 Competition G15
S63
BirminghamTurbocharger inspection2
enquiries
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