BMW 8 Series G14Convertible2018–2026
Every powertrain fitted to the G14-generation BMW 8 Series Convertible — from the 3.0-litre B58 six-cylinder to the 4.4-litre N63 V8 and the fire-breathing S63-powered M8 Competition.
Real UK costs, honest failure data, and the hot-vee V8's defining role in this generation's ownership story.
Backed by real UK 8 Series enquiries in 2025 [BMW-VERIFIED].
Engine Database
Every powertrain fitted to the G14 8 Series Convertible specifically — from the 2018 launch through the LCI facelift and the introduction of the M8 Competition.
Overview
The BMW G14 is the convertible variant of the second-generation BMW 8 Series, a range of grand tourers produced since 2018. Launched in November 2018, the G14 features a folding cloth roof that operates in 15 seconds at speeds up to 50 km/h (31 mph). Built on BMW's CLAR platform, the G14 shares its powertrain line-up with the G15 Coupé and G16 Gran Coupé.
The engine range includes the 3.0-litre B58 six-cylinder petrol (840i), the 4.4-litre N63 V8 petrol (M850i xDrive), the high-performance S63 V8 (M8 Competition), and the 3.0-litre B57 six-cylinder diesel (840d xDrive), all mated to an 8-speed automatic transmission. An LCI facelift in 2020 introduced updated engines and mild-hybrid technology on the diesel variant. The 8 Series range is scheduled to end production in 2026.
Key Facts
Market Intelligence
The G14 8 Series Convertible is a low-volume luxury grand tourer; enquiries are concentrated on the B58 six-cylinder and N63 V8 petrol engines, with values ranging from £35,000 for early 840i examples to £80,000+ for well-specced M8 Competitions.
04Best & Worst Engines
Best Petrol Overall
B58B30M1 3.0-litre six-cylinder (840i) — The 335 bhp B58 is widely regarded as BMW's most reliable modern six-cylinder. Delivering 500 Nm of torque and 0-62 in approximately 5.2 seconds, it offers genuine grand touring performance with proven durability. The TU2 version (2020+) adds mild improvements without compromising reliability.
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Who it's for: buyers who want the sweet spot of performance, refinement, and reliability.
Best Diesel Overall
B57D30B 3.0-litre six-cylinder (840d xDrive) — The 315–340 bhp B57 six-cylinder diesel delivers effortless torque (680–700 Nm), excellent refinement, and genuine durability for high-mileage grand touring. The mild-hybrid system adds efficiency without complexity.
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Who it's for: high-mileage buyers who want effortless torque and 40+ mpg economy.
Highest Risk
N63B44D/N63B44T3 4.4-litre V8 (M850i xDrive) — The hot-vee V8 design places turbochargers in the engine's 'V', generating significant heat. Valve stem seal wear and cooling system leaks are well-documented on N63-family engines. The updated TU3 version (2020+) is improved but still demands meticulous maintenance.
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Who it's for: buyers who accept the premium and keep the car under warranty.
Most Expensive Failure
S63B44B/S63B44T4 4.4-litre V8 (M8 Competition) — The 625 bhp S63 is the most expensive powertrain in the G14 line-up to replace, with reconditioned units costing £8,000–£14,000. The high-performance V8 shares the N63's hot-vee architecture with additional stresses from higher boost and output.
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Who it's for: collectors and enthusiasts who accept the premium.
Best Value
B58B30M1 3.0-litre six-cylinder (840i) — The 335 bhp B58 offers the best balance of performance, refinement, and running costs. It's the most common G14 engine, parts are widely available, and the recon cost is significantly lower than the V8 options.
“”
Who it's for: the majority of G14 buyers who want genuine grand touring performance without V8 running costs.
Most Overlooked
B57D30B 3.0-litre diesel (840d xDrive) — Often overlooked by buyers who default to the petrol six or V8, the 840d delivers effortless torque, 40+ mpg economy, and genuine long-distance capability. With 680 Nm of torque and 0-62 in approximately 5.1 seconds, it's the ultimate motorway cruiser in the G14 range.
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Who it's for: buyers who prioritise long-distance touring economy without sacrificing performance.
Best Petrol Overall
B58B30M1
3.0-litre six-cylinder (840i) — The 335 bhp B58 is widely regarded as BMW's most reliable modern six-cylinder. Delivering 500 Nm of torque and 0-62 in approximately 5.2 seconds, it offers genuine grand touring performance with proven durability. The TU2 version (2020+) adds mild improvements without compromising reliability.
“”
Who it's for: buyers who want the sweet spot of performance, refinement, and reliability.
Best Diesel Overall
B57D30B
3.0-litre six-cylinder (840d xDrive) — The 315–340 bhp B57 six-cylinder diesel delivers effortless torque (680–700 Nm), excellent refinement, and genuine durability for high-mileage grand touring. The mild-hybrid system adds efficiency without complexity.
“”
Who it's for: high-mileage buyers who want effortless torque and 40+ mpg economy.
Highest Risk
N63B44D/N63B44T3
4.4-litre V8 (M850i xDrive) — The hot-vee V8 design places turbochargers in the engine's 'V', generating significant heat. Valve stem seal wear and cooling system leaks are well-documented on N63-family engines. The updated TU3 version (2020+) is improved but still demands meticulous maintenance.
“”
Who it's for: buyers who accept the premium and keep the car under warranty.
Most Expensive Failure
S63B44B/S63B44T4
4.4-litre V8 (M8 Competition) — The 625 bhp S63 is the most expensive powertrain in the G14 line-up to replace, with reconditioned units costing £8,000–£14,000. The high-performance V8 shares the N63's hot-vee architecture with additional stresses from higher boost and output.
“”
Who it's for: collectors and enthusiasts who accept the premium.
Best Value
B58B30M1
3.0-litre six-cylinder (840i) — The 335 bhp B58 offers the best balance of performance, refinement, and running costs. It's the most common G14 engine, parts are widely available, and the recon cost is significantly lower than the V8 options.
“”
Who it's for: the majority of G14 buyers who want genuine grand touring performance without V8 running costs.
Most Overlooked
B57D30B 3.0-litre diesel (840d xDrive) — Often overlooked by buyers who default to the petrol six or V8, the 840d delivers effortless torque, 40+ mpg economy, and genuine long-distance capability. With 680 Nm of torque and 0-62 in approximately 5.1 seconds, it's the ultimate motorway cruiser in the G14 range.
“”
Who it's for: buyers who prioritise long-distance touring economy without sacrificing performance.
Engine
Typical Mileage
Common Major Failure
Repair Cost (Specialist)
Replacement Cost (Recon)
Ownership Verdict
Best Petrol -reliable six-cylinder
Highest Risk -V8 ownership is expensivePremium Choice -M car running costsBest Diesel -effortless torqueEngineTypical MileageCommon Major FailureRepair Cost (Specialist)Replacement Cost (Recon)Ownership VerdictBest Petrol -reliable six-cylinderHighest Risk -V8 ownership is expensivePremium Choice -M car running costsBest Diesel -effortless torque
The G14 rule
A £6,000–£10,000 N63 recon against a £30,000–£40,000 early G14 sits at 15–33% of the car's value -safe. Against a well-kept £50,000+ example, the same recon drops to a routine 12–20% -a repair decision, not a financial one. For a high-value luxury car, the 70% BMW threshold rarely applies, but we'll still honestly tell you when a repair doesn't make sense.
Key Takeaways
Which G14 Costs Least To Own?
B57 840d -best economy, lowest recon cost.
Which G14 Costs Most To Own?
S63 M8 Competition -valve stem seals and turbo repairs are expensive.
Which G14 Is Best Value?
B58 840i -the sweet spot between performance and running costs.
N63/S63
V8 Valve Stem Seal Wear
Affected models:
G14 M850i xDrive (N63B44D/N63B44T3) and M8 Competition (S63B44B/S63B44T4).
Typical failure mileage:
30,000–60,000 miles.
Root cause:
The hot-vee V8 design generates significant heat, causing valve stem seals to harden and wear prematurely. Blue smoke on startup is the classic warning sign; ignored, it leads to oil consumption and potential catalytic converter damage.
Tiered Cost Table
Preventative (Repair)
Dealer
Specialist
£4,000–£6,000
£3,000–£5,000
Valve stem seal replacement
Catastrophic (Replacement)
Dealer
Specialist
£8,000–£12,000
£6,000–£10,000
Full reconditioned engine
Our Recommendation
On any M850i or M8, watch for blue smoke on startup -a £3,000–£5,000 preventative repair is cheaper than the £6,000–£10,000+ replacement that follows if ignored.
B58
High-Pressure Fuel Pump (HPFP) Failure
Affected models:
G14 840i (B58B30C/B58B30M1), all years.
Typical failure mileage:
40,000–80,000 miles.
Root cause:
The high-pressure fuel pump on B58 engines can fail, leading to long cranking times before the engine starts, noticeable loss of power, engine misfires, and even stalling. The issue can be intermittent, making diagnosis challenging.
Tiered Cost Table
Preventative (Repair)
Dealer
Specialist
£1,200–£2,000
£800–£1,500
HPFP replacement, fuel system flush
Catastrophic (Replacement)
Dealer
Specialist
£5,500–£9,000
£4,800–£8,000
Full reconditioned engine
Our Recommendation
If your G14 840i exhibits long cranking or power loss, have the HPFP inspected immediately -an £800–£1,500 preventative repair is cheaper than the £4,800–£8,000 replacement that follows if ignored.
B57
Diesel DPF & EGR Issues
Affected models:
G14 840d xDrive (B57D30B).
Typical failure mileage:
60,000–120,000 miles.
Root cause:
DPF blockage is common on cars used predominantly for urban driving, with fault codes indicating DPF sensor failure. EGR cooler issues can also develop. The mild-hybrid system adds some complexity to the diesel powertrain.
Tiered Cost Table
Preventative (Repair)
Dealer
Specialist
£800–£1,500
£500–£1,200
EGR cooler replacement, DPF regeneration
Catastrophic (Replacement)
Dealer
Specialist
£4,000–£6,000
£2,800–£5,000
Full reconditioned engine
Our Recommendation
On any G14 diesel, ensure regular motorway runs for DPF regeneration -a £500–£1,200 preventative service is cheaper than the £2,800–£5,000 replacement that follows if ignored.
VariantEngine Code
Used (Supply)
Reconditioned (Supply)
Rebuilt (Supply)
Labour Hours
Diesel Variants
Petrol Variants
Scope note: All variants listed are G14 8 Series Convertible only. The G15 Coupé and G16 Gran Coupé share these powertrains but are covered on their own Generation Pages. The M8 Competition is the high-performance variant powered by the S63 V8. The LCI facelift (2020) introduced the updated B58B30M1 and N63B44T3 engines, along with mild-hybrid technology on the 840d.
Most Requested Engines (2025)
Most Requested Variants
Average Replacement Cost
£4,500–£14,000 supply-only
[BMW-QUOTE]
Most Common Failures
Live FeedBMW 8 Series 840i G14
B58
London
Long cranking on startup4
enquiries
Updated MonthlyBMW 8 Series M850i G14
N63
BirminghamBlue smoke on startup3
enquiries
Updated MonthlyBMW 8 Series M8 Competition G14
S63
ManchesterOil consumption, valve stem seals2
enquiries
Updated MonthlyMost Requested Engines (2025)
Most Requested Variants
Average Replacement Cost
£4,500–£14,000 supply-only
[BMW-QUOTE]
Most Common Failures
Most Requested Engines (2025)
Most Requested Variants
Average Replacement Cost
£4,500–£14,000 supply-only
[BMW-QUOTE]
Most Common Failures
Diesel Variants
Petrol Variants
Scope note: All variants listed are G14 8 Series Convertible only. The G15 Coupé and G16 Gran Coupé share these powertrains but are covered on their own Generation Pages. The M8 Competition is the high-performance variant powered by the S63 V8. The LCI facelift (2020) introduced the updated B58B30M1 and N63B44T3 engines, along with mild-hybrid technology on the 840d.
Most Requested Engines (2025)
Most Requested Variants
Average Replacement Cost
£4,500–£14,000 supply-only
[BMW-QUOTE]
Most Common Failures
Most Requested Engines (2025)
Most Requested Variants
Average Replacement Cost
£4,500–£14,000 supply-only
[BMW-QUOTE]
Most Common Failures
Most Requested Engines (2025)
Most Requested Variants
Average Replacement Cost
£4,500–£14,000 supply-only
[BMW-QUOTE]
Most Common Failures
Live FeedBMW 8 Series 840i G14
B58
LondonLong cranking on startup4
enquiries
BMW 8 Series M850i G14
N63
BirminghamBlue smoke on startup3
enquiries
BMW 8 Series M8 Competition G14
S63
ManchesterOil consumption, valve stem seals2
enquiries
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