What a B58 swap actually costs in an E9x — the 2026 breakdown

Forum threads throw numbers from $5,000 to $40,000 at this question and none of them agree, because most threads are missing half the build, are years out of date, or never reconciled a single invoice. This dispatch does it line by line: what each system costs, what most people forget to budget, and where the money actually goes.

Where the money goes

It is not "engine + transmission + go." A realistic cost sheet has eight line items, and the one that sinks budgets is almost never the engine:

• Donor engine and accessories — the long block plus everything bolted to it
• Transmission — a ZF 8HP matched to the donor engine
• Wiring, DME and integration — the section most budgets forget
• Cooling system — split-circuit reroute, electric pump, thermostat
• Fuel system — pump, lines, pressure management for direct injection supply
• Exhaust — downpipe fabrication is where the B58's layout bites in an E9x
• Mounts, driveline, hardware — custom driveshaft is near-certain with the 8HP
• Tuning and coding — immobilizer, VIN, TCM adaptation, then calibration

1 · Donor engine and accessories — $4,500 to $9,500

The B58 is one of the most abundant modern BMW engines in salvage right now. It left the factory in the F-series 340i/440i/240i (pre-update), the G-series M340i and 540i (the Technical Update engines), the Z4 M40i, the X3/X4/X5 40i models, and the Toyota Supra. For an E9x swap, the common pick is a wrecked RWD 340i or 240i with the ZF 8HP: the engine, transmission, harness, ECU and accessories come out as one matched set, which matters more than any single component price.

2026 salvage ranges:

• Complete B58 pull-out with harness, ECU, accessories: $4,500–$7,500
• Technical Update (2019+) pull-out: $6,500–$9,500
• Low-mileage (<15k) pull-out with accessories: $7,000–$9,500

Buy the most complete pull-out you can stretch to. Rebuying the starter, alternator, and accessory drives separately typically costs $1,200 or more, and not every accessory carries across B58 variants.

Before money moves, verify on the donor:

1. Compression — all six within 5% of each other
2. Oil — no coolant mixing, no metal in the filter housing
3. Turbo actuator — play in the wastegate rod means a rebuild budget
4. Known leak points — valve cover and oil filter housing on higher-mileage units
5. Crank sensor ring — damaged reluctor teeth will haunt first start

2 · Transmission — $2,000 to $5,500

The ZF 8HP family is the transmission the B58 was designed around, which is exactly why it's the right answer for this conversion. The 8HP45 carries stock B58 torque with room; if the plan includes serious torque targets, the 8HP70 from a 540i/740i donor is the upgrade path. Get the engine and transmission from the same car so the TCM and DME already speak the same language — piecing them together means coding work later.

• Used 8HP45 (340i/240i): $2,000–$3,500
• Used 8HP70 (540i/740i): $3,000–$4,500
• Rebuilt 8HP45 with warranty: $4,000–$5,500
• Replacement TCM if needed: $400–$700

3 · Wiring, DME and integration — $800 to $3,500

This is where most budget estimates fall apart, because this is where the swap actually happens. The B58's engine management needs to talk to the transmission controller, the chassis modules, and a minimum set of signals to run properly — and the E9x side has to be bridged into that world cleanly. There are two routes:

Route A: standalone harness — $800 to $1,500

Strip the engine down to what it needs to run with standalone management. Simpler wiring, but you trade away factory driveability and diagnostics. It's a valid path; know what you're giving up before you choose it.

Route B: factory DME, full integration — $1,500 to $3,500

Keep the factory DME, run the full harness, and merge it into the E9x chassis with a purpose-built adapter or merge harness. This preserves cold-start behavior and full diagnostics, and it's the route this desk recommends for a car that has to live on the road. The work splits across the merge harness or adapter ($600–$1,200), DME coding ($300–$800), immobilizer and start-request handling ($200–$500), and connectors and sensors ($200–$500).

4 · Cooling system — $600 to $2,200

The B58 runs a split cooling circuit with an electric water pump, an electronic thermostat, and a coolant pipe through the engine valley. In an E9x, everything gets rerouted. Budget:

• Radiator (OEM-spec or upgraded): $400–$800
• Electric water pump: $250–$400
• Electronic thermostat: $120–$200
• Hoses and custom pipes: $150–$400
• Expansion tank and coolant: $140–$250

Technical Update engines use different pump and thermostat spec than pre-update engines. Match parts to your exact donor variant, not to "a B58."

5 · Fuel system — $300 to $1,200

The B58's high-pressure pump does the injection work, but it needs adequate supply pressure and volume. A healthy pump upgrade ($150–$250), proper lines and fittings ($100–$300), and filtration ($30–$80) cover a stock-power build. Bigger power plans change this line item significantly — size the fuel system to the power target before ordering.

6 · Exhaust — $500 to $3,500

The widest range on the sheet, because it's the most fabrication-heavy. If you weld and have a lift, materials alone run about $500–$1,000. Paying a shop for a quality custom system is $2,000+. The specific E9x headache: the B58's downpipe exit geometry means fabricating around the steering column and crossmember area — the part of this job that trips up the most DIY builds.

7 · Mounts, driveline, hardware — $300 to $1,500

• Engine mounts (conversion-specific): $150–$400
• Transmission mount: $50–$200
• Custom driveshaft (length and output flange differ with the 8HP): $300–$800
• Hardware, brackets, fasteners: $100–$300

8 · Tuning and coding — $400 to $2,000

Immobilizer deletion, VIN correction, and TCM coding run $300–$800. An off-the-shelf calibration covers a stock-engine build ($400–$1,000); modified engines with turbo work need custom calibration ($800–$2,000). For anything past modest power targets, professional calibration is not optional — it's the difference between a build that runs and a build that runs reliably.

The grand total

Tier 1 · Budget build — DIY, used parts, own labor

Tier 2 · Mid-range build — DIY with quality upgrades

Tier 3 · Full shop build — low-mileage donor, paid labor

The hidden line items that sink budgets

These are the ones that never appear in a forum-thread budget:

• Valley coolant pipe o-rings — do it while the engine is out ($50–$100), because doing it later is hours of misery
• Seals and gaskets — rear main, valve cover, oil filter housing while everything is apart ($150–$400)
• AC line routing — custom lines or a delete decision ($200–$600)
• Power steering adaptation — the B58's electric pump needs integration with the E9x rack ($150–$400)
• Brake booster vacuum — electric vacuum pump integration ($100–$300)
• Broken fasteners and small parts — every build eats some; budget $100 and thank yourself later

Where the B48 fits

If the budget is the deciding factor: the B48 family is related but not identical, and donors tend to cost less. You trade displacement and power ceiling for a smaller purchase price — but the integration work (wiring, controller, cooling) is not meaningfully simpler, so don't assume a proportionally cheaper build. Verify the delta on current salvage prices before choosing.

Bottom line

A B58 + 8HP E9x conversion is not a cheap project, and anyone quoting you a flat $5,000 is quoting you a fantasy. But it's a bounded project: every line item above is knowable before the first wrench turns. That's the whole point of planning the swap on paper first — the cheapest mistake to fix is the one you catch on the spreadsheet.