Turbocharging a BMW M20 motor

Turbocharging a BMW M20 engine

This page describes the various paths that can be followed in the process of turbocharging a BMW M20 motor, from mild to wild. Most of the information pertains to the M20B25 2.5litre version.The information is arranged thematically, with no guarantee of its accuracy.

Engine block

The BMW M20 motor comes in various flavours:

M20B20, 2.0 litre, 75mm stroke, 81mm bore,140mm rods
M20B25, 2.5 litre, 75mm stroke, 84mm bore,135mm rods
M20B27, 2.7 litre, 81mm stroke, 84mm bore, 130mm rods
Connecting rod dimensions for M20B25:

C-C length: 135mm
Big end bore: 48.0mm
Big end width: 21.8mm
Pin diameter: 22mm
Small end width: 21.8mm

Stock internals have been proven to be reliable at high output levels, with the rods being the weak link if detonation is avoided. At elevated power output, detonation will be unforgiving towards the stock cast pistons. Various options to use for the bottom end are:

Stock rods and pistons in various combinations, which yield the following ratios:

M20B25 8.8 to 1
M20B20 7 to 1
M20B27 7 to 1

Stock rods and pistons, with 3.5mm metal headgasket, will lower the compression by approx 1 point
Aftermarket forged steel rods, with forged aluminum pistons. Bore size up to 87 mm possible if the block is crack-free, but for turbo avoid more than 85mm bore. Diesel/eta 81mm crankshaft can be added with this combo, but not recommended for very high rpm operation.

Listed below are the dimensions of some pistons and rods that can be mixed and matched to get various displacements in an M20:

Pistons (CH is compression height):

M44, 30.4mm CH, 84mm bore
M42, 31.65mm CH, 84mm bore
M43, 31.65mm CH, 84mm bore
M40, 31.65mm CH, 84mm bore
M20, 34.09mm CH, 84mm bore
M50B25 (VANOS), 32.55mm CH, 84mm bore
M50B25 (non VANOS), 38.2mm CH, 84mm bore
M52B28, 31.82mm CH, 84mm bore
M54B30, 28.32mm CH, 84mm bore
S50B30 (USA), 32.8mm CH, 86mm bore
S50B32 (USA), 31mm CH, 86.4mm bore

Crankshafts:

M20B25, 75mm
M20B27, 81mm
M50B25, 75mm
M50B28, 84mm
M52B28, 84mm
M54B30, 89.6mm

Rods:

M50B25 (VANOS), 135mm
M50B25 (non VANOS), 140mm
M20B27, 130mm
M20B20, 130mm
M20B25, 135mm

Block:

M20B25, deck height = 206.7mm
M20B27, deck height = 206.2mm

Stock headgasket is 1.75mm when compressed and accomodates 85.2mm bore

One documented setup is:

M50 pistons
81mm crankshaft
M20B25 block
remove 1mm from top of pistons

Another documented setup:

M20B25 block
M43TUB19 oversize 85.5mm pistons
M20B25 135mm rod
324td 81mm crankshaft
remove 0.3mm from deck height

Use some simple mathematics to determine if your piston/rod/crank combo will fit inside the block, and then start worrying about piston shape (for valve reliefs and compression ratio's sake).

Cylinder head

M20B20 has small valves and smaller chamber (37cc?)
M20B25 has bigger valves and combustion chamber (40cc?)

Engine hardware

Use 12.9 metric blue bolts

Intake manifold

Use stock up to 400+hp

Exhaust manifold

Use Y-adapter between turbo flange and original twin manifolds
Custom log type manifold
Custom header style
Cast manifold TCD or cartech

Turbocharger

Use any suitable garrett with ~60mm compressor inducer and at least 0.58 AR T4 housing
At least Holset HX35 with nothing lower than 12cm^2, and up to 19cm^2

Fueling

The engine can be fueled in an infinite number of ways, but the recommended paths are:

Standalone engine management with at least 300cc/min injectors, and preferably 440cc or higher
Most stock BMW fuel pumps good for just over 250hp of fuel, but an inline Bosch or Walbro in conjunction with the stock pump is the safer option

Ignition

Most modern vehicles have very good ignition systems, and the BMW is no exception. Some recommendations:

Use colder plugs. For most applications, a Bosch 5 or an NGK 7 are fine, usually gapped to 0.7-0.8mm. For higher boost, it might be safer to go with NGK 8's.
Proper gapping of the plugs is necessary if using the stock coil. Depends on plug type (copper/iridium etc) and boost levels. For wastespark conversions, a gap of 0.8mm can usually be used, otherwise 0.7mm should be fine.
Aftermarket ignition leads are usually a good thing as the stock ones normally tend to be old and tired. Magnecor KV85 are fine.