Turbocharging a BMW M50 motor

Turbocharging a BMW M50 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, 84mm bore,135mm rods
M20B25, 2.5 litre, 75mm stroke, 84mm bore,140mm rods
M20B25 eta, 2.7 litre, 75mm stroke, 84mm bore, 140mm rods

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
M20B25 eta 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 84mm crankshaft can be added with this combo, but not recommended for high rpm operation.

Engine block ancillaries:

2.0 oil pumps supposedly higher flowing than 1.8 ones, so preferable
ABF crank pulley can be used to convert to serpentine accessory belt system, or alternatively a 2.0-8v crank pulley can be slightly machined to correct any belt alignment issues
Crankshafts with 60-2 trigger wheels attached (e.g. ABF) are only found on 2.0 tall-blocks, so an external wheel+sensor must be fabricated for the standard height blocks if crankshaft trigger is required (e.g. waste-spark coils)

Cylinder head

Either small intake port (ABF, 9A) or the larger intake port heads (some PL, KR) can be used. Porting and polishing doesn't hurt if done properly. Possible additions:

Stiffer valve springs to allow higher rpm safely and to overcome the newfound positive manifold pressure
Titanium retainers are half the weight of stock steel retainers (~20g each) to allow higher rpm safely
Lightweight lifters from newer VW models (p/n 050 109 309 H) to save some valvetrain weight (saves ~10g each)
With no clearance ABF camshafts are about 254° duration and about the same for the KR
ABF camshafts have higher lift so can potentially injest more air (10,76int/10,73exh mm for the ABF and only 9,2int/10,21exh mm for the KR)
Here are the timing figures (for 1mm lift)

ABF : 1-38/39-1 so 2° of overlap
KR : 3-35/43-3 so 6° of overlap

The 35mm ABF valve guide is shorter than the 38mm 9A/KR/PL, hence causes less air restriction
Aftermarket or customs cams with internal vernier pulley to adjust overlap can be beneficial for some setups
Adjustable camshaft pulley can be useful to move the powerband around

Engine hardware

Use stock VW metal headgasket (p/n 050 103 383 A) for bores up to 83mm
Use ARP head studs (p/n ARP-204-4204 or ARP-204-4702) to help avoid blown head gaskets. Torque to ~70-85lbft using ARP lube
Use ARP rod bolts (p/n ARP-104-6004) to help keep the rods INSIDE the cylinder block. Can also use Ford 302 rod bolts (p/n ARP-154-6002) with some slight grinding of the head or if the bolts are pushed into the conrod hard enough. Torque to 25-40lbft using ARP lube.

Intake manifold

Various options possible when it comes to choosing an intake manifold:

Use stock K-Jetronic 16V manifold with custom fuel rail to accept EFI injectors
Use ABF manifold but swap the 225cc/min ABF injectors (p/n 0 280 155 606) for some bigger 279cc/min Nissan Primera ones (p/n 0 280 155 607 or 0 280 155 611)
Use a cut down Audi S2 manifold with 1.8T fuel rail (transverse or longitudinal will work)
Use custom intake manifold with 1.8T fuel rail (transverse or longitudinal will work). To reuse the stock flange, CIS injector seats will need to be swapped to digifant EFI seats (p/n 037 133 555 A)

Exhaust manifold

Various options possible when it comes to choosing an intake manifold:

Use stock 16V manifold with different flange welded on (places turbo very low)
Use stock 16V manifold with adapter plate
Use a cut down Audi S2 exhaust manifold Audi external wastegate
Use custom exhaust manifold (log-type or header-type), with or without external wastegate

Turbocharger

Use anything suitable for a 2.0-16v motor running a maximum RPM of 7000. The choices are many:

For good low-rpm response and reasonable power, a Garrett 60 trim T3 with stage 2 wheel and 0.48AR housing works well
The next stage up would be a hybrid T3/T4 50 trim compressor with stage 2 or stage 3 0.63AR turbine housing
A stage 3 0.82AR turbine housing will build boost at ~5000rpm and make good top end

Fueling

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

G60 Digifant I ECU and loom with custom programming and bigger 300 or 440cc injectors
Standalone engine management with at least 300cc/min injectors, and preferably 440cc or higher
Most stock VW 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 16V is no exception. Some recommendations:

Use colder plugs. Stock 16V is F6DTC for KR/PL/9A and F5DP0R (platinum) for ABF. 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.