M20

Upgrades

Intake

For most Stage 0.5-1.5 M20, an upgraded drop-in filter is usually perfectly fine and offers a similar power gain to a full intake system. The stock intake box handles about 200 WHp relatively well. The M30 intake box offers a 62% increase in filter surface diameter and handles over 300 WHp relatively well. You can upgrade to a full intake system if you wish. A popular option is the KAMotors intake.

The stock M20B25 is equipped with what's known as a VAM (Vane Airflow Meter/Volume Airflow Meter), VAFS (Volume Air Flow Sensor), AFM (Air Flow Meter), or VAFM (Vane/Volume Air Flow Meter) -- all of which are correct and perfectly acceptable terms. Programmatically this sensor can handle up to about 350 Hp. Realistically it's not recommended to take it over 250 Hp.

The M30B35 VAM (P/N BMW: 13621286064, Bosch: 0280203027) is a common upgrade choice. It has a larger diameter which allows for improved airflow. Programmatically it is capable of reading up to nearly 400 Hp. Realistically it's not recommended to take it over 300 Hp.

There are no retail-level MAF kits on the market that correctly translate a MAF signal into something the stock Motronic 1.1/1.3 ECUs can read correctly. Yes, even that one.

Exhaust

The best bolt-on upgrade for any M20 engine is the exhaust manifold. The stock unit is heavy and very poorly designed for a performance application. A popular option is the Shmeidmann longtube system (shipping to the USA can be expensive however).

A cat-back exhaust offers the least gains in comparison to intake and exhaust manifold. 57-76 mm (2.25-3 inch) diameters are ideal, though there is minimal difference beyond 65 mm (2.5 in) diameter at lower modification levels. Please consider the weight difference to power gain and decide what works best for you.

Head

A popular upgrade is +2 mm oversized valves on both intake/exhaust, with matching angle grinds. A port and polish is also worthwhile when modifying the head.

Camshaft

Common street camshafts range in the 260-268 range on both intake and exhaust. Race camshafts can have as much as 294 degrees of timing.

Read the camshaft page for more information.

Valvetrain

+1 mm oversized valves are a worthwhile upgrade. If you plan to rev high, the most important upgrades are valve springs and rockers.

Forced Induction

You can expect between 6-12 Hp per pound of positive pressure added for most forced induction setups. All M20 variants are able to handle 6-8 PSI relatively well without any internal modifications.

The M20 in all forms reacts quite well to forced induction. The B27 with original electronics requires extra work and components, please read the B27 section below.

A tuned Motronic 1.1 or 1.3 ECU can handle 14-18 PSI relatively well without large modifications. However, it may be necessary to change the intake metering device.

The VAM (Vane Airflow Meter) on the M20B20, B23, B25, and S-ETA B27 are scaled to handle a maximum of 350 WHp, though it's not recommended to exceed 300 WHp for reliability.

The M30 VAM is scaled to handle a maximum of about 400 WHp, though it's not recommended to exceed 350 WHp.