For most Stage 0.5-1.5 M20, an upgraded drop-in filter is usually perfectly fine and offers a fair gain of a few Hp. 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.
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. It is, however, NOT a MAF (Mass Air Flow sensor), which operates on an entirely different principal.
Programmatically the stock 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.
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.
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.
+1 mm oversized valves are a worthwhile upgrade.
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.
If you plan to rev high, the most important upgrades are valve springs and rockers.
M20B27 vehicles equipped with ML1.0 (Motronic Limited 1.0; 1984-1987) often wish to upgrade to M1.1/M1.3 (Motronic 1.1/1.3) electronics for a far superior system. This allows the usage of modern high impedance injectors, a vastly improved ECU tuning platform, and vastly improved control electronics.
M1.1-equipped vehicles (1988 325e with S-ETA engine, and some 1988-1989 325i) can swap directly to M1.3 with no modification. There is only minor benefits to M1.3 systems as compared to M1.1. The major upgrades were to closed-loop fuel control, a massive increase in number of tunable maps, and a completely redesigned and improved circuit board. There is no measurable output power benefit to one system or the other, and both have nearly identical processing power.
The stock 325e (M20B27) injectors/ECU are low impedance systems which are not compatible with most modern injectors, which are high impedance. They flow approximately 15 lb/hr at their default 2.5 bar, or 18 lb/hr at 3.0 bar.
The stock injectors for 320i (-B20), 323i (-B23), and 325i (-B25) with M1.1/1.3 systems flow 14.5-15 lb/hr at their default 3.0 bar. They are generally considered poor in terms of fuel dispersal pattern and performance. They will handle upwards of 150-155 WHp, which is enough to support the 135-145 WHp most stock M20B25 tend to make.
A common minor upgrade to the -B20/23/25 engines are M50B25 injectors (0-280-150-714), which flow 15 lb/hr at 3.0 bar -- a common misconception is that they will flow 17.5 lb/hr, which they only do on 3.5 bar fuel systems. These injectors have an improved fuel dispersion in comparison to the stock M20 injectors. They have a multi-hole (4-hole) nozzle, rather than the single exposed pintle. See the difference on the injector page. Flowing 15 lb/hr means they are compatible with the stock tune, although it's still best practice to have a tune made for the specific injectors you're using, as they all have different response times and flow patterns. These will support 150-155 WHp.
As you go beyond the stock power level, it becomes critical to upgrade the injector flow rate. 19 lb/hr injectors will support 200 WHp. 24 lb/hr injectors will support 245 WHp. 42 lb/hr injectors will support 430 WHp.
Read more about injectors!
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 250 WHp for reliability.
The M30 VAM is scaled to handle a maximum of about 400 WHp, though it's not recommended to exceed 300 WHp.
Read more on the Forced Induction page!