For most Stage 0.5-1.5 M42, 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.
There are no retail-level MAF kits on the market that correctly translate a MAF signal into something the stock Motronic 1.7 ECUs can read correctly. Yes, even that one.
The stock exhaust manifold is relatively good for OE. Upgrading this component is still a worthwhile investment, and can reduce weight, temperatures, and overall engine efficiency.
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.
The stock M42 fuel injectors are 18.25 lb/hr @ 3.0 bar (E30), or 18.5 lb/hr @ 3.5 bar (E36) and support 125 WHp safely, or 145 WHp maximum.
The stock M42 fuel pump flows 130 liters per hour. This tends to handle upwards of 220-250 WHp.
Read the camshaft page for more information.
+1 mm oversized valves are a worthwhile upgrade. If you plan to rev high, the most important upgrades are valve springs and rockers.
You can expect between 6-12 Hp per pound of positive pressure added for most forced induction setups. All M42 variants are able to handle 6-8 PSI relatively well without any internal modifications.
A tuned Motronic 1.7 ECU can handle 400 WHp relatively well without any sensor changes or large modifications.
The VAM (Vane Airflow Meter) on the M42B18 is scaled to handle a maximum of 350 WHp, though it's not recommended to exceed 300 WHp for reliability.