For most Stage 0.5-1.5 M40, an upgraded drop-in filter is usually perfectly fine and offers a similar power gain to a full intake system. At higher power levels, upgrading the intake system will be more worthwhile.
The stock exhaust manifold is relatively good for OE. Upgrading this component is still a worthwhile investment, and can reduce weight, temperatures, and increase 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 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 6-10 Hp per pound of positive pressure added for most forced induction setups. All M40 variants are able to handle 6-8 PSI relatively well without any internal modifications. With just ARP head studs and a quality head gasket, an otherwise stock engine can handle 12-15 PSI relatively well.