Tuning the BMW N42

"Comprehensive guide to performance parts and tuning the BMW N42 engine!"

The BMW N42 offer good returns when tuned and with the ultimate parts like a remap, turbo upgrades and camshafts you will dramatically increase your driving enjoyment.

TorqueCars will examine the options for your N42 tuning and show the premier upgrades.

The greatest N42 upgrades on an engine are obviously the ones that give the best value for money.

We won't be swayed by popular N42 upgrades, they need to be cost effective.

The cam profile plays a big part in the engines power output so cam upgrades make quite a large difference. The intake and exhaust durations will alter depending on the chosen cam profile, so large power band gains are on offer for cam upgrades.

NB: Fast road cams normally boost the bhp and torque throughout the rpm range, you could drop a little bottom end bhp but the high end rpm power will improve.

Race cams, boost the high end rpm power band but as a result the car will not idle smoothly and low end power nearly always suffers.

For a road car must carefully try to optimize your power band to your driving style.

I would be surprised if you find a N42 Race cam is a pleasure to live with when on the daily commute.

Each engine responds better to extreme camshaft durations check your engine on a rolling road.

The ecu map and fuelling also will make differences on the torque gains you'll get.

Extending exhaust or intake durations can alter the torque band and on most engines the exhaust and intake durations do not need to match, although most cams and tuners use matched pairs there are some advantages to extending the intake or exhaust durations.

Stage 1 modifications: Drilled & smoothed airbox, Sports exhaust manifold, Remaps/piggy back ECU, Fast road camshaft, Panel air filters, Intake headers.

Stage 2 modifications: Fast road cam, Ported and polished head, induction kit, high flow fuel injectors, Sports catalyst & performance exhaust, fuel pump upgrades.

Stage 3 modifications: Twin charging conversions, Engine balancing & blueprinting, Adding or Upgrading forced induction (turbo/supercharger), Internal engine upgrades (head flowing porting/bigger valves), Competition cam, Crank and Piston upgrades to alter compression.

The N42 engines are fantastic to work on and we see that there is a growing number of parts and tuning parts around.

Mapping should help to release the full potential of all the parts you've fitted to your N42.

It will usually give around 30% more power on turbocharged vehicles and you can expect to see around 15% on NASP engines, but the end result may vary depending on the parts you've done and the condition of your engine.

Forcing air into the N42 engine is the whole point to any engine modification task.

Intake manifold transmit the air during the suck phase from the intake filter and allow it to be pulled into the engine cylinders with fuel for the squish phase.

The shape and rate of flow of the Intake manifold can make a large effect on to fuel mixing and power on the N42.

Commonly we find the intake are begging for aftermarket tuning parts, although some car makers provide well optimised intake.

Larger N42 valves, getting N42 port enlargement and head flowing will also lift bhp and torque, the fantastic side effect is it will raise potential for a better bhp and torque increase on other upgrades.

Turbo upgrades

NASP engines need quite a lot of work when you add a turbo, so we have a separate guide to help you take into account the pros and cons of going this route on your N42

The more air you can get into an engine, the more fuel it can burn and uprating the induction with a turbocharger upgrade makes significant power gains.

When an engine has a turbocharger tuning mods are relatively easy and you will discover turbo charged engines are built with uprated components.

However most engines will need better parts at higher power limits

It is important to find these restrictions and install stronger pistons, crank and engine components to survive the power.

We've seen people spending a lot of money on turbo upgrades on the N42 only to see the engine block go up in smoke just after it's finished.

Big turbos tend to suffer a bottom end lag, and low capacity turbos spool up much more quickly but don't have the peak end engines power gains.

Thankfully the selection of turbos is always improving and we commonly find variable vane turbos, permitting the vane profile is altered according to speed to lower lag and increase top end performance.

Twin scroll turbos divert the exhaust flow into a couple of channels and feed these at differently angled vanes in the turbo charger. They also help the scavenging effect of the engine.

It is common that there is a limit in the air flow sensor AFM/MAP on the N42 when a lot more air is being drawn into the engine.

Going up you'll find 4 bar air sensors coping with quite large power gains, whereas the OEM air sensor sapped power at a much lower level.

Adding a supercharger or additional turbo will make large torque gains, although more challenging to get working. We have this guide to twinchargers if you want to read more.

Fuelling upgrades and mods

You will need to ensure that the engine is not starved of fuel so need to increase the fuelling when you start extending past 20% of a bhp and torque increase.It makes sense to over specify your flow rate on the injectors.

The rule of thumb is to add 20% to the flow rate when buying an injector, this takes into account injector deterioration and affords a little spare capacity should the engine require more fuel.

We think this one is common sense, but you'll need to match your fuel injector to the type of fuel your car uses as well.

Exhaust upgrades and mods

Only look to boost your exhaust if the existing exhaust is creating a restriction in flow.

On most factory exhausts you'll see the exhaust flow rate is still ok even on modest power gains, but when you start pushing up the power levels you will need to get a better flowing exhaust.

Sports exhausts generally help improve air flow out of the engine but avoid an exhaust that is too large or you may end up will reduce the flow rate. Stick to 1.5 to 2.5 inches for best results.

Common exhaust restrictions can be traced to the catalysts installed, so adding a freer flowing race alternative will help avoid this restriction.

Weak spots, Issues & common problem areas on the N42

The N42 engines, if regularly serviced and maintained, are generally very reliable and have few issues. There are plenty of horror stories around about the N42 but in reality, if looked after it is a very reliable engine.

Overheating has been noted, which is usually down to thermostat issues or silt clogging up the cooling system.

The timing chain will stretch over time, and when you hear a rattle it's time to get it seen to but we'd recommend doing this every 5 years or so anyway.

Regular oil changes are vital on the N42, particularly when the engine has been modified and is putting down more power than the manufacturer intended.

History of the Engine

Produced between 2001-2004 the N42 engine  DOHC valvetrain, VANOS (variable valve timing) Valvetronic (variable valve lift)

The non VANOS N40 engine was based on the N42


  • 114 bhp @5,500 rpm 175 Nm (129 lbft) @3,750 rpm


  • 141 bhp @6,000 rpm 200 Nm (148 lbft) @ 3,750 rpm

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