BMW M42 Tuning

"All you need to know about tuning the BMW M42 engine!"

Let us look at M42 tuning and outline the best modifications for your car. BMW M42 is a really good project engine, and with the right mods like remaps, turbo improvements, and camshafts you will definitely maximize your driving experience.

Replaced with the M44B19 engine block the M42 production runs from 1989-1996.

Realistically you can get around 205bhp from a 1.8 block bored out to a 2.1L capacity, or more if you go the turbo route, but this will cost quite a bit of money to achieve.

BMW didn't do a bad job on the M42, so improvements to it are going to be costly. The DISA (dual length intake) was applied toward the end of the production run of this engine.

History of the M42 Engine

M42B18 engine specs 1,796 cc DOHC straight-four

  • 100 kW (134 hp) at 6,000 rpm 172 Nm (127 lbft) at 4,600 rpm 1989-1992 (E30)
  • 103 kW (138 hp) at 6,000 rpm 175 Nm (129 lbft) at 4,500 rpm 1992-1996 (E36)

There was an interesting motorsports version of the M42. The S42B20 which was essentially a rebored/stroked 1.8 with individual throttle bodies

  • 206 kW (276 hp) to (315 hp)

Tuning the BMW M42 and best M42 performance parts.

Best M42 tuning parts

Just because particular upgrades are popular with M42 owners it doesn't mean you should fit it, instead we will focus upgrades that will give your M42 the best value for money to power increase.

Ideally you'd need internal mods, strengthening, increasing the compression ratio, and reboring and stroking.

An S50 engine swap is usually a cheaper power upgrade because mods to increase the power of your M42 will be quite costly.

A lighter flywheel improves engine response and is a good pairing with a remap and head work, but not so good with a fast road camshaft.

The usual boy race magazine tips such as  "add a sports exhaust" and "induction kit" will really not help your power at all, and in more than a few cases these mods have robbed some low end power.

Altering your M42 cam will make a dramatic difference to the engine torque. Choosing a higher performance cam profile raises the torque accordingly. It remains the best power mod you can do on a NASP engine, but you really need to get other mods done as well.

Fast road camshafts commonly push up the bhp throughout the rpm range, you may lose a little bottom end power but your higher rpm power will improve.

Typically most M42 engines work well with a cam profile around 265 to 279but you'll need a remap to fully release the potential of this mods.

If you can't find performance cams for your M42 then you might find a local supplier who can regrind your stock cams to a more sporty profile.

Competition camshafts, push up the higher rpm power band but as a result the car will not idle smoothly and low end power nearly always suffers.

In a road car you need to optimize your torque band to your typical driving style.

You'll never have found a M42 Race cam is a pleasure to live with when on the daily commute.

Different M42 engines respond better to more aggressive camshaft durations check your engine on a rolling road.

The ECU mapping and injectors and fuel pump also will make differences on the power gains you'll make.

Longer valve durations can alter the power 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.

M42 Tuning modifications.

Please watch our video which covers the 5 principles of tuning your M42. Be sure to subscribe and support our new channel.

Best mods for your M42

  1. Turbo upgrades - A New Turbo is the most efficient approach to improve intake air supply, allowing you to combust more fuel and make power gains. It is one of the most complex upgrades but provides the best gains.
  2. Lighter flywheels - a lower mass flywheel will improve the engines free revving nature. But not always suitable for all M42 engines.
  3. Fast road cams are generally the biggest mechanical mod upgrade, but they must be setup by someone who knows what they are doing and they are not always easy to source but there might be a local firm to regrind a stock cam .
  4. Intake and Sports Exhausts - NB: on their own these mods won't ADD POWER , but they can help release power after other mods by removing the restriction.
  5. Head work - The goals of porting and flowing the head are to get air flowing into the engine while removing flow restrictions and turbulence.

M42 Tuning Stages

Typical stage 1 mods often include: Panel air filters, Drilled & smoothed airbox, Intake manifolds, Fast road camshaft, Sports exhaust manifold, Remaps/piggy back ECU.

Typical stage 2 mods often include: Ported and polished head, high flow fuel injectors, induction kit, Sports catalyst & performance exhaust, Fast road cam, fuel pump upgrades.

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

The M42 engines respond well to upgrades and thanks to their popularity there is a lot of mods and performance parts out there.

Mapping helps unlock the full potential of all the parts you've done to your M42. The Bosch Motronic M1.7  is well known to tuners and fairly accessible.

It will usually give around 15% on NASP engines, but the end result often depends much on the parts you've done and the condition of your engine.

It is the aim to any engine upgrade job to pull more fuel and air into each cylinder

Intake Manifolds carry or channel the air from the filter and allow it to be pulled into the engine cylinders.

The size of bore and shape and rate of flow of the Plenum can make a substantial improvement to fuel atomisation and engine efficiency on the M42.

Most intake manifolds are improved through motorsport experience and OEM ones are usually lacking, although a few manufacturers provide reasonably well designed intake manifolds.

Big valve conversions on the M42, carrying out M42 port enlargement and head flowing will also increase torque, this will give you an improved torque increase on other parts.

M42 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 M42.

Forged rods and low compression pistons are the way to go if you want to add a turbo, but you will need 640cc injectors for something like a Garrett GT28.

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, you will probably see power around 280hp, but your results will vary depending on the turbo you fit and how it is setup.

There are common areas of failure for every engine, with some being very over engineered and some only just able to handle stock power

It is important to find these limits and fit better quality crank and pistons to survive the power.

We see many tuners spending a lot of money on turbocharger upgrades on the M42 only to watch the car explode when it's finished.

Big capacity turbos often suffer a bottom end lag, and small turbos spool up quickly but do not have the peak end engines power gains.

In the last 10 years the selection of turbo units is always developing and we are seeing variable vane turbo units, where the vane profile is altered according to speed to lower lag and increase top end bhp.

Twin scroll turbo units divert the exhaust flow into a couple of channels and flow these at differently profiled vanes in the turbo. They also improve the scavenging effect of the engine.

You'll commonly see there is a limit in the air flow sensor AFM/MAP on the M42 when considerably more air is being fed into the engine.

We see 4 bar air sensors coping with quite large power gains, whereas the OEM air sensor sapped performance at a much lower level.

Adding a supercharger or additional turbo will make large performance gains, although more challenging to configure. We have this feature on twinchargers if you want to read more.

M42 Fuelling

When you improve the torque you will need to pay attention to to the fuel system.

We've seen good results from a set of Mustang injectors and a remap, this helps release quite a bit of power from a stock M42.

More torque needs more fuel and individual throttle bodies can be a great way to deliver more fuel.

We would recommend you to be generous with your injector capacity.

As a rule of thumb add 20% when fitting an injector, which takes into account injector deterioration and provides 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.

All the following flywheel power targets will assume an injector duty cycle of 80% and a base of 58psi of fuel pressure at idle.

4 Cylinder turbocharged engines

  • 58 PSI 340cc/min 200hp
  • 58 PSI 511cc/min 300hp

4 Cylinder NASP engines

  • 58 PSI 285cc/min 200hp
  • 58 PSI 426cc/min 300hp

4 Cylinder supercharged engines

  • 58 PSI 312cc/min 200hp
  • 58 PSI 468cc/min 300hp

M42 Exhaust Upgrades

You should look to increase your exhaust if your current exhaust is actually creating a restriction in flow.

On most factory exhausts you'll see the 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.

Don't expect power gains from a performance exhaust, the stock system flows quite well and can handle quite a power upgrade before you hit a restriction.

Sports exhausts can usually airflow out of the engine but avoiding 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.

Typically exhaust restrictions are in the catalyst installed, so adding a freer flowing sports alternative will help avoid this restriction.

Weak spots, Issues & problem areas on the M42

The M42 engines are generally reliable and solid as long as they are regularly serviced and maintained.

Early engines led to reports of failures of a profile gasket, the one that seals the lower cam chain case to the cylinder head, leading to main bearing failure as steam mixes with oil in the sump. Most engines have had this repaired with a new gasket material so it shouldn't be a problem.

Regular oil changes are vital on the M42, especially when tuned and will help extend the life and reliability of the engine.

Chain guides and hydraulic tensioners suffered from excessive wear so were later redesigned.

For more information on Tuning your M42 engine please join us in our friendly forum where you can discuss tuning options in more detail with our M42 owners. It would also be worth reading our unbiased tuning articles to get a full grasp of the benefits and drawbacks of each modification.

Please help us improve these tips by sending us your feedback in the comments box below.

We love to hear what our visitors have got up to and which modifications work best for them on each model of car. Comments are used to improve the accuracy of these M42 articles which are continually updated.

Check out my YouTube channel, we're regularly adding new content...


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