BMW s38 Tuning

"All you need to know about tuning the BMW s38 engine!"

The BMW s38 have loads of potential and with carefully picked modified parts like remapping, turbo upgrades and camshafts you will really improve your driving experience. We were emailed and people have asked questions requesting more information on tuning and building a modified S38 and particularly which turbo upgrades work best, so this web page has been assembled which covers our go to parts for this engine and what we feel are the best value parts you can do.

The S38 replaced the M88 and the aim was reliability and better fuel economy, and as catalytic converters arrived we see shorter cam durations and better engine management thanks to the Bosch Motronic ECU.

Here we look at s38 tuning and highlight the greatest mods that work.

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History, Power & Specs of the s38 Engine

S38B35

3,453 cc (210.7 cu in) 190 kW (255 hp) @6,500 rpm 330 Nm (243 lbft) @4,500 rpm 1986–1989

  • 1986-1987 E28 M5 - Canada, Japan and USA
  • 1986-1988 E24 M6 - Canada, Japan and USA
  • 1987-1989 E24 M635CSi - catalyst

S38B36

3,535 cc (215.7 cu in) 232 kW (311 hp) @6,900 rpm 360 Nm (266 lbft) @4,750 rpm 1989–1993

  • 1988–1992 E34 M5 (worldwide)
  • 1989–1993 E34 M5 - Canada and US

S38B38

3,795 cc (231.6 cu in) 250 kW (335 hp) @6,900 rpm 400 Nm (295 lbft) @4,750 rpm 1991–1995

  • 1991-1995 E34 M5 - European-specification

Tuning the BMW s38 and best s38 performance parts.

Best s38 parts

When talking about the ultimate modifications for your s38 engine, we are going to concentrate on the ones that give the biggest return for your cash.

Altering your s38 cam will make a dramatic difference to the engine bhp. Choosing a higher performance cam profile raises the bhp accordingly.

Fast road cams tend to boost the torque over the rev band, you could drop a little low down bhp but your higher rpm power will be better.

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

A Race cam is not great on the daily commute, because the lumpy idle will make the car prone to stall and smooth driving at low rpm becomes impossible. If you are developing a track car this doesn't matter as you are in the high end of your RPM range anyway and that is where you want the power to be.

You should ideally optimize your bhp range to your cars usage so for a road car stick with a shorter duration s38 cam

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

The map and injectors and fuel pump also have an effect on the bhp gains you'll achieve.

Extending exhaust or intake durations can alter the bhp 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.

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Best mods for your S38

  1. Intake Mods and Exhaust - Note that on their own these mods will NOT ADD TORQUE in most cases, but they permit you to enhance power after other modifications by losing a restriction.
  2. Low mass flywheel - a reduced weight flywheel will significantly improve the engines rev changes. In some cases flywheel lightening is not not a great upgrade for all S38 engines.
  3. Flowing and porting the engine head - for larger gains, you will get better flow and make a more efficient engine if you do this to support your other mods.
  4. Forced induction upgrades - forced induction is the most efficient approach to increase your intakes air supply, which permits you to use more fuel and make more power. Typically one of the most challenging mods it does provide the largest gains.
  5. Fast road cams are one of the most significant mechanical changes, but TorqueCars recommend they be setup by someone qualified to set them up and you might struggle to find one but you might have a local firm to regrind a stock camshaft .
  6. Tunes - S38 remapping offers the most power for the money, aftermarket ECUs, and inline Tuning boxes are all alternatives.

S38 Tuning Stages

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

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

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

The s38 power plant are fantastic to work on and thanks to their popularity there is an increase of upgrades and tuning parts about.

Mapping will help to establish the full potential of all the upgrades you've done to your s38.

(In some cases, as the factory ECU is locked flashing is not an option, so an aftermarket ECU is the route to take, and many of these will outperform factory ECU's but make sure it has knock protection and that you get it setup properly.)

It will usually give you around 30% more power on turbocharged vehicles and you can expect to see around 15% on NA (naturally aspirated) engines, but the end result usually differs on the upgrades you've carried out and the condition of your engine.

Feeding more fuel and air into your s38 is the aim to any engine upgrade task.

Intake manifolds take the air during the suck phase from the air filter and allow it to be drawn into the engine and mixed with fuel.

Design and rate of flow of the Intake manifold can make a large improvement to fuel engine efficiency on the s38.

Many mass produced engine air intake manifolds are improved through an upgrade, although a few car makers provide reasonably well designed air intake manifolds.

Adding a s38 larger valve kit, doing some s38 port enlargement and head flowing will also improve performance, the fantastic side effect is it will raise potential for a better performance increase on other tuning mods.

s38 Turbo upgrades

NA (naturally aspirated) 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 s38

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 your car has forced induction upgrades are going to make more power and we find turbo engines are built with stronger components.

There are reliable limits for every engine, with some being very over engineered and some just sufficiently able to handle stock powerWe recommend you find these limits and upgrade to stronger pistons, crank and engine components to utilize the power.

It's not unheard of drivers spending a loads of money on turbo upgrades on the s38 only to experience the engine block explode on it's first outing after it's used in anger.

Big turbochargers often experience low end lag, and smaller turbochargers spool up really quickly but do not have the peak rpm power band gains.

In the last 10 years the world of turbos is always evolving and we are seeing variable vane turbos, allowing the vane profile is altered according to speed to lower lag and increase top end bhp.

Twin scroll turbos divert the exhaust flow into two channels and push these at differently angled vanes in the turbo charger. They also boost the scavenging effect of the engine.

You'll commonly see there's a limit in the air flow sensor MAP/MAF/AFM on the s38 when considerably more air is being drawn into the engine.

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

Adding a supercharger or additional turbo will make large power gains, although more complex to install. We have this in depth look at twinchargers if you want to read more.

Fuelling Upgrades on the S38

Don't omit to ramp up the fuel system when you are increasing the bhp and torque - it makes the car more thirsty. Don't forget to over specify your flow rate on the injectors.

The accepted safe increase is to add 20% to the flow rate when fitting an injector, which takes into account injector deterioration and gives 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.

s38 Performance Exhausts

You only need to uprate your exhaust if your exhaust is actually creating a flow problem.

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

Note that with the largest exhaust you can find this will reduce the exhaust flow rate - the best for power gains are usually between 1.5 to 2.5 inches. It is the shape and material more than the bore size.

Usual exhaust restrictions are traced to the catalyst and filters installed, so adding a higher flowing race alternative such as a sports catalyst pretty much removes this restriction, thanks to it's larger size and surface area, and will effectively raise the performance to levels you would expect without having a catalyst installed, but keeps the car road legal.

Weak spots, Issues & problem areas on the s38

The s38 engines are generally reliable and solid units, as long as you follow the manufacturers service schedules, and use a good quality oil to ensure longevity. Few problems should happen as long as they are regularly serviced and maintained.

Carbon build up in the head, particularly around the valves which will sap power or create flat spots, this is a larger issue on direct injection engines but should be looked out for on all engines. We have tips on removing carbon build up.

Some of our members have had issues with flat spots or glitches after applying mods and upgrades or tuning, this is not usually related to this engines design, so instead see our article on diagnosing flat spots and problems after tuning which should help you get the bottom of this issue.

Regular oil changes are vital on the s38, especially when tuned and will help extend the life and reliability of the engine.

If you would like to know more, or just get some friendly advice on Tuning your s38 engine please join us in our car forums where you can discuss s38 tuning options in more detail with our s38 owners. It would also be worth reading our unbiased BMW tuning articles to get insights into each modification and how effective they will be for your car.

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 parts work best for you on your car. Which helps us keep our guides and tips up to date helping others with their modified car projects. Your feedback and comments are used to keep this page up to date, and help improve the accuracy of these s38 tuning guides which get regular updates and revisions.

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