Tuning the BMW S65B40 / S65B44

"Comprehensive guide to tuning and performance parts on the BMW S65B40 / S65B44 engine!"

The BMW S65B40 / S65B44 are popular engines and with carefully chosen parts like a remap, turbo improvements and camshafts you will enhance your driving fun.

This pages aim is look into S65B40 / S65B44 tuning and summarise the optimum modifications.

Just because particular mods are are common on S65B40 / S65B44 it doesn't mean its worth having, instead we'll focus mods that will give your S65B40 / S65B44 the best power gain for you spend.

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

NB: Fast road camshafts commonly raise the bhp through the rev band, you could sacrifice a little low down torque but high end rpm power will be higher.

Competition camshafts, raise the high end 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 driving style.

You will never have found a S65B40 / S65B44 Competition cam is a pleasure to live with when on the daily commute.

Different S65B40 / S65B44 engines respond better to more or less aggressive camshaft durations than others.

The engine timing and fuelling also will make differences on the power gains you'll hit.

Extending exhaust or intake 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.

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

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

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

Carefully think through your options and then buy your tuning parts and set yourself a power target to void expensive mistakes.

A remap will help to establish the full potential of all the tuning mods you've fitted to your S65B40 / S65B44.

It will usually give around 30% more power on turbocharged vehicles and you can expect to see around 15% on NASP engines, but your mileage usually rely on the tuning mods you've done and the condition of your engine.

Feeding fuel and air into each cylinder is vital to any engine modification task.

Air Intake manifolds take the air during the suck phase from the filter and allow it to be drawn into the engine cylinders.

Shape and rate of flow of the Intake can make a substantial change to fuel atomisation on the S65B40 / S65B44.

We often see air intake manifolds are in dire need of a performance upgrade, although a few car makers provide well optimised air intake manifolds.

Fitting big valve kits, getting S65B40 / S65B44 port enlargement and head flowing will also lift bhp and torque, and more importantly will allow you to get a greater bhp and torque increase on other modifications.

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 S65B40 / S65B44

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

If an engine is turbocharged, upgrades are more reliable and we find turbo charged engines are built with many forged and stronger components.

However you will find an engines have limits

See where you'll find these restrictions and fit stronger pistons, crank and engine components to survive the power.

We see many tuners spending a a stack of money on turbocharger upgrades on the S65B40 / S65B44 only to have the motor go up in smoke just after it's been completed.

Large turbo units commonly suffer no power at low rpm, and low capacity turbo units spool up quickly but do not have the peak rpm engines power gains.

Over the last 20 years the selection of turbo chargers is always evolving and we commonly find variable vane turbo chargers, allowing the vane angle is altered according to speed to lower lag and increase top end bhp.

Twin scroll turbo chargers divert the exhaust gases into two channels and direct these at differently profiled vanes in the turbo charger. They also improve the scavenging effect of the engine.

It is common that there's a limit in the air flow sensor MAF/MAP on the S65B40 / S65B44 when considerably more air is being fed into the engine.

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

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

Fuelling upgrades and mods

When you boost the torque you will need to uprate to the fuel delivery.

More torque needs more fuel. We would recommend you to over specify your flow rate on the injectors.

The rule of thumb is to add 20% to the flow rate when specifying an injector, helps cope with injector deterioration and allows some spare capacity should the engine need 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

You should look to increase your exhaust if the existing exhaust is actually causing a flow problem.

On most factory exhausts you'll find the exhaust flow rate is good 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 can help balance the flow of gases through the engine.

But if your exhaust is too large, ie: over 2.5 inches bore, you will lose a lot of your flow rate and end up sapping power and torque.

Typically exhaust restrictions come around the filters installed, so adding a higher flowing sports alternative will help avoid this restriction.

Weak spots, Issues & common problem areas on the S65B40 / S65B44

The S65 engines, if regularly serviced and maintained, are generally very reliable and have few issues.

Don't drive it hard until it's warmed up and don't drive it like a high revving turbo car on the redline all the time and it'll last forever.

Gaskets on the valve cover can leak a little oil, you might even smell burning oil as it drips over the hot engine. The heat from the engine causes this and is not a major cause for concern.

Regular oil changes are vital on the S65, particularly when the engine has been modified and is putting down more power than the manufacturer intended.

There was a rob bearing wear issue that led to a few engines failing, this problem is not as common as you'd think looking around, but it is very well documented. If the engine has a tick tick noise at tickover or at low to mid rpms get this checked.

Throttle bodies can fail, and you'll get the EML and DSC lights come up if you didn't realise the car was in Limp home mode.

Rough idling is often down to a faulty idle control valve, which can also cause cold start issues and misfires when under load.

History of the Engine

It was a lovely sounding NASP V8 fitted to the M3. The S65 replaced the S54

It came as a 4.0l and 4.4 version, both sounded fantastic and perform really well.

  • S65B40 (414 hp) at 8,300 rpm 400Nm(295 lbft) at 3,900 rpm
  • S65B44  (444 hp) at 8,300 rpm 440Nm(325 lbft) at 3,750 rpm

There was also a motorsport version of the S65 named P65B44

For more information on Tuning your S65 engine please join us in our friendly forum where you can discuss tuning options in more detail with our owners. It would also be worth reading our unbiased tuning articles to get a full grasp of the benefits and drawbacks of each modification.

We need your help improving this article, so please send us your feedback in the comments box below and pass on any tips, points or facts we have wrong or have not covered.

We really like hearing from our readers, and hearing about which upgrades were the most effective for them, it helps us improve our recommendations and articles to reflect current trends in modifications and ensures that our guides and tips are kept up to date.

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