This short tuning guide will briefly outline the upgrades needed to tune your MG Rover L-series diesel car.

In order to improve the performance of the L-series diesel engine fitted to the Rover 200, 400, 600, 25, 45 and MG ZR and ZS we need to make changes to it to enable more fuel and air to get into the engine and allow the exhaust gases to exit more easily.

Rover L-series engine


Preparation to tuning

Before we start tuning it is important to make sure that the car is in good health – that it is up to date with servicing and cambelt changes.

We have a limited selection of genuine MG Rover service parts available here.  For a full range of MG Rover spares and service parts and to buy online visit Discount MG Rover Spares.



We also recommend fitting a Digimafam as the MAF (Mass Air Flow) sensor is prone to degradation (not necessarry on the Rover 200, 400 & 600). The Digimafam amplifies the MAF signal returning the car to full performance, and with early 25/45/ZR/ZS cars it can actually increase performance.


Digimafam - Digital MAF Compensator

If the performance of the car is very poor in the low revs or ‘hits a brick wall’ at 4000rpm it is likely that MAF (mass air flow) sensor has deteriorated. This is very common. Fitting a new MAF sensor or a Mafam / Digimafam will restore performance and is essential to getting the most from all the other modifications. The Digimafam with a Low Gain Pierburgh MAF will also help control fueling and smoke when fitting SDI injectors.


Increasing air flow

Increasing the amount of air into the engine will allow the fuel to be burnt fully and the engine to produce more power and be more efficient. There are 2 ways to do this; removing air flow restrictions and increasing flow of air from the turbo. To increase the flow of air from the standard turbo we adjust the wastegate to increase the boost pressure. The wastegate is a device that stops the turbo from overspooling or spinning too fast and creating too much boost. The recommended boost level for the standard turbo is 19psi.   Ensure the boost is set accurately using a boost gauge or our OBDII reader.


L-series Diesel Turbo and wastegate


Another way we can increase the amount of air going into the engine is by removing/ blanking or disabling the EGR valve. This valve allows waste exhaust gas to enter the air intake system. This waste exhaust gas has very little oxygen in it so cannot help with combustion of the fuel. It also reduces fuel economy by making the burn inefficient and clogs up the air intake system with soot. The positive aspects of the EGR valve is it helps the car warm up quickly and reduces some harmful exhaust gases. Disabling it may put on the EML (engine management light) on post 2004 cars. You can temporarily disable the EGR valve by simply pulling off the vacuum pipe off the top and then blocking the vacuum tube with the supplied blanking plug.

EGR blanking kit  EGR blanking kit


The OEM (original) turbo outlet pipe is another restriction in the air intake system. It has tight bends and squashed restrictive sections. Replacing it with a performance turbo outlet pipe increases torque and power and helps the turbo spool quicker. This results in a noticeable improvement in performance.  The standard vs the performance turbo outlet pipes are shown below.

Fitting a cone filter or panel filter will help get air quickly and easily into the engine.    The cone filters have approximately twice the area of a panel filter.

Foam Cone Filter for L-series Diesel Engines

The rocker cover (the metal top engine cover) vents oily gases into the air intake system. This oil coats everything in the air intake system and can block up airways. Fitting an oil catch can and venting the pipe to atmosphere can help keep the engine and airways clean.

Oil catch can

This oil also seeps through the boost hoses and makes them weak and liable to split.


Increasing fueling

In order to increase the power we must provide more fuel for the engine to burn. To do this we can either remap the car which involves changing the engine’s control over how much fuel it lets into the engine at any given rev range or by fitting new injectors which will allow more fuel into the engine. Uprated injectors include SDI injectors. These injectors have a break pressure of 200bar compared to the 210bar injectors of the later 25, 45, ZR and ZS and have different nozzles. This allows more fuel into the engine thus giving more power. In order to make the most of this extra fueling we must increase the amount of air going into the engine. This can be done by removing restrictions in the air intake system and allowing the turbo to make more boost.

SDI Injector Conversion Service

There may be an increase in smoke when accelerating with the SDI injectors fitted. This can be reduced by increasing boost to 19psi, blanking off the EGR valve and fitting a Digimafam. Fitting a Digimafam with a Low Gain Pierburgh MAF will allow you to control the amount of fueling in the low rev range which is essential to stop off boost smoking. An engine remap can also be done which can control the amount of off boost fuel injected.  We recommend Dynamic ECU Tuning (formally Reidy Remaps) as Matt Reidy has been remapping the L-series engine for years and has agents all over the UK.


Removing exhaust restrictions

The catalytic converter in the L-series engines is a simple oxidative type designed primarily to remove the smell from the exhaust gas. It is necessary for MOT’s here in the UK as of 2014.   It is a major restriction in the exhaust system to getting rid of waste exhaust gases. Replacing it with a 2.5” decat can help increase turbo spooling, acceleration and actually help fuel economy.

 L-series decat

The original exhaust system is 2.25” and use draw bends which produce narrow sections and a restrictive baffled backbox. Fitting a 2.5” stainless steel mandrel bent (i.e. no kinks, squashed or narrow sections) cat back (an exhaust from catalytic converter back to the backbox) exhaust system with a free flowing backbox can give good gains and as essential to get the most from the all the other modifications.  Stainless steel exhausts usually have a lifetime transferrable guarantee and changeable exhaust tips.

Stainless 2.5" Exhaust System for MG Rover 200/25/ZR

Intercoolers and Boost hoses

The job of the intercooler is to cool the air entering the engine.  Cold air is more dense and so there is more oxygen to burn the fuel.  Only the early 220 and 420 D and SD cars did not have an intercooler as standard.  Fitting an intercooler to these cars is a great improvement. You can fit the original intercooler from any later MG Rover or a FMIC (front mounted intercooler). An original MG Rover intercooler will simply bolt on and replace the intake manifold on the SD.

A FMIC (front mounted intercooler) will help to reduce the intake air temperatures over the OEM intercooler but will require some skill and modification to fit as there are no off the shelf kits for MG Rovers. There are numerous ways to put them on, although some require some cutting of the slam panel.  A FMIC may increase turbo lag due to the increased volume of the intercooler and pipework.




Replacing the OEM boost hoses is recommended as the old rubber hoses are notorious for splitting resulting in power loss and excessive black smoke. These hoses can be replaced with silicone hoses or rigid hoses both available from our store.


Handling Upgrades

There are several ways of improving the handling of the MG Rover 25, ZR and 45, ZS.

Strut Braces -   Fitting strut braces  will stiffen up the body shell to make handling more immediate .  Strut braces are a great way of improving handling without negatively upsetting ride comfort.  Our strut braces are one piece with no weak joining bolts or pivots.  They are used by numerous MG ZR race teams.  We are also the only company that produces strut braces designed exclusively for the 45 and ZS that fits the L-series, K-series and KV6 engines.  Brands include Dark Ice Designs, OMP and E-Tech (no longer manufactured).

Lowering Springs - Lowering springs will reduce body roll and make a bif difference in the handling of the car (as well as improving looks).  You are limited to an approximately 30mm drop in ride height (based on standard ZR/ZS ride height).  Ride comfort will be negatively affected.  Manfacturers include SPAX, APEC, PI, Direnza, AVO as well as 'no brand' lowering springs.

Coilovers - The ultimate way of lowering your car to improve handling. Coilovers allow adjustment of ride height and damping adjustment but come at a cost of £600+.  Brands include GAS and AVO.

Anti-roll Bars - MG Rover did not include a rear anti-roll bar for the 25 and ZR although the holes to fit one of the rear H-frame were left in.  You can fit our 19mm rear anti-roll bar to reduce the body roll under hard cornering.  This is a particulary good upgrade to improve handling when running stock suspension.




The L-series engine is currently limited by the small size of the GT1549 turbo to roughly 155bhp with 240ftlbs of torque.   On the positive side this limit has been proven to be safe for many years with no engine upgrades needed to cope with this power and torque.  

We are currently developing a gt1752 turbo kit capable of 185bhp.  See our facebook page for developements.

We hope to continue to push the tuning potential of these engines whilst making high quality performance parts available off the shelf.


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