BMW M135i / M235i Tuning Options

Cars such as the BMW M135i and M235i (and their less popular relatives 335i and 435i) are great sports cars offering great performance without breaking the bank. Equipped with the N55 engine they offer around 320 BHP and 450 Nm of torque out of the factory, making them one of the greatest value performance cars on the market.

But what if this doesn't cut it any more? How tunable are these engines? How do I tune my BMW? Well, you are lucky enough as there are many different options  available for the N55 engines nowadays. We will cover each of them in this article including expected gains, benefits and drawbacks and compare them so you can find the right solution that will suit you best.

Custom Remap

Custom remaps are the most classic way of increasing your car's power. As long as the remap is carried out by a reputable garage this method will most likely make the biggest impact on car's performance. Custom remap means that all changes are tailored to your car exactly. At the end you should get a dyno printout showing the gains and healthy curves.

On the other hand though, custom remaps offer very little if any user control. Once the map is uploaded user is unable to tweak anything and there is no ability to switch the map off unlike with other methods listed later in this article. This will put off customers who care about their warranty as the BMW dealerships will be able to see that there were changes made to the ECU which may result in rejected claim even if the remap was removed prior to the dealership visit.

With custom remap, depending on the car and tuner you can expect the results of 410-435 bhp with decat downpipe and 370-390 bhp with standard down pipe.

Photo Credit: Auto Evolution

Tuning boxes

When it comes to the tuning boxes, the clear winner on N55 platform is the JB4. Over the years it was proven to be reliable and trusted among thousands of users. Its greatest benefit is the fact it is almost untraceable in case of any warranty work - you simply remove the JB4 hardware and you're ready to go. It is essential if you are planning to run meth for the extra performance. Also, since it is a physical product you can sell it on once you change your car and get some money back.

It comes standard with a few maps where each is designed for a different, typical hardware setups. On top of this it allows tuners to add maps of their own which are a bit closer to custom remaps tailored to your needs. For an additional fee you can purchase a Bluetooth controller for your JB4 which allows you to switch between the maps, read and clear the codes and to data log.

But even the JB4 has its drawbacks. Many consider it quite dated with the standard maps not offering enough gains over the alternative tuning methods. The auto code clearing feature is great for getting rid of the Engine Management Light for drivers with a decat downpipe however if you have other issues, you may not always be aware of them due to the EML being forced off.

Some people even claim that BMW dealerships will be able to see that the engine was running higher boost if they decide to dig deep enough into the car's logs. This information has not been officially confirmed though.

The official UK website for JB4 claims that users can expect gains of up to 50 horsepower and 65 torque.

Photo credit: Uro Tuning

Flash tuning

In recent years flash tuning has become increasingly popular. Two major players on N55 platform - bootmod3 and MHD made it very easy for customers to upload the maps to the car's ECU by them selves, right on their driveway with no need to book in and travel to a tuner garage.

What are the advantages of flash tuning? Like mentioned earlier, you do it when you want and where you want, you pay for the (digital) product and you carry out the service yourself, obtaining the complete freedom. Additionally, both MHD and bootmod3 offer extra settings such as cold start delete, burble control and much more. If you decide at any point you don't like your current settings you can easily change them on your flashing device (laptop / iPhone / iPad / Android device for bootmod3 and Android device for MHD) and reupload the map. These devices can also be used for data logging.

Both brands offer standard OTS maps which are designed for vehicles with assumed hardware modifications. You can however request a custom map tailored to your vehicle for an additional fee. There are third-party tuners offering maps on both of these platforms if you do not wish to choose the developers' service.

However, even though flash tuning is being made simpler for the end user, there is still a chance of making a mistake and bricking your ECU during the upload process. The standard maps are only generic and not optimised for an individual car so it will not be using its full potential and there could more cost involved in order to make the most out of it. Finally, this may be a deal breaker for many as even when the map is removed it can be traced back and void the warranty.

Example of flash tuning gains based on official MHD website:

Photo Credit: Bimmerpost


Each of the tuning methods has its benefits and drawbacks. Luckily there are many options available to suit each group of consumers. In our opinion if you are someone who is concerned about their warranty - JB4 is the right option for you. If you want to get most out of your car and don't care about freedom and being able to tweak certain settings yourself - custom remap is the one. If you want to get both of best worlds, performance and control you should definitely look into flash tuning.

Whichever tuning method you choose, you should never forget about the supporting mods - splitting charge pipe being the most common issue on N55 engines. Therefore upgraded charge pipe is always recommended when tuning these cars. Upgraded intercooler is highly recommended too when expecting high gains. Standard brakes on these cars are okay but not excellent which should also be considered for heavy road and track usage. These cars come equipped with open differentials as standard so LSD should be on your list too in order to be able to put the power down fully and to improve stability.


This article is based on knowledge widely available on the internet. It puts together information found on dealers' websites and experiences expressed by users on forums. It is not aimed to discourage or encourage you to use one method or brand over another but to show you objective advantages and disadvantages of each. Before choosing either way to tune your car we recommend further research and use of reputable garage. More details on each tuning method will be explained in future in more detailed articles.


JB4 Website:

MHD Website:

PTF - BM3 Website:

BMW M135i M140i M235i M240i UK Forum on Facebook: