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- The OPF/GPF - The killer of any decent exhaust tune

The OPF - Pandora's Box For Many Petrolheads

30w ago

37.5K

By Marc Rutten

Many of you may know that the automotive world, especially in Europe is changing rapidly. Words like WLTP, Euro 6 and OPF filters are becoming more important every single minute right now, while for many of you these subjects don't immediately ring a bell. Personally I feel that we all should be made aware of what is ahead of us and be prepared for what is to come. Let me start with a brief explanation of these terms before I dive more into the impact of these changes and where we are moving to over the coming years.

First of all, WLTP stands for the Worldwide Harmonised Light Vehicle Test Procedure (WLTP), a laboratory test which is used to measure fuel consumption and CO2 emissions from passenger cars, as well as their pollutant emissions. The WLTP is replacing the NEDC, which was designed in the 1980s. Due to evolutions in technology and driving conditions, it became outdated. The European Union has therefore developed the new WLTP test.

The difference between NEDC and WLTP

While the old NEDC test determined test values based on a theoretical driving profile, the WLTP cycle was developed using real-driving data, gathered from around the world. WLTP therefore better represents everyday driving profiles. The WLTP driving cycle is divided into four parts with different average speeds: low, medium, high and extra high. Each part contains a variety of driving phases, stops, acceleration and braking phases. For a certain car type, each powertrain configuration is tested with WLTP for the car’s lightest (most economical) and heaviest (least economical) version. This is a lot of work for car manufacturers like Audi, BMW, Mercedes-Benz, Volkswagen, Toyota and Fiat, since they have a large number of variations.

The WLTP applies to all new car registrations in Europe from September 2018 (more on this later) and this is where the release of for instance my new BMW M2 Competition comes into play. The release of the car and the sales throughout the period that the WLTP is in play meant that all EU-spec M2C's had to be made compliant to the new regulations. In case of the EU-spec version of the car it meant that BMW had to outfit it with two, Otto Particle Filters (OPFs) in the exhaust system. Positioned between the downpipes and the mid damper, these passive filters are the key to making almost all petrol-powered cars in the European market right now compliant to the Euro 6 emission rules, which have been brought forward due to Dieselgate.

OPF filters fitted on the BMW M2 Competition

What is an OPF and what does it do? The OPF can also be referred to as the GPF or Gasoline particulate filters (GPF). They are an emission after-treatment technology based on diesel particulate filters (DPF), developed to control particulate emissions from gasoline direct injection (GDI) engines. The filter applied to European and also Chinese cars has a honeycomb structure with alternately sealed inlet and outlet channels. This forces the exhaust gas to flow through a porous filter wall, which traps the soot or carbon particles. The filter can be continuously regenerated under corresponding driving conditions. The particulate filter is optimised for backpressure, has a high filtration efficiency and is also maintenance-free and self-regulating.

You might ask yourself right now; what is so important about all this information? Let me be clear! If you are living in the EU, it means that from next year onwards, almost every new car on the road will simply sound "shit". The OPF is killing any nice, loud and/or high pitched exhaust sounds and are an obstruction for any petrolhead who likes a proper soundtrack or wants to tune their car. Let me explain this with two examples!

Preparing a Christmas Turkey Shmee150-style

Example 1: Lamborghini

Lamborghini is known for their high pitch natural aspirated engines screaming away and shooting flames galore. To comply with the older NEDC, the V10's and V12's in the lineup were always homologated based on their quieter STRADA mode, allowing for CORSA to be loud and in your face. Exactly what a Lamborghini should be! From 2019 onwards, this is all over. The next generation Lambo V10's and V12's only sold in Europe (and maybe China) will sound a lot quieter, are homologated on CORSA mode and are outfitted with OPF's in the exhaust line. This means additional weight added to the car and a more quiet sound track without flames or hardly any pops & bangs. CORSA mode is only allowed to be as loud as STRADA was before in the old situation!

You might ask yourself why Lamborghini is affected and why do I use this brand as an example. The reason is quite simple. Lamborghini is part of the VAG group (remember Dieselgate) and a decision was made within the group that across the whole group all cars should receive a OPF filter even if they did not need one. Bentley's W12 engine is for instance a great example of this. It complies with the WLTP even without the filter, but the VAG group has made a clear decision and doesn't want to end up in another Dieselgate situation. The Aventador SVJ Coupe will be the last Lamborghini without an OPF and therefore a decent exhaust tune.

Other sports car brands like Ferrari, McLaren, Koenigsegg, TVR or Morgan won't be affected by the WLTP because of their low volume production of less than 10,000 cars per year and the fact they are not affiliated to any major car manufacturer. Luckily their cars will still sound properly. What does this mean for the aftermarket world and their availability of exhaust systems, downpipes and linkpipes? Let's discuss that in my second example!

Akrapovic Slip-on Line Titanium Exhaust (prototype) with Titanium Linkpipe (prototype) for the BMW M2 Competition

Example 2: Aftermarket exhaust systems

To explain the future of aftermarket exhausts in more depth, I have taken my own BMW M2 Competition as the main subject of my story, but there are many other cars I could have opted for. The implementation of the OPF is a market wide in Europe and it affects cat-back exhaust systems and the necessity of downpipes. For the sake of making it easy for you I will go from the rear of the car to the front touching on each segment of the exhaust line:

Slip-on replacement: This is the easiest upgrade for everyone with a EU-spec M2C. Replace only the ugly looking rear damper connected to all four tail pipes and buy an aftermarket system that is homologated to comply with local emission regulations. You will have hardly any power increase, improved looks and a louder exhaust across the rev range, but never louder than legally allowed. Optional exhaust valve control is not legal, but you can add it if you prefer. The slip-on is the only option for future EU-spec cars with OPFs, if you want to keep it legal and don't want a ECU tune on your car. Of course, you can also do a backbox delete, but that is not legal at all.

Linkpipe replacement: Not legal and only for track use, but an option if you feel like going illegal and don't have problems tuning your car. The linkpipe replaces the stock midpipe which often holds the OPFs and centre damper or resonator. A ECU tune might be required to disable the OPF sensors that monitor the back pressure before the OPFs, which are in this case passive and not actively monitored like for instance diesel particle filters. In case of the M2C if you don't do this, then your car goes into limp mode.

Sport downpipes fitment: Only a possible option if your preferred ECU tune requires it, not an option when you leave the OPFs installed on the car. It is a waste to install an improved set of downpipes when the OPFs are still there, because the OPFs are the biggest obstruction in the exhaust flow.

Catless downpipes fitment: Totally illegal, and only smart when you have a full cat-back system that replaces both the stock slip-on and the centre midpipe with the OPFs. Installing a catless downpipe without removing the OPFs is not possible! Reason why... Your flow will change triggering the pressure sensors and you will simply burn through the OPFs, which are placed after the downpipes.

The GPF or OPF are ending interesting exhaust tunes

These examples show exactly what is going to happen. Where in the past you were able to replace manifolds, downpipes and full cat-back exhausts and still be fully homologated with local European emission and sound regulations, the future will not allow this anymore. The future will be a lot more quiet when it comes to screaming exhausts, pops & bangs and flames. The next step in this process is that engines will also become a lot smaller. The next generation of engines will either be fully electric (like a Tesla or a Audi E-tron) or offer a combination of a natural-aspirated low-cylinder powerplant accompanied by hybrid assistance allowing for enough power under your right foot (like for instance a BMW i8).

For those of you, who are planning the removal of the OPFs of your beloved new automobile. Be aware that it will make your vehicle non-homologated to the new WLTP and Euro 6 regulations. If you still want to opt for a better exhaust note, then the only way to do this is by working your way from the rear to the front. A slip-on system which only replaces the rear muffler or backbox is the sweet spot for us in the EU! It can often stay within limits and offers a more sporty sound track across the rev range. For anyone else in the rest of the world, you are lucky because you have way more room to play still. For you nothing has changed yet, but that is only a matter of time.

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