R56 GP 2 Diffuser

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With the track day season over for another year for all but the bravest (or possibly foolish) drivers I’ve started to think about jobs to be done before the spring comes. With the power upgrades now done I’m looking at making the car quicker through the corners. My plan was always to add some descrete aero upgrades to the car without making it shout race car too loudly. I make no excuses for the fact that I am in many ways inspired by Nigel Pinder’s awesome Mk2 Golf the Pinderwagen. Nige has really gone the whole hog with his car but it’s trailered to the track.

One of the most under utilised areas for improvement is the underside of the car, maybe becase it’s more of a black art or doesn’t have a visual impact of adding a big wing. Fortunately the R53 GP and later R56 Minis have a flat floor from the factory which fits the R50/R52/R53 cars. The ultimate factory R56 (the GP2) also has a rear diffuser, I’d decided to do down this route as the parts are still available and considerably cheaper than the Mini Challenge parts. The theory is that by smoothing the airflow under the car the air moves more quickly than the air flowing over the body work (given the brick like shape of the Mini there is clearly a difference!) causing a low pressure area that means the car is pushed into the ground. It also doesn’t increase the drag unlike a large rear wing.

Before I get into any details of the steps needed to get the R56 GP2 diffuser to fit on the R53 let me make it very clear, if you don’t like the idea of cutting up new parts you’ve just brought this is not the job for you! Secondly my car has been running a straight R56 style exhaust (now using a R56 JCW back box) for some time, if you’ve not done the conversion you’ll have more work to do. If you are interested I wrote a post about an earlier version of the exhaust here.

Before you start you’ll need the R56 GP2 Diffuser (it comes in 5 parts: a rear section, two tunnels and two brackets) the nice folk at Orranje can supply you with the full kit of parts including the fittings. I’ve brought a fair number of parts for my car from them and the service is always excellent.

If you plan to do the necessary modifications yourself you will need some basic hand tools:

  • Hand drill and drills
  • Needle files
  • Hawksaw
  • Masking tape
  • Marker/pen/pencil
  • Trolley Jack
  • Axle Stands
  • Screw drivers
  • Torx drivers
  • There are four areas that need modification, the rear bumper, the rear part of the diffuser, the tunnels and the original bumper trim.

    Removing the bumper trim
    The first job is to remove the original bumper trim, this is held in by two screws at either end and then clips into a channel in the bumper itself. The least painful way of doing this is to remove the entire bumper but this requires removing the rear wheels and rear arch liners. Instead I jacked up the rear of the car secured it with axle stands and then used a large screwdriver to unclip the trim. Put the trim in a safe place as this will need modifying later to fill the gaps either side of the diffuser.

    Modifying the bumper
    The slots in the bumper for the original trim don’t line up with the clips on the rear section of the diffuser, new slots need to be cut into the bumper. To work out the position use masking tape to cover the channel in the bumper where the original trim went. You can then offer up the rear section of the diffuser and mark where the slots need to be, making sure you center the trim before marking out the slots. To make the slots I used a drill to create a number of holes along the length of the slot and then opened it out using a combination of needle files and the a regular flat file. Once you have cut the slots out the rear diffuser needs modifying as there are a number of vertical bars inside the channel that will prevent if from fitting flush. I sanded down both the clips on the diffuser and the longer trim sections as well as cutting some pieces out with cutters so it would fit. Finally the bumper needs two sections cutting out of it where the diffuser curves downwards. It’s difficult to describe the entier process as it required a continual process of trial fitting modifying and trial fitting again unti it was right.


    Modifying the tunnels and brackets
    This is the part where you need to take a hacksaw to your lovely new difffuser… If you have the rear section mounted onto the bumper it won’t take very long to realise that with the tunnels in place you are going to have issues with the rear trailing arms. The rear end of the R53 and R56 are the same in terms of the suspension layout but are very different when it comes to the bodywork, the rear bumper is at least 30mm closer to the rear suspension on the R53. The brackets that come with the kit are designed to bolt up to the R56 exhaust hangers (the mounting holes are present on the R53) that fortunately I have on my car. If you don’t have the exhaust hangers you’ll need an alternative method. I’ve seen some examples of this conversion on the internet where people have cut the brackets but I chose to lengthen the mounting holes and move them as far back as I could so I could retain 3 points of contact on the tunnel. With the brackets bolted up it’s time to offer up the tunnel so you work out how much needs to be cut away. The rear section of the diffuser has an aluminium plate rivet to the plastic tunnel, I chose to remove the rivets by drilling them out so could cut the plastic around the rear trailing arms and then modify the metal part afterwards. This is another part of the job where a bit of trial and error comes in, I suggest making small cuts test fitting and then repeating the process until it works. If the tunnel isn’t far enough back you’ll keep pushing the rear section out of the bumper. Because of the amount of material that needs to be removed from the tunnels the three mounting holes need to be moved too. The original mounting holes are recessed I used some large “repair” washers to make up the difference in height and to prevent bending the plastic.



    Modifying the rear bumper trim
    With the rear section clippped into the rear bumper and the tunnels attached the last job is to cut and shape the original bumper trim. The simplest approach is to make a cardboard template and then cut and shape the trim to suit. Because of the compound shape of the trim it is a bit arkward and like many of the other jobs needs a bit of trial and error.

    I’ve not attempted to describe every step or provide detailed pictures of every modification but rather give a flavour of the work required. If you aren’t confident enough to take a drill, file, saw to your bumper and new diffuser this isn’t the job for you. You also need a fair amount of both time and patience as this will take quite a few hours (I’d estimate at least 10-12) to complete and will require a lot to test fitting and modification to get it to fit. Given this is a track car and not a show car I wasn’t that obsessed with it being perfect at the expense of durability, if you are particularly anal about the look of your car the job may take you a lot longer.