Works style rally car preperation for the clubman budget

The Fuel System

As explained earlier we have decided to dump the orignal fuel tank in favour of an aftermarket smaller aluminium one fitted in the boot. The reasons for this are 2 fold, the orignal tank sits underneath the floorpan at the rear of the car and as we intend to do quarry sprints in the car the standard tank would be quite exposed to getting hit by flying rocks and debris thrown up by the wheels, this could have been dealt with by making up metal tank gaurds to cover and protect the standard tank but the increase in weight these would add would be undesireable. Another problem a fuel injected car can suffer from in competition is called fuel surge. And there now follow a series of ropey diagrams to illustrate how fuel starvation can occur and how we have choose to try and cure it.

   Now I realise this could be asking a lot but if you can stretch your imagination for a minute and pretend that the picture above is a rear view of the bmw showing the petrol tank in the boot and the car is taking a hard right hand corner and as such the car is leaning over to the left. You can see that the fuel will tend to flow over to the left hand side of the tank also. Now if pipe A was our fuel return line to the tank and pipe B was our fuel feed to the engine, you can see how for even a short second the suction pipe B can be left to suck fresh air. This results in no fuel or reduced fuel pressure up at the injectors on the engine and is felt as a loss or reduction in power depending on how long the pipe is left to suck up fresh air. Not the nicest of things to happen when your trying to give the car a boot full of rev's to pull out of a corner. In reality the above diagram shows an extreme situation, as BMW being the smart people they are have designed a small bowl around the bottom of the fuel pick up pipe to try and prevent this situation from occuring. However this system is only 100% effective if the tank is reasonably full of fuel and as you may imagine the tank on a 325 is fairly large and we want to run as small a fuel load as possible to reduce weight.
   So........... the solution we have decided to go with is to fit a small (18litre) fuel tank in the boot coupled with a low pressure pump, high pressure pump, and a swirl tank. How do these work ? Time for another fantastically detailed diagram.


      Hopefully the little black arrows should help show which way the fuel is flowing and make the above a little easier to follow. Basically the fuel is sucked from the tank by a low pressure fuel pump and pushed into the swirl tank. The swirl tank is also fed by the fuel return line bringing the majority of fuel back from the engine unused. These two feeds should ensure that the swirl tank is mostly always full and the pipe from the top of the swirl tank allows it to overflow back into the main tank and vent any air trapped in there back to the main tank aswell. A high pressure pump then sucks fuel from the bottom of the swirl pot, which is always full no matter how far over the car is lying in a corner, and pumps it under high pressure up to the injectors ensuring that there's always a good supply of fuel at the injectors.

   And this is how it looks in real life.........


As you can see for the actual plumping of the fuel we have used standard rubber high pressure fuel hose in the boot to keep costs down, but to comply with the regulations set out by Motor Sport Ireland the plumbing inside the passenger compartment is steel braided with threaded end fittings as opposed to jubliee clips.


   and then returning to rubber in the engine compartment passing through some fabricated bulkhead fittings.......


 with the fuel taken care of its now time to move on to plumbing up the braking system.

                                             Click on the link below to follow the progress,