Works style rally car preperation for the clubman budget
The Fuel System
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.
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.........
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,