Hi everyone,
Ok so I thought I would put this guide up as to my knowledge there are a few people out there including myself who have been having big problems sorting out the fuel system on the fn2 when running turbos or superchargers at 450 bhp+. This may also work for some of the U.S cars but I personally do not have the experience to say if the fuel collector units are the same as the European FN2.
The in tank fuel pump unit in the fn2 is actually a fairly complex little assembly so unfortunately running a high power uprated pump and fuel return system is not quite as simple as in the older days :)
So just to cover all the bases if people are looking for information on what to do and they have no prior knowledge as to what is required or what problems you may encounter when running high power outputs on these cars.
The standard FN2 runs a dead head fuel system. this means that the fuel pressure is regulated inside the fuel take sender unit at 4bar pressure and it is sent to the engine via a single fuel line that feeds the fuel rail and injectors. The first problem that you will encounter with this setup is that the fuel regulator is fixed at 4 bar pressure. When we convert these cars to run forced induction the boost pressure in the intake manifold actually fights against the fuel pressure, so for example if you are using a 4 bar pressure regulator but you are also running 1 bar boost pressure you would effectively be running 3bar fuel pressure. This is fine upto a certain power output as you just keep increasing the injector duty cycle to compensate for the rop in fuel pressure. However...for larger power engines we need to go a little further.
The first stage is to note that we need to ditch the standard fixed rate regulator in favour of an adjustable rising rate regulator, By this we mean a fuel regulator that has a vacuum line attached to the intake manifold so that it can see boost pressure. these regulators will then increase the fuel pressure to compensate for the boost pressure. I.e a 4 bar static fuel pressure at idle would increase to 5bar fuel pressure when the engine is running at 1bar boost pressure.
To do this you will need to run a fuel return line from the engine back to the fuel tank and mount a rising rate regulator in the engine bay. if you have an aftermarket fuel rail then they usually have the option to run a feed and return line at either end. if you are using the stock fuel rail then you will just have to cut the fuel feed line just before the fuel rail, fit a t-piece and run the new line to the regulator. the the exit fitting of the regulator needs to run into the new return line that you can fit under the car. I just use some 8mm copper brake line and clip it along the route of the standard fuel line with some rubber fuel hose at each end.
Now, the tricky bit starts here. the stock fuel pump system runs a special little blue diverter valve on the side of the fuel collector pot in the tank. If you try to run your new fuel return line back into the fuel tank as people usually do then what tends to happen is once the fuel tank is less than half full the fuel collector can not refill as fast as the up-rated fuel pump can empty it, On my 500+ bhp civic It tends to give you about 1 gears worth of acceleration before you just hit a fuel cut (not a very clever idea with forced induction as its a very high risk of frying a piston)
The other problem with the standard fuel collector is that it just isn't very big, so with an up-rated fuel pump and a set of 1000cc injectors running flat out it just doenst stand much chance :(
So, here is what I did to solve the issue in the most cost effective way before just getting silly with running a whole new fuel tank etc.
Take the standard intank fuel pump unit and carefully disassemble it.
The centre part in the this photo is what we no longer need. it takes up a huge amount of volume inside the fuel collector pot and we wont be using it anyway as it just contains the stock fuel pressure regulator and post filter etc.
drop the stock pump filter back inside the collector pot. The top of the filter neck will need cutting down slightly to get the new pump to sit inside.
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drop the new fuel pump in place. This is a 340 ltr/hr pump. Aeromotive sell them as well as many other suppliers.
here is the trick bit. With the standard filter/regulator housing discarded I used a new cnc plate to replicate the slider mounting boss. Fit the top half of the sender unit into the new cnc plate and use the original c-clips to hold the shafts in place with the springs in place.
drop the unit into the stock collector. You will need to drill 3 holes in the collector casing in order to screw in the 3 securing bolts that hold it all in place. Be sure to remove any swarf after drilling.
drill a hole and fit the fuel return line fitting into the top of the sender unit. (the aluminium fitting in the top of the picture)
next the high pressure fuel feed line needs to be modified to fit directly onto the new pump. Take a sharp knife and cut the plastic pipe half way along the smooth section.
use a hose clip to secure the hard line onto the top of the new pump
next we need to route the return line. The line needs to be split via a t-piece so that one half goes onto the brass fitting found on the top of the new cnc (this is the main fuel return into the collector) and the other needs to go to the flap valve jet. As with the main fuel feed line you can use the stock plastic pipe and just use a clip to secure it onto the t-piece.
The routing is important and you need to make sure that no hoses will kink when the unit is fitted back into the fuel tank.
To give a bit of background on the operation on the return line. The flap valve jet is required as it causes a stream of fuel to enter the swirl pot. Without this the fuel level in the swirl pot will not exceed that of the rest of the fuel tank. This causes a big problem when you have 1” of fuel left in the tank as the fuel system would consume that in a second! By running with the flap valve the swirl pot will be able to almost completely fill itself to the brim when on anything but 100% throttle as some fuel will always be returning to the tank and activating the flap. When on 100% throttle we now have the volume in the collector pot because we have removed the stock internals which allows us to pretty much double the volume of fuel within the collector.
The brass fitting in the cnc plate is drilled to a restricted size to achieve the correct balance of fuel so that the flap jet has enough pressure to open the flap and so that there is also enough pressure relief in the return line so that the pressure can be dropped to 4 bar at idle.
This has been tried and tested in my own car running 500+bhp and There is no problem running flat out even with 10% fuel in the tank.
Please note that if making these mods you must always check your fuel pressure is set to 4 bar at idle and adjust the regulator correctly If it is not. And you must also get your Air/fuel ratio checked on full throttle as fuelling will change compared to the standard setup.
Hope this info is of use to some people as I know there are plenty of people who have been struggling with these issues.
I will make these adaptor parts available for sale in the near future, so probably best to contact TTS performance directly if you are interested in these conversion parts.
Enjoy.