WHY REDUCE FUEL PRESSURE SLIGHTLY ?

WHY REDUCE FUEL PRESSURE SLIGHTLY ?

There is a good reason for opting for slightly reduced fuel pressure on the R53 Cooper S . The idea of the fuel pressure regulator is to maintain a CONSTANT fuel pressure over and above the inlet manifold pressure. That is on a std car 3.5 bar . Eg it will give out 3 bar Rail pressure on light throttle when the inlet manifold is -0.5 bar ( ie part vacuum), but 4 bar Rail pressure when on light boost of +0.5 bar in the inlet manifold . This is because with vacuum the fuel is being partially sucked out of the injector anyway therefore less fuel pressure is used, however at positive boost the fuel is being pushed back into the injector therefore higher fuel pressure must be used to compensate. This works fine at std boost or cars running up to 17psi of boost .
Once this is understood you can start to understand the problems at high boost . The std pump produces max 4.65 bar, if the boost is increased ( with pulley change) to any more than 1.15 bar ( 17psi) the fuel pressure drops off !!. You should not resort to compensating for this with mapping . Now if we increase the injector size to say 440cc and run the fuel line pressure at say 3.15 bar reduced from the std 3.5 bar, ( 4.65 Bar pump pressure minus 3.15 fuel rail pressure = 1.5 Bar ). Yep we can now maintain constant fuel pressure up to 1.5 Bar Boost pressure (22 psi) ….. It also allows 440cc injectors to be used without remapping , killing 2 birds with one stone .I don’t expect all to understand it first read, but if read slowly it should make sense.

One last point……. DO NOT have someone set up your fuelling on a Rolling Road only ! It must be set up on the real road, as it always runs leaner (weaker) on the real road with these cars . Set up fueling on real road using an on board Wide Band Air Fuel Ratio Meter. Failure to do this can cause over high combustion temps leading to detonation resulting in pistons or rings or spark plug or cylinder or valves , cracking / melting etc. So if you’ve gone through 4 engines already, all mapped by the same people it does beg the question.