The last thing I did was to fit a regulator and a pressure gauge to the fuel line, between the TB fuel rail and the regulator. The later was bought from Burton Power and allows you to adjust your regulator (FPR) to the pressure you want. The injectors that I had are quite large, rated at 384cc/min @ 55psi (found out by calling Bosch - and checking on Blatchat), and this was more than I wanted to run them at. The problem is if your injectors flow too much fuel you are forced to run very short duration around idle speeds and the result is that a change in opening time of 0.1ms can effectively be the same as adding half as much fuel again into the engine. This makes setting up a nice idle virtually impossible. I therefore needed to reduce the flow of the injectors by reducing the fuel rail pressure, meaning I had longer opening times at idle, which in turn means you have finer control of your idle fuelling. This was all detailed on the ECU set-up pages far more accurately that I can explain here – so if you want to understand this better I suggest you visit the injector pages here. Using the expected outputs of my engine (plus a small amount to be safe – 210bhp) I could get a close approximation of the amount of fuel I needed at max power/revs (324cc/min). I could then use a calculator to work out what pressure I would need to supply my injectors with to achieve this. The calculation is:

New flow rate = old flow rate × SQRT[new pressure÷old pressure]

So, in my example, 324cc = 384cc x SQRT [ ? ÷ 55psi ]. This works out as 39.1psi. However, I was pretty paranoid about running the engine lean at the top end so I played around with a few figures and soon settled on a value of 43.5psi, which is a nice, convenient 3bar. This gives lots of spare capacity, but importantly it gave a high enough idle pulse width to allow for finer control. This can be checked using a rule of thumb calculation:

PW = REQ_FUEL * VE * MAP + Injector_open_time

Without running the engine I didn’t know what my idle VE (the volumetric efficiency) or the idle manifold pressure would be, but example figures of 30% and 35% were given. Using the Megatune tuning software I could pump in my chosen flow rates from above and get my required fuel figure. My calculation therefore looked like this:
PW = 9.9 * (30/100) * (35/100) + 1ms. This equals 2.1ms, which is a good figure. If it had only been 1.2 or 1.3, then when you remove the opening time, any adjustment of opening time (in 0.1ms steps) is close to halving or doubling the fuelling – not good for fine control.

So I knew the pressure I could run for the final engine, but first I had to repeat the process for my initial run on the standard cyl head and cams. I guessed that a standard engine with TB’s would give around 180bhp, this is the figure that Raceline offer as their standard engine. This gave a fuel requirement of 278cc/min and for this I chose 35psi which gave an idle pulse width of 2.2ms. This was therefore my initial setting on my fuel pressure regulator. This was set by means of a screw and locknut on the top of the unit whilst I connected the fuel pump to a spare battery. The ECU only runs the pump for 2secs when it primes the system until the engine is running.

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