Fuel pressure is important for heavy stage 2 / stage 3 tunes for a simple reason. There aren't any good injectors which can flow more, the only way to flow more fuel through the same size injector holes (before the exhaust stroke) is to increase the pressure of the fuel. If you know how to do this without tripping the cut out conditions then it's not that hard to do. Obviously its easier to do this with a good fuel pump.
The theory is all there, and this has been confirmed several times by TB206 who has pushed the HDI engines the furthest out of everyone. I've also found it to be very beneficial on the heavier tunes, assuming the fuel pump is good. When the pump cannot meet the demand it's kind of pointless asking for it and just creates extra resistance for not much benefit.
It's not too hard to work out when injection will end based on the start time, engine speed, fuel pressure and the injector calibration time. With this it's clear that end of injection is far too late when compared to stock values. Yes, you can push it quite a bit later, but this results in high EGTs as your fuel is burning as it goes through the turbo on the exhaust stroke.
With the bigger turbos this isn't so much an issue as it is when pushing the stock turbo hard, the bigger turbos seem to provide enough air to burn most of the fuel cleanly, if you want to push it really far you need more fuel pressure otherwise you'll have smoke. Burning the fuel in the cylinders rather than the exhaust also gives the benefits of reduced exhaust manifold pressure and temperature.
And Graham, you don't want to rev much past 5 on the HDI, you'll just end up with a con rod shooting through the side, it's just not designed to rev that high! I thought you need reliability still? lol
The theory is all there, and this has been confirmed several times by TB206 who has pushed the HDI engines the furthest out of everyone. I've also found it to be very beneficial on the heavier tunes, assuming the fuel pump is good. When the pump cannot meet the demand it's kind of pointless asking for it and just creates extra resistance for not much benefit.
It's not too hard to work out when injection will end based on the start time, engine speed, fuel pressure and the injector calibration time. With this it's clear that end of injection is far too late when compared to stock values. Yes, you can push it quite a bit later, but this results in high EGTs as your fuel is burning as it goes through the turbo on the exhaust stroke.
With the bigger turbos this isn't so much an issue as it is when pushing the stock turbo hard, the bigger turbos seem to provide enough air to burn most of the fuel cleanly, if you want to push it really far you need more fuel pressure otherwise you'll have smoke. Burning the fuel in the cylinders rather than the exhaust also gives the benefits of reduced exhaust manifold pressure and temperature.
And Graham, you don't want to rev much past 5 on the HDI, you'll just end up with a con rod shooting through the side, it's just not designed to rev that high! I thought you need reliability still? lol