I also think the tune is the problem. When you go WOT at low rpm, what rpm are you talking about. I never go WOT below 3 to 3.5k rpm. Otherwise I feel like I would be lugging the engine which doesn't help bearing life.
I don't understand how a higher pressure prv would help with your problem. Aluthman and Charles correct me if I'm off base, but all the prv does is maintain the pressure in the fuel rail (available to the injectors) at a prescribed level, and relieves the excess pressure by lifting until the pressure drops to the prescribed level. It's all mechanical. It should do this at all rpm levels from idle to redline. The pressure sensor measures the rail pressure and trips a code if it is not being met either due to a clogged fuel filter, malfunction of the LPFP or HPFP, or a faulty prv, etc.
Theoretically, the stock system should be able to meter fuel at the prescribed pressure up to redline. The so-called "upgrade" HPFP would not help the stock tune system, and a higher prv pressure wouldn't either although the higher rail pressure might result in a slightly better injector spray pattern. At higher rail pressure the injectors would be capable of delivering a greater volume of fuel, but in a stock system the spray impulse duration would be electronically shortened by the ECU. Otherwise, you'd be running too rich a mixture. The point here is that the higher pressure wouldn't be affecting the volume of fuel supplied to the cylinder at any rpm.
Having said that, a tuned system is capable of delivering much more air to the cylinders due to the higher boost pressure recipe of the particular tune, which is where the horsepower increases come from at higher rpm. Therefore, at higher rpm the ECU would be calling for longer injector pulse duration to achieve the correct fuel/air ratio. This mandates a higher fuel volume than the stock HPFP is capable of delivering. As I understand it, all the "upgraded" HPFP does is deliver a greater volume of fuel, again, at the prescribed pressure. The HPFP does this by simply increasing the diameter of the pump piston. I believe the APR pump piston area is 30% greater than stock (don't know about the other HPFPs on the market) which should meet the fueling requirements at the highest rpm and the highest boost pressures. I can see that a higher rail fuel pressure might help the situation of the tuned system at the higher rpm levels until the capability of the injectors is exceeded, which I believe is why some of the higher tunes required the larger fuel injectors (S4).
The down side of higher prv is as follows: 145 bar is about 2100 psi. The diameter of the HPFP piston is about 0.3 inches. Therefore at 145 bar the fuel is pushing back against the piston with a force of 150 pounds plus the 20 pounds of the HPFP spring pressure equals about 170 pounds force. This is the same force transmitted to the cam/cam follower junction. The "upgraded" HPFP force would be 213 pounds of back pressure. A 165 bar prv would be about 240 pounds of force with the higher capacity HPFP. That's a lot of contact pressure between the cam follower and the cam. Check your cam followers often!! The stock system would be about 135 pounds of force calculated using a fuel rail pressure of 115 bar.
Sorry to meander around the subject but would appreciate any comments or corrections on the above logic.
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Originally Posted by aluthman
and I dont know anyone in the Westchester area who does either. Does anyone happen to know what the requested bar is for a stage 2 unitronic tune?
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