5 bar fuel pressure regulator vs. 60 or 80lb  injectors on a 996tt

Turbo owners always want more power and 996 Turbo owners have asked which is better betwen a 5 bar fuel pressure regulator vs. 60 or 80lb  injectors on a 996tt.

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Sure, a 5 bar fpr is a quick drop in, and should gain about 15% in fuel flow and power, but there’s a few hidden issues that shouldn’t be overlooked!The most prominent issue is the factory 996 turbo fuel pump that has a hidden little surprise!  The OEM fuel pump in the 996 turbo has a 6 bar over-pressure protection feature built into it as do the first gen Walbro 450s and many other popular pumps.   When you exceed that, it opens up this port and bleeds the excess pressure back into the tank robbing your engine of that power increase you’re after.  That’s fine, you say, since we’re only running 5 bar fuel pressure, right?  Well, no.  Since these regulators operate on a 1 to 1 ratio with fuel pressure to maintain a constant pressure drop across the fuel injectors, your fuel pressure will raise and lower according to the intake manifold pressure or vacuum.  Running stock boost or 0.8 bar would raise the 5 bar static pressure up to 5.8 bar.  Running 1.0 bar would hit 6 bar and be right at the protection of the pump and 1.2 bar of boost would try to make 6.2 bar of fuel pressure!   But with the bleed off happening at 6 bar you’ll get stuck there and any extra fuel demand will simply bleed back into the tank.   Now if you ran 3.8 bar base which the stock fpr runs, at 0.8 bar you’d be at 4.6 bar, while 1.0 would produce 4.8 bar and 1.2 bar of boost would be at 5 bar fuel pressure.  Which is well under the 6 bar over pressure protection limit built into the stock pump.  So, what was initially a 15% gain using the math of  SQR(5.0/3.8) = 1.147 or just under a 15% gain of fuel flow, we’re now looking at 6 bar of fuel pressure at 1.2 bar of boost when using the 5 bar fuel pressure regulator vs. 5 bar of fuel pressure  at 1.2 bar of boost when using the 3.8 bar fpr. A loss of 0.2 bar fuel pressure which would be more like using a 4.8 bar vs. 5 bar fuel pressure regulator reducing the gain almost 3% to 12.4% over the 3.8 bar flow rate.  For the math nerds and engineers out there that’s SQR((5.0 base – 0.2 loss) / 3.8 base) = 1.1239 or 12.4%.  But that’s not considering the lost fuel coming out of the bleed off valve in the tank which will drop the flow increase yet again.   Trying to run even higher boost on a 5 bar setup continues to lose ground.   While the 5 bar sounds great in theory, in practice with a stock pump, it’s not gaining  a whole lot.  It is gaining, It does provide a flow increase and can be tuned for more power, but a better way is to bump the fuel injector size up to 60 or 80lbs and maintain the oem fuel pressure to avoid this issue.We can get around this issue like we do with a series’d bosch 044 fuel pump in the engine bay.  This takes the pressure and splits it between each pump making the 6.2 bar demand now only 3.1 bar per pump.  This also increases the flow of the oem in-tank pump greatly since it’s no longer having to push all that pressure!   At only 3.1 bar, it flows substantially more than it would at 6.0 bar, probably 30 to 40%.  I don’t have the charts on the factory pump but that would be typical based on flow charts from 044 and other high performance pumps like this one below for an 044 Bosch Fuel pump (Superseded by the 0580 464 200)

  The other advantage of the series’d rear pump is you’ll lower the current draw on the front pump.  As these cars tend to have issues with the electrical bulkhead overheating, this is a great help by reducing the current draw through the bulkhead thus extending the life of the electrical component and pump.  See the current draw decrease from almost 14 amps down to 11 amps in the chart below (6.2 vs. 3.1 bar)

Another issue you’ll run into running high pressure on the single front pump is with the age of the oem accordion lines in the tank feeding the jet pumps you’ll tend to crack or split them.  They hold plenty of pressure brand new, but with the current fuels with E10 and other blends it’s making them brittle and cracking them.  Running high pressures can cause them to fail prematurely.  Using the series’d 044 in the engine bay also helps this issue with the lower pressure requirement of the front pump by dividing the fuel pressure increase between the pumps.  Either that or running the larger fuel injectors and maintaining stock fuel pressures.A quick note on a nice difference between the 996 and 997 turbo fuel pressure regulators. With the 997.1TT coming with a dual fuel pump setup, Porsche increased the return flow of the oem fuel pressure regulator.  It’s still 3.8 bar, but the return orifice is now 3mm vs. 2mm.   This allows a tremendous increase in return flow on high flow fuel systems.   This will negate the high fuel pressure many see at idle and light throttle conditions due to the restrictive return orifice in the stock 996 turbo fuel pressure regulator.The moral of this story is while raising the fuel pressure does increase the fuel flow and potential power to the engine, it doesn’t come without a cost and there are better ways to do it.   For a quick upgrade, the 5 bar regulators are great.  For a little more, you should consider upgrading the injectors and leaving the stock fpr in place.