Fuel line pressure drop (fwd)

[David Piper]

>I'm forwarding along a question from a friend of mine who's having fueling
>problems with a supercharged V8 (LT1 Camaro Z28).  He recently rebuilt the
>engine and has been chasing insufficient fuel problems since.  Estimated
>crank horsepower at redline is ~600hp - the car has been on a DynoJet
>chassis dyno and was making 450 rear-wheel horsepower at 5000rpm when the
>stock injectors would lock up .  Redline on the car is 6500rpm and the
>engine is centrifugally supercharged, so there's a lot more boost/power
>lurking in the higher rpm range.
>
We have observed 560 to 600 HP on our Dynojet with 10 psig boost on 110 octane.

>The original problem was injector lock up at 5000rpm with the stock 24
>lb/hr injectors and the ~90psi of fuel pressure that the FMU was creating.
>The stock injectors were replaced with Ford SVO (Bosche) 30lb/hr injectors
>and the static fuel pressure decreased.  A Vortech SuperFMU (adjustable
>rate) was installed at the same time and has been tuned for appropriate
>fuel pressure at full boost.
>
The max press we have used for these inj is 62 psig which will increase fuel
flow 12%.  You need 36 pph inj for 600 HP at 0.50 #/BHP-hr.  The above
examples were using 36 pph inj and the 600 HP one was maxed out at 0.85 lambda.

>The current problem is that with the engine off, fuel pumps on, the
>boost/fuel-pressure curve can be dialed in correctly and the desired
>full-boost fuel pressure set (using a regulated air supply to simulate
>boost).  With the engine running and the car actually making a
>wide-open-throttle run, the fuel pressure starts dropping above 5500rpm
>and things lean out (leading to audible detonation - good thing there's
>forged pistons in there now!).

You have a fuel supply restriction; pump, filter or regulator or too much
pressure set at FMU.  Is the FMU installed backwards?
>
>Current suspicion is that either (or both) the fuel lines are too small,
>or the intank (stock) fuel pump and the T-Rex in-line fuel pump are
>insufficient.  The original thought was that it was cheaper to try
>increasing the fuel supply line size, but after doing the following
>calculations, we're not so sure and current thinking is leaning towards
>putting a larger Bosche in-tank pump in to replace the 2 currently in use
>(stock + external in-line).

The T-Rex is good for 190 lpm or 49 gph or 300 pph (=560 HP) but not at
higher pressures.  Stay below 60 psig with this pump, get 50 pph inj or repl
pump.
>
>If the following math is correct, it would appear that larger fuel lines
>don't buy you much.  Yet, on most serious forced-induction setups, you see
>larger fuel lines in use.  Comments?

Some feel that with a light, taller geared combination that the near
instantaneous demand would starve the pump, but we don't believe it, since
the fuel supply system is a closed loop in solid hydraulic lock.  Inertial
forces come into play in Top Fuel.  You don't need to worry; 3/8" lines
should be adequate.

Do not detonate your motor. 

Your calcs assume too much, but expect 2-5 psi drop in a 3/8" line.
There are entrance and exit losses, velocity heads and assumptions re fuel
viscosity vs temp and the L/D for each bend in the hose.

Keep me posted on your results.  I will consult with our patrons.

TurboDave(aka DynoDave)