Fuel line pressure drop (fwd)

[Jody Shapiro]

Sorry if people see this twice, I sent it out once before but never saw it
come through.

-Jody


---------- Forwarded message ----------
Date: Tue, 06 Jan 1998 11:46:40 -0800
From: Jody Shapiro <jshapiro at bit-net.com>
To: jshapiro at bit-net.com
Subject: Fuel line pressure drop

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.

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 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!).

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).

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?

Thanks,
-Jody

---------- Forwarded message ----------
Date: Fri, 02 Jan 1998 08:24:55 -0800
From: Jeff Stevens <jstevens at idt.com>
Reply-To: ba-f-body at f-body.org
To: ba-f-body at f-body.org, f-tech at colossus.net
Subject: [ba] Prof. Jeff does Fluid Mechanics (long)

Spent some quality time with my old college Fluid Mechanics textbook
studying Reynolds numbers, dynamic viscosity, friction factors, and other
cool stuff.  If there are any Mech Eng in the audience, I'd greatly
appreciate a sanity check on this.

Assumptions:

inner diameter (ID) of stock fuel supply tube: 0.34" (.028')
ID of proposed -8AN braided steel hose: 0.45"
length of stock line, T-Rex pump to fuel rail: 15'
length of rerouted -8 hose: 10'
   (routed up firewall instead of over fenderwell)
max fuel useage: 330lb/hr
   (600hp with BSFC of 0.55)
specific gravity of gasoline: 0.69
specific weight of water: 62.4lb/ft^3
relative roughness of tube: 0 (smooth pipe)
dynamic viscosity of gasoline: 2*10^(-5)
   (any petrol. eng. out there?  please check this!)
density of gasoline: 1.3 slug per ft^3

Step 1: velocity in stock tube:

(330 lb)   (   hr   )   (     ft^3    )   (       1        )
 ------  *  --------  *  -------------  *  ----------------
(  hr  )   (3600 sec)   (.69 * 62.4 lb)   (PI * .014^2 ft^2)

equals 3.4 ft per second.

Step 2: Reynolds number:

(1.3 slug)   (3.4 ft)   (.028 ft)   (   ft-sec  )
 --------  *  ------  *  -------  *  -----------
(  ft^3  )   ( sec  )   (   1   )   (2*10^5 slug)

equals about 6200.

Step 3: friction factor:

from textbook graph, with smooth pipe & Reynolds # of 6200:

equals about .034.

Step 4: lost head, "h"

h = friction factor * (length/diameter) * (vel^2 / 2g)

       ( 15 ft )  (3.4^2 ft^2)  (   sec^2   )
..034 *  ------- *  ---------- *  -----------
       (.028 ft)  (   sec^2  )  (2 * 32.2 ft)

equals 3.3 ft

Step 5: pressure drop, delta-p

delta-p = specific weight * h

      (62.4 lb)  (  ft^2  )  (3.3 ft)
..69 *  ------- *  -------- *  ------
      (  ft^3 )  (144 in^2)  (  1   )

equals about 1 psi

Conclusion:

A 600hp motor pulling 330#/hr of gasoline through the puny stock fuel line
only causes a 1 psi drop in fuel pressure from the pump discharge to the
fuel rail.  If this is true, why bother increasing the size of the fuel
line?  A 1 psi drop is nothing.  I must have an error somewhere.  Can
anybody help me find it?


Jeff
96Z M6, Blackstone shortblock/heads/intake, Vortech s-trim/SuperFMU, 30#/hr,
CompCams Xtreme/1.6 RR, AS&M headers/58, Random cat, Borla, Crane HI-6/PS-91,
McLeod Street Twin, 3.73, poly from tranny back, IRS panhard/RTAs