Modifications - Fuel Delivery

Fuel Delivery

 

Correct fuel pressure and fuel volume is necessary to safely operate a forced injection, fuel injected engine.  In general it is best to err on the conservative side.  That is, you want a surplus of available fuel rather than a shortage.  Surplus fuel can be tuned around, a shortage cannot and can result in serious engine damage.

The fuel injectors in the 6G72 require approximately 36 PSI net at the injector.  At 0 in/Hg, 36-40 PSI is desired.  Each one PSI increase in boost requires one PSI additional fuel pressure to maintain a net of 36 PSI at the injector.  So, to run 20 PSI of boost the injectors require 56-60 PSI of fuel pressure behind them.  The stock fuel pressure specification is 45 PSI.

The stock fuel pump is rated to flow 180 lph @ 43 PSI assuming the standard 12 volt wiring.   As fuel pressure demands increase, the ability of the pump to flow its rated capacity decreases.  At 56 psi that 180 lph pump can flow roughly 140 lph.  What you really need to maintain the 180 lph is a pump rated at roughly 235 lph @ 43 PSI.

This is calculated as:

required flow rate = required pressure ÷ rated pressure x flow at rated pressure

= 56 PSI / 43 PSI x 180 lph

= 235 (approximately -- remember to err on the conservative side)

From this, to be safe, you would likely select a 260 lph @ 43 PSI fuel pump to allow for moderate changes in fuel requirements.

The fuel requirements for a given target HP can be calculated as follows:

Fuel requirements (gal/hr) = HP x BSFC ÷ #inj x IDC

BSFC

= Brake Specific Fuel Consumption
HP = theoretical horsepower
#inj = number of injectors
IDC = injector duty cycle expressed as a fraction

BSFC is roughly 0.6 for a forced induction engine.  IDC is expressed as a ratio.  For example, 80% IDC is 0.80. This formula assumes a proper state of tune.

Useful conversion factors:

cc/min = lbs/hr x 10.5

gals/hr = cc/min ÷ 6

liters/hr = gals/hr x 3.785

The stock fuel filter can also be a restriction point in your fuel system.  The stock banjo fittings are generally inadequate for high fuel flow applications.  Several excellent hihgh flow filters are available on the aftermarket.  A clear filter is useful so that it can be visually inspected.  A filter with a replaceable element will help keep maintenance costs down.

Fuel Pumps

Fuel pumps are usually rated for flow at either 36 PSI or 43 PSI.  Pumps also have a ceiling capacity where flow efficiency begins to drop off and pressure cannot be maintained.  It is critical to consider all of these factors when analyzing fuel needs and selecting a fuel pump.

Denso in-tank replacements

Many different Denso (formerly Nippon Denso) fuel pumps are available.  They typically are more or less physically identical in exterior design and fit usually unmodified in the stock location.  The pumps are often found in other Japanese applications on the US market or originate from Japan-only vehicles.  Expect to pay quite a bit for these pumps since many of them are not in Denso USA inventory and must be ordered directly from Japan.

For proper cooling, in-tank pumps rely on being immersed in fuel surrounding them most of the time.  For this reason it is not wise to routinely run on less than 1/4 of a tank of fuel.  Below that level the pump may be exposed and therefore suffer premature failure from reduced cooling.

HKS in tank fuel pump HKS Intank Fuel Pump

Booster Pumps

This style of pump operates in-line to the original pump usually external to the fuel tank.  These pumps may be adeqaute for some applications, however a full replacement pump of the proper specifications is the preferred approach.

External Pumps

External fuel pump options will require totally new plumbing and mounting.  The stock lines and fittings will typically be insufficient.  The noise of an external pump is usually more audible than in-tank counterparts.  External pumps have cooling fins similar to heat sinks used on electronic components.

SX External fuel pump SX External Fuel Pump

Fuel Pressure Regulators (FPR)

A good quality boost dependant rising rate fuel pressure regulator is a must.  The stock FPR may be adequate for most needs.  Just do not ignore the output capabilities of the pump.

SX FPR SX Boost Dependant Fuel Pressure Regulator

Fuel Injectors

Most experts do not recommend operating above 80% IDC for extended periods of time.  Injector failure can result.  Some injectors are not even properly tested for flow at fuel operating capacity so be very aware when purchasing injectors how they were calibrated and the manufacturers recommended operting limits.  For short bursts, 90% IDC should be fine.  Much above that and you should be considering larger injectors.

Injectors are available from HKS and RC Engineering, with RC offering a wide variety of styles and sizes.

SX FPR RC Engineering Fuel Injectors

Fuel Filter

A free flowing fuel filter is a must.  The stock filter uses restrictive 90 degree banjo bolts.  Numerous aftermarket options are available.  They will usually require changing the fuel line and fittings.

SX Filter SX Fuel Filter

Throttle Body

A larger bore throttle body may sacrifice some low-end throttle response at the benefit of top end performance.  This may be desirable for high horsepower applications where larger turbos and aftermarket airmeter or VPC is employed.  RC Engineering offers throttle bodies which are bored and fitted with larger throttle plates.


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