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FI Fuel Pump Ratings

MECHANICAL FUEL PUMP FOR MFI

Mechanical fuel injection uses a positive displacement fuel pump to load the jetting distribution system.   Proper fuel pump size and jetting is vital for good performance and good spark plug readings.

In the past, Hilborn fuel pumps were rated by the manufacturer at 1,800 RPM fuel pump speed.  Enderle, Kinsler, and most other fuel pumps were rated at 4,000 RPM fuel pump speed.

As a result, Hilborn pumps rated at the slower speed would appear smaller.  Yet, when they were run at higher RPM, they were comparable in size to other manufacturers.

FUEL PUMP SPEED

Most installations gear or belt drive the fuel pump to 1/2 engine speed.  A Hilborn fuel pump at 1,800 RPM would correlate to an engine speed of 3,600 RPM.  Other fuel pumps rated at 4,000 RPM would correlate to an engine speed of 8,000 RPM.  The later 4,000 / 8,000 RPM combination is commonly used as a standard by most MFI flow bench services world wide.

EXAMPLE OF BUZZING THE FUEL PUMP FOR MORE POWER

A 20 GPM fuel pump for nostalgia nitro racing would be at an engine speed of 8,000 RPM.  Many of the competitors gear the wheel speed on these racer cars to run higher engine speeds.  Over 10,000 RPM is common.  The same 20 GPM fuel pump flows upward of 25 GPM at the higher RPM.  Fuel mixtures for the class running over 85% nitro produce a lot more power at the higher operating RPM.

Technical: About 50% of the combustion is stoichiometric.  About 50% is monopropellant.  As engine speed is increased, the monopropellant portion of combustion increases, and power goes up.

REFERENCE: Fuel Injection Racing Secrets, Blown Nitro Racing on a Budget.

LATEST NEWS: REALLY HIGH PRESSURE

One of our friends is currently developing a very high pressure fuel pump for MFI using an industrial design.  The purpose is greater atomization such as what occurs in high pressure gasoline direct injection as well as diesels for highway use.  We will update as more info is produced.

Fuel Injection Bypass Circuits

Racing mechanical fuel injection jetting is made up of nozzles to the engine and one or more bypass pathways back to the fuel supply.  The positive displacement fuel pump with the jetting layout makes a linear ‘fuel curve’ with engine RPM.  That is, the fuel to the engine per revolution is essentially constant.  As RPM goes up, fuel goes up.  Typical fuel pumps are usually bigger than necessary.  Tuning is done by changing one of the bypass circuits.  That is done to reduce or increases the fuel bypass for one of several different purposes.

Main bypass

The main bypass is the most common one used for tuning.

>> A bigger main bypass is used to reduce engine enrichment

>> A smaller main bypass is used to increase engine enrichment.

Idle bypass

Most fuel injection systems use an idle bypass.  It is enabled at low throttle positions.  It bypasses extra fuel for the idle needs where the throttle valve is closed down.  Only a small amount of air is going into the engine.  Only a small amount of fuel is needed for idle.  The idle circuit shuts down as the throttle is opened.  That is to deliver the full amount of fuel to the engine at full throttle.

High speed bypass

A high-speed is used to provide an added bypass for various functions.  For a normally (naturally aspirated) racing engine, it is commonly used to open at high engine RPM.  That reduces fuel per revolution.  That is used to match the engine fuel need.  In a normally aspirated engine, that usually goes down at higher engine RPM because of a reduction in volumetric efficiency at higher RPM.

For a blown engine, older blowers would lay down at high speeds.  The high-speed bypass was used in the same manner.  It would open at higher RPM to reduce fuel per revolution.  That would match the declining fuel need with the blower that would pump less air at higher speeds.

The high-speed bypass can also be used for extra fuel enrichment at the low end to reduce power.  Drag racing with a torque converter may benefit from that.  Power reduction on the hit can help to avoid tire spin on the launch.

Pump relief bypass

Some FI barrel valves are equipped with an added bypass circuit for pump pressure relief.  That is usually only enabled at low to mid throttle positions.  When the throttle is partially closed at high RPM, the fuel pump is delivering a lot of fuel.  However, the barrel valve is closed down.  Without pump relief, the fuel pressure can go too high.  This circuit is disabled at mid to full throttle.

6,000 HP on Methanol

Our 5000 Horsepower on Methanol tech manual features specs on some very high output methanol racing engines.  A 6,000 horsepower combination is detailed on p. 132. It is based on a top fuel drag racing V-8 long block with two stage compressors.  A minimum fuel pomp size is 32 GPM at 4000 pump RPM.  Boost is 7 atmospheres.  At a mechanical compression ratio of 10 to 1, approximately 0.23 cubic inches of fuel volume occupies about 4% of the combustion chamber volume at top dead center.  Going up in boost with more fuel pump can yield 12,000 horsepower also featured in this section.

Several 5000 horsepower combinations are described in Chapter 28 including Gene Sharber’s tractor pulling two stage turbo methanol engine combo gets 110 psi manifold boost.

Other high output engine combos are featured in appendix 29.  highlights are the GM Ecotec drag racing 4 cylinder at 1,450 HP and Smokey Yunick’s 209 ci turbo small block pulling 1,200 horsepower for the Indy 500  We thank GM Corp and Carbon press for insider details of these combinations.