Welcome to Bob’s Blog! This is where Bob will post tips, tricks, and experiences he has with other racers. Check back often to read what Bob has been up to.

Fuel Injection Bypass Circuits

Monday, November 22nd, 2021

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

Thursday, September 9th, 2021

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.


Fuel Injection Jetting for Ram Air

Sunday, August 15th, 2021

For a mechanical fuel injection (MFI) engine with a forward facing air scoop, ram air into the scoop increases with speed.  At about 200 MPH, a 14% increase in air was determined in our drag racing car as an example.

A corresponding increase in fuel was the best way to maintain an optimum air to fuel ratio.  In our case, that was 3.4 to 1 for blown methanol.

For MFI, that increase can be done in several ways.


The high speed bypass can be shut off for ram air at high speed for the enrichment if it is the proper size.  The shut-off function can be done with a manual/valve, pressure/solenoid/valve, or timer/solenoid/valve.


For MFI fuel systems with a main bypass, an extra nozzle can be added.  It is off at low speed.  As the speed increases, the nozzle can be enabled with a manual/valve, pressure/solenoid/valve, or timer/solenoid/valve.

Note: Some MFI fuel systems are run without a main bypass.  Nostalgia Nitro drag racing is an example.  This method will not work in that case.  With the extra nozzle, only a drop in fuel pressure would occur.  No added fuel would be delivered with ram air increase.


The unique Engler stumble valve can be added to a bypass circuit.  It operates opposite to a high speed poppet.  As the fuel pressure increases, the stumble valve will shut down.  In the bypass circuit, that will richen the engine.  The stumble valve by itself will automatically initiate with fuel pressure.

Note 1: That would occur in low gear as well if the engine is revved high enough.  To disable that, a manual or timer controlled shut-off valve would be needed.

Note 2: The Engler stumble valve bypass requires an inline jet to restrict the fuel flow when the valve is open.  That jet restriction is part of the bypass circuit for tuning.


Jetting for proper enrichment for high speed ram air can be done with our ProCalc MFI jetting calculator.  Lining up the nozzle or bypass jet sizes can be done to maintain an optimum air to fuel ratio throughout a race.  Enabling pressures can be determined as well.  We can also provide this setup service and training with our ProTune & ProTune Advance services.


In a 225 MPH top dragster, a 0.060 in diameter high speed bypass was initiated during the low speed portion of the run.  At 200 MPH the high speed bypass was turned off to maintain a 3.5 to 1 air to fuel ratio above 200 MPH for the blown methanol engine.

Note: Without this function, the spark advance had to be reduced about 10 degrees as a band aid to the leaner running engine from ram air.  The car ran more consistently with fuel enrichment for ram air than with ignition retard for ram air.


Especially with low percentages of nitro for match racing, enrichment for ram air is better for tuning than an ignition retard alternative.  An optimum air to fuel ratio can be mapped & maintained throughout a run with proper enrichment for ram air.


Drag Racing in Hungary

Tuesday, June 29th, 2021

Kiskunlachaza is the latest racetrack addition to


Rons Fuel Injection FAQ’s

Tuesday, June 22nd, 2021

Ron Fuel Injection makes cost effective throttle body kits, throttle assemblies, fuel pumps, & other parts for both normally aspirated and supercharged engines; and EFI setups with HALTECH engine management systems.

Excerpts from their site to common questions about fuel injection:

… makes the car a more consistent performer.
… mechanical fuel injection does not utilize a venturi (restriction) to draw fuel into the engine … Without this restriction, and with no heavy fuel to deal with in the intake …
… can be outfitted with the correct sized throttle body to work on virtually any combination …
… RONS prices their systems as complete units.  These kits include the throttle body, combination barrel valve/fuel shut-off, …
… in most cases the system will need parts replaced or changed for your particular application …”

Click on for the details.

Volumetric Efficiency in Normally (Naturally) Aspirated Racing Engines

Wednesday, May 19th, 2021

Normally (naturally) aspirated racing engines usually operate over an engine RPM range.  Tuning for performance is often needed especially with air density changes.  That usually involves two different engine speeds that are key to high power.  These are the torque peak RPM and the horsepower peak RPM.  Engine design establishes the specific engine speeds for both.

Note: A normally (naturally) aspirated engine designed to operate at only one engine RPM may have a torque peak and horsepower peak at or near the same RPM.  That design would establish optimum features making both better.  However, the engine would perform poorly at other RPMs.  As a result, the application is limited.  Engines for power generator operation or liquid pumping with RPM control are examples.

Torque Peak RPM

The torque peak of a racing engine is the RPM where air pumping through the engine is optimum.  That is established by intake size and camshaft timing.  Typical intake port velocity peaks at 400 feet per second.  Slower port velocities at lower RPM are less efficient due to air and fuel separation.  Essentially the ports and camshaft timing are too big for that lower speed, and torque is lower.  Higher port velocities at higher RPM are also less efficient due to the increased pressure drop from pumping losses.  Essentially the ports and camshaft timing are too small, and torque is also lower.

Horsepower Peak RPM

With higher engine speeds however, the number of torque pulses per second goes up.  That overcomes the reduction in torque up to a point.  That is where the horsepower peak occurs.  Above that, torque is so low that the increased torque pulse frequency cannot overcome it.  Some engines also reach other RPM limits such as valve float, an electronic rev limiter, or pumping losses from exhaust backpressure or simply small ports, valves, and valve timing.

Different Engine Speeds Example

Methanol racing engines are common in circle sprint car racing and drag racing.  Racing camshaft timing and large intake ports / valves make torque peak values typically around 5,000 RPM.  Horsepower peak values are achieved at a lot higher engine speed, typically around 7500 RPM.  Torque at 7500 RPM is lower, but horsepower is greater due to the higher torque pulse frequency.

Fuel System Delivery Example

Fuel system tuning considers both engine speeds.  For methanol engines, torque peak air/fuel ratio is typically 4.8 to 1 with a volumetric efficiency around 100%.  Horsepower peak air/fuel is typically 5.2 to 1 with a volumetric efficiency around 85%.  Fuel injection jetting is unique for both engine speeds with a smooth transition from one to the other.  In addition, jetting must be adjusted for different air densities.  Trial and error tuning can be difficult because of the two key engine RPMs, the transition, and differences from air density changes.

Added References

More information is in our books: High Horsepower Tuning for Mechanical Fuel Injection, Jetting for Racing Mechanical Fuel Injection (for Small Block V-8s), Jetting for Racing Mechanical Fuel Injection (for Big Block V-8s) 2nd Edition.

Roots Blower Sizes

Tuesday, May 18th, 2021

Different blower case lengths, often bigger than standard sizes, are available through several of the manufacturers. This can lead to misleading information about how big a blower is with a larger case length. In addition, two of the blower manufacturers provide CFM claims that translate to blowers that are a lot bigger than the standard geometric displacements. CFM values are temperature dependent. As air is compressed in the blower, it heats up. Output CFM values can be greater in volume due to that temperature increase, but the weight of air remains the same. Air weight is dependent on the geometric displacement of the blower. For fuel system tuning, the weight of air from the blower is key, not volume.

Standard blower designations and sizes common in motorsports for both standard & high helix rotors

blower designation

rotor length

blower displacement with rotor seals


cubic inches






















Misunderstanding from Longer Blower Case Lengths

Case lengths may be longer than these rotor lengths.  Some are over 22 inches.  The difference is made up with an internal plug filling the case volume, usually on one end of the case, shortening the case length.  That plug may have additional ports.  Added ports may be feeding from the top or discharging out the bottom.  One or more plugs may be used to make up the difference between the rotor length and the case length.  A plug or plugs may be located at the drive end or rear of the blower.

Geometric Blower Displacement from Rotor Length is Key

The presence of plugs with ports does not increase the geometric displacement of the blower.  The geometric displacement is the same for standard or high helix blowers.  That is defined by the geometric limits of the interlocking rotors.  Some blowers are equipped with front and rear gear drives that may also appear to extend the case length.

6-71 Example

The 6-71 blower with a 15-inch rotor length is specified in many motorsports’ classes.  Many of these are equipped with longer cases with internal plugs and/or rear gear drives.  They may appear illegal; however, the 15-inch rotor resides in a legal blower regardless of the external appearance.

14-71 Example

Likewise, the 14-71 blower with a 19-inch rotor length is specified in many other motorsports classed.  Again, longer cases may be used with various internal features hugging around legal 19 inch rotors.

Fuel System Tuning

Fuel delivery need is based on several features: blower displacement, blower efficiency features, and ram air (if so equipped with a forward mounted air inlet).

Blower features such as high helix or internal ports affect blower efficiency or pumping horsepower.  The gain in power, reported for these, is from a reduction in drive horsepower from the crankshaft, not from an increase in displacement.  Some inlet features are added to increase air flow from interrupts that would otherwise reduce the flow from the geometric air flow.  But air flow from the geometric volume is not greater.

For ram air from a forward mounted air intake, air flow increases with greater racer speeds.  This can be considerable.  At high speed, the pressure going into the air scoop or injector hat can be significantly above atmospheric pressure.  That is multiplied by the boost ratio of the blower to the engine.  For example, assume a blown alcohol engine with 2 atmospheres of boost.  If an increase of 2 psi pressure occurs in the inlet hat from high racer speed, that can increase boost by 4 psi.  Fuel should be increased in proportion.

Drag Racing ProMods & Ram Air Example

Drag racing ProMods with Roots blowers and large inlet blower hats are popular.  Dyno horsepower is typically around 3,000.  Yet closer to 4,000 horsepower is needed to get the speeds and ETs generated by these racers.  The extra horsepower is generated by ram air as the speed increases with appropriate fuel system jetting.

Summary of Fuel System Delivery

Fuel system delivery for different blowers:

  • Fuel system delivery needed for the blower displacement would be the same for standard or high helix rotors.
  • Fuel system delivery needed for blowers with improved inlet, end port, and outlet features may be greater where these features increase air flow that overcome losses that would otherwise reduce the geometric air flow.
  • Fuel system delivery needed for ram air increases with speed; the increase in the weight of fuel for ram air is in proportion to the increase in the weight of air from ram air.

More information is in our books: Fuel Injection Racing Secrets, 5000 Horsepower on Methanol, and Blown Nitro Racing on a Budget.

Latest Racing Resumed

Sunday, March 14th, 2021

DRAGS — FUNNYCAR CHAOS 2021 (source:
EVENT #1 – March 26-27th – TEXAS MOTORPLEX – Ennis, TX
>>> air density, density altitude, grains, forecast, historic, wind speed & track map/orientation
EVENT #2 – April 23-24th – AMARILLO DRAGWAY – Amarillo, TX

— March 20, 2021; test & tune
— March 26, 2021; slicks & radials test session
— March 27-28, 2021; flashlight shootout for 6 classes
— April 3, 2021; truck invation
— April 10, 2021; IHRA Summit Super series
— April 11, 18, 2021 test & tune
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