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.

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
bobszabo is online now

Bakersfield Good Vibes March Meet Date Change

Friday, March 12th, 2021
Good Vibrations March Meet, changed to May 27-30, 2021, Fomoso, Bakersfield, Ca air density hourly forecast for tuning; historical, metric, DA, AD, water grains; site Map location…ield_Speedway/


Sunday, March 7th, 2021

Drag Racing
Gainsville Raceway, Fl; NHRA Baby Gators, Mar 4-7, ’21; NHRA Gatornationals, Mar. 11-14, ’21; AIR DENSITY for tuning, map for wind/track direction

Edinburg Motorsports Park, Edinburg, TX; IHRA Sportsman, Mar 20, ’21; AIR DENSITY for tuning, map for wind/track direction

Farmington dragway, NC; 336 Presents Lucky 13 Track, Mar 13, ’21; Test & Tune, Mar 20, ’21; IHRA Sportsman, Mar 27, ’21; Farm Points, Mar 28, ’21; Test & Tune, April 2, ’21; Annual Easter DAD DAWG 4.50 & Farm Points April 3, ’21; AIR DENSITY for tuning, map for wind/track direction

Onawa Dragway, IA; Test & Tune, April 17-18, 2021; IHRA Sportsman, Sat June 12, 2021; AIR DENSITY for tuning, map for wind/track direction

Empire Dragway, Leicester, NY; IHRA Sportsman, Sat June 19, ’21; AIR DENSITY for tuning, map for wind/track direction

Beacon Dragway, Paducah, KY; TNT, June 5, Bracket June 6, ’21, various other events …; IHRA Sportsman, Sat, July 31, ’21; AIR DENSITY for tuning, map for wind/track direction

Maryland Internat’l Raceway, Mechanicsville, MD; Test & Tune, Mar 7, ’21; various other events …; IHRA Sportsman, Sun, Aug 22, ’21; AIR DENSITY; for tuning, map for wind/track direction


Road Racing
Button Willow Raceway Button Willow, Ca; Mar 18, May 1, 2021; ’21; AIR DENSITY for tuning, map for wind/track direction, aero …, traction, rain?

Chuckwalla Valley Raceway Desert Center, Ca; Mar 20-21, ’21; AIR DENSITY for tuning, map for wind/track direction, aero …, traction, rain?

Auto Club Speedway Fontana, Ca; Speed Ventures road racing Mar 13-14, 2021; AIR DENSITY, for tuning, map for wind/track direction, aero …, traction, rain?


Formula One, Mar 26-28, 2021; AIR DENSITY for tuning, map for wind/track direction, aero …, traction, rain?



Monday, March 1st, 2021

Latest News about Hilborn from Mike Chilando & staff, Alkydigger:

“… Alkydigger has developed all the Hilborn Barrel Valves, Hoses, Nozzles, including a ½-20 Thread Sprint Car Nozzle with a drop in changeable jet to keep replacement costs down to $10 per change.    We are also going to take their (Hilborn) EFI 8 Stacker for the BB Chevy and reverse engineer it to become MFI using our barrel valve and nozzles.

We won’t let it die!  Thank you: Mike, Toni, & Shannon”






Note: Alkydigger is a stocking distributor for our fuel injection books and dealer for our ProCalc jetting & AFR calculator.


Low Cost 760-800 HP Injected Racing Engine Combo

Wednesday, February 17th, 2021

Disclaimer: All of the following engine profiles cannot be guaranteed to work adequately right out of the box.  In any of these engine profiles, jetting combinations were determined from similar running engine profiles previously analyzed with ProCalc.  Spark plug readings and engine temperature measurements are needed from incremental runs.  That is to determine if jetting adjustments may be further needed from these baseline profiles for your particular setup.  Incremental runs are necessary, first with spark plug readings (such as described in our books and other sources) and second with temperature checking during or after a run.  Appropriate adjustments to jetting or ignition advance are all needed before full power, full runs should be done.  Adequate performance without engine failures is also dependent on appropriate engine assembly and parts selection not addressed by these descriptions.  Additional qualified expert engine building / tuning skills and information is necessary.

Note: Many racing engines are available with higher horsepower levels, but look at the prices and shipping costs.  Higher powered engines are quite costly.  Shipping of complete engines can be an additional significant cost.  In addition, high powered engines on gasoline can be more expensive to run because of  more expensive $20+/gal high octane gasoline.  Here is an alternative for the budget racer, especially one who may already have access to lower cost parts.  More expensive components can be substituted especially if you already own them although changes in engine size affect the fuel injection setup.

Low Cost Injected Alcohol V-8 Description

The following is an example of a low cost engine combination for land or water racing.  It starts with a low cost injected alcohol big block V-8 racing engine.  Keys to low cost are the following:

>> factory cast iron block and square port heads such as BBC’s

>> mechanical fuel injection

>> methanol fuel.

This is a ‘high power per dollar’ engine that is based on a low cost block/cylinder head platform: big block GM, FoMoCo, or Mopar V-8; running on relatively low cost methanol fuel; with the following engine specs:

  • engine platform: 454 big block GM; 460 Ford, Ford FE bored / stroked to 440+ ci; 440 Mopar, or cast iron Early Hemi bored / stroked to 440+ ci; amount of overbore may be limited by cylinder wall thickness; (smaller displacement engines such as Ford FE or Early Hemi’s can be done, however the fuel injection jetting combinations featured below would need adjustment to smaller values; larger displacement engines such as stroker big blocks can be done also, however fuel injection jetting combos would need adjustment to larger values; our FI books and jetting calculator provide info for those changes)
  • engine RPM: not to exceed 8,000 RPM
  • displacement: 440-460
  • crankshaft: factory forging such as the GM steel crankshaft, FoMoCo steel truck/motor home crankshaft, Mopar steel truck/motor home crankshaft for 440; OEM crankshaft for early Hemi; or aftermarket low cost forging made by one of several crankshaft suppliers
  • rods: premium high strength steel
  • premium soft bearings: main & rod clearances approx. 0.003 inches for mostly OEM parts that may need this extra clearance for when they move around from more power
  • pistons: 12.5 to 1 compression ratio piston with piston-to-wall clearance as recommended by the piston manufacturer; typically 0.006 inches for forged pistons
  • steel top ring, cast iron 2nd, medium tension oil ring
  • wet sump oiling system with extra oil capacity
  • premium multi layer oil filter
  • premium multi viscosity engine oil such as 15w50; this higher viscosity is recommended for the increased bearing clearance
  • piston pins: premium steel
  • camshaft: roller tappet with 0.800 inch lift, about 275 deg intake duration, about 285 deg exhaust duration (hemi about 280 deg duration both intake & exhaust), lobe centers 106 to 109 for wedge / poly cylinder head; cam index usually about 4 deg advanced to line up the overlap centered around top-dead-center; 109-110 for hemi head; hemi cam index: straight up; run a steel camshaft core, not cast
  • premium high strength roller lifters, pushrods, rocker arms, rocker arm mounts
  • cylinder heads: hand ported cast iron BBC square port for GM engine; Max Wedge head for Mopar wedge; ported 331 or 354 head suggested for early Hemi
  • intake valves: 2.15 to 2.4 inches
  • exhaust valves: 1.75 to 2 inches
  • valve spring seat pressure: 250-300 pounds with premium valve retainers, keepers (check with camshaft supplier for more specifics)
  • ignition: 2 to 6 amp magneto: Vertex, Mallory, or Cirello
  • spark advance: approximately 36 deg for 2 amp magneto; 34 deg. for 6 amp magneto
  • spark plug gap for magneto: 0.016-0.020 inches
  • headers: individual pipes where rules permit with a small reduction in med range power; otherwise, four-into-one collector should increase mid range power with a small reduction in top end power; low restriction mufflers could be added for reduced noise although top end reduction estimate of about 50 horsepower may occur for any type of baffled mufflers
  • manifold:

>>> stack type: 2.75 dia. or more throttles, ram tube length for 5,000 to 8,000 RPM range

>>> tunnel ram with Enderle hat, Hilborn, or two RONs throttle bodies with an air scoop

>>> single plane, largest volume runners with RONs throttle body with air scoop.

This engine can be assembled for around $6,000 to $11,000 with mostly new and premium used parts.  It would be almost street-able with an automatic transmission & high stall speed torque converter.  Although it would have very high fuel consumption and not emissions compliant in many parts of the world.  The main bypass would need to be adjusted for different air densities.

Intake & Fuel System

Racing mechanical fuel injection ProCalc analysis for normally aspirated 450 ci V-8:

  • at 95% air density, about 800 HP at 7,500 RPM
  • fuel pump: approx 4.9 GPM at 4,000 pump RPM; 8,000 engine RPM; RONs, Enderle, or Hilborn -1/2 size fuel pump
  • nozzles: 0.030 inch dia; more nozzles size info in our Jetting Big Block book
  • main bypass at 0.085 inches for air density around 95%
  • torque peak at 5,000 RPM
  • volumetric efficiency at torque peak: 103%
  • air to fuel ratio at torque peak: approx. 4.8 to 1
  • fuel pressure at 5,000 launch RPM: approx. 55 psi
  • high speed bypass poppet or diaphragm valve recommended opening RPM: 5,800
  • fuel pressure at 5,800 RPM: 74 psi
  • high speed bypass poppet or diaphragm valve pressure setting: 74 psi
  • high speed bypass jet size: 0.060 inch dia
  • volumetric efficiency at horsepower peak: 90%
  • air to fuel ratio at horsepower peak: 5.2 to 1
  • horsepower peak: approx 7,500 RPM
  • fuel pressure at 7,500 RPM with high speed bypass open: 79 psi
  • fuel to the engine: 1.84 gallons per minute; 110 gal/hour; 729 #/hour (methanol) with above fuel pump and jetting combo for 95% air density
  • brake specific fuel consumption: 1.1 pounds per horsepower per hour
  • for dragster or altered weight: 1,750 pounds, 1/4 mile ET: 7.6 seconds; 1/8 mile ET: 4.86 seconds; quarter mile speed: 180 MPH; eighth mile speed: 144 MPH at 95% air density.

Same setup at lower air density; all the same except:

  • at 90% air density, about 760 HP at 7,500 RPM
  • main bypass: 0.090 inch dia
  • the high speed bypass poppet or diaphragm valve opening pressure: 66 psi to maintain a 5,800 RPM opening point
  • fuel to the engine: 1.75 gallons per minute; 105 gal/hour; 693 #/hour (methanol)
  • brake specific fuel consumption: 1.1 pounds per horsepower per hour
  • for dragster weight of 1,750 pounds, quarter mile ET: 7.7 seconds; eighth mile ET: 4.93 seconds; quarter mile speed: 177 MPH; eighth mile speed: 142 MPH at 90% air density.

Note the following recommendations for different conditions:

  • for higher air density, run a smaller main bypass to maintain the air to fuel ratio and brake specific fuel consumption for best power; however, that will raise the fuel pressure; without adjustment, that will cause the high speed bypass to open at a lower RPM; that will cause the engine go lean at the top end; engine damage could occur as a result; instead, the high speed bypass opening pressure needs to be adjusted to a higher value to maintain the same opening RPM.
  • for lower air density, run a larger main bypass to maintain the same air to fuel ratio and brake specific fuel consumption for best power; however, that will lower the fuel pressure; without adjustment, that will cause the high speed bypass to open at a higher RPM; that will cause the engine go rich at the top end; power may be way down as a result; instead, the high speed bypass opening pressure needs to be adjusted to a lower opening pressure to maintain the same opening RPM.
  • our ProCalc fuel injection jetting calculator can determine the appropriate pressure settings for different air densities and different main bypass jet sizes.
  • at high altitude race tracks, the nozzles need to be changed to smaller ones to keep fuel pressure up; with the appropriate main bypass, the same fuel curve can be maintained for any altitude; horsepower will be the best for that altitude, but it would be lower as the altitude goes up, decreasing the air density.
  • our ProCalc calculator can be used to determine the best nozzle size change for higher altitudes.
  • our ProTune and ProTune Advance services can provide further analysis and training for setting up the calculator for most specific fuel injection and engine combinations.

Ram Air Effects w/o High Speed Bypass

Add to that considerations for ram air effects at high speed.  For racecars or boats that exceed 150 MPH with forward mounted hat throttle bodies or air scoops facing the air flow, ram air will add more air into the inlet.  Jetting may need adjustment for ram air provisions such as a smaller high speed bypass for high speed operation.  More information about ram air gains is in our High HP tuning book.


More details of normally aspirated fuel injection engine combinations are in our books: Fuel Injection Racing Secrets, 5000 Horsepower on Methanol, and many others.

Adding Toluene for more octane

Saturday, February 13th, 2021

Excellent article in Alkydigger web site about adding toluene to gasoline for more octane:

The Surfers top fuel dragster team in the ’60s was known to explore toluene with nitromethane for their racing fuel.

However different fuel mixtures, especially those with nitromethane, are touchy.  They should be avoided without expert consultation!

The Alkydigger article indicates toluene has limitations as well.