Latest Nitro News – circa 1968

Jerry Gleason’s fuel injected Big Block drag race boat set the latest (at the time) APBA unblown fuel record at 130 MPH.  That was done on 30% nitro (Ref. HRM, April, 1968, p. 107).

Keep in mind that was with typical OEM Big Block cylinder heads and block.  It was with cam lift and timing, although radical for the time, that was a lot less than today’s setups due to valve train material limits.  It was however record setting for the parts that were available.

Straight Methanol Baseline

For Jerry’s engine on a straight methanol baseline, our Big Block Jetting manual identified the following stacker fuel injection info:

  • weight of air at 100% air density: 0.0105 pounds per revolution
  • air to fuel ratio: 5.1 to 1
  • weight of fuel for this air demand: 0.00205 pounds per revolution
  • engine nozzles: 0.034 inch diameter
  • recommended fuel pump size: 4.9 GPM @ 4,000 pump RPM
  • main bypass: 0.095 inch diameter
  • fuel split: 51% to the engine, 49% bypassed
  • conservative high speed jet size: 0.035 inch diameter.

Tuning for Weather Corrections

For 95% air density such as on a 100 F temperature day, increase the main bypass to 0.100 inch diameter to maintain the air to fuel ratio.

For 80% air density such as at a race course elevation of a few thousand feet, decrease the nozzle size to 0.032 inch diameter.  Increase the main bypass to 0.110 inch diameter.  Both changes are necessary to maintain the fuel pressure and the air to fuel ratio.


Page 38, 71, & 78 of our Big Block Jetting manual describe flatlining.  This method uses a high speed bypass jet size to flatten the fuel curve as engine speed goes above the torque peak.  Flatlining is a common setup in a normally aspirated FI engine that is highly tuned.  Using Pro-Calc, a 0.070 inch diameter high speed bypass jet size was determined to further flatten the fuel curve.  That size in an unrestrictive circuit is also similar to the fuel flowing through the restrictive path of a high speed poppet or regulator without a jet.  That flat characteristic is necessary to reduce the fuel for the reduced engine volumetric efficiency that occurs as the engine approaches the HP peak. Flatlining is common in most normally aspirated fuel injection engines that are highly tuned.

30% Nitro Addition by Weight

For a 30% nitro mixture by weight, page 92 of our Nitro book indicates a potential of a 35% increase in power from the nitro addition over a straight methanol setup.  The increase would be dependent on the proper increase in fuel to the engine. Information on that is also provided in the publication.

The engine, at that time, would make around 700 HP on straight methanol. With the nitro addition, it would make over 900 HP. Information from page 87 and Appendix 1 of the Nitro book indicate the specific increase in engine fuel weight over methanol that is necessary for that amount of nitro addition.   While the HRM article did not provide any further engine jetting details, the setup was reconstructed on our Pro-Calc’s fuel injection jetting calculator.  Typical jetting for 95% air density, common to locations where this record holding race boat competed, would be as follows:

  • air to fuel ratio entry (derived from our Nitro book info): 3.4 to 1
  • engine nozzles determination: 0.036 inch diameter
  • fuel pump revision: 7 GPM @ 4,000 pump RPM
  • main bypass determination: 0.110 inch diameter for 95% air density (80 deg. F, 40% humidity, 30 inch HG barometer)
  • high speed size would remain the same as one of the methanol setups summarized as follows:
    • 0.034 inch diameter for a conservative setup
    • 0.070 inch diameter for a setup approaching a leaner flatlining characteristic
    • no high speed jet with a restrictive pressure poppet or regulator that would further flatten the fuel curve
  • high speed poppet or regulator opening pressure would be about 56 psi to flatline just above the torque peak.

Tuning Tip

Flat lining in a nitro engine can be mostly developed and tested on lower cost straight methanol. That curve can be easily transfered to a nitro mixture setup with larger nozzles. Pro-Calc makes it easy to determine a straight methanol setup for starting out. It is then easy to add nitro in small increments with the proper jetting and fuel pump changes determined for each increment.

Development Time

The time it took to determine the methanol baseline from our Big Block Jetting manual was a few minutes.  The time to determine the nitro conversion from our Nitro manual was a few more minutes.  With this information, the time to determine nozzles, bypass jetting, fuel pump size, and fuel pressure on Pro-Calc was again only a few more minutes.  The information in our manuals and the Pro-Calc calculator make it all a matter of metrics (by the numbers).  Setups for other changes are simple as well.  Examples are:

  • a bigger or smaller engine
  • straight methanol or a mixture with different percentages of nitro
  • more or less air temperature, more or less humidity, & more or less air pressure.

With this baseline, testing would start out close with simple fine-tuning for temperature, humidity, and air pressure changes.

Note: Pro-Calc can be used to determine FI setups for blown engines, different numbers of cylinders, 2 cycle engines, rotary engines, & turbocharged engines.

30% Nitro by Volume Instead of By Weight

For 30% nitro “by volume” in Jerry’s engine, our Nitro manual indicates approximately a 45% power increase potential above straight methanol (over 1,000 HP). That requires a specific increase in fuel, also identified by our Nitro manual that is substancially different than the increase for 30% by weight.  That amount in Pro-Calc reveals a different setup, starting with larger 0.039 inch diameter nozzles.

The alternative of ‘guestimating’ instead of the metrics is much more risky.  There are too many variables that affect the setup.  Differences such as nitro ‘by weight’ or ‘by volume’ are as important to get it right as the engine size.

Pro-Calc Flexibility Saves Parts Purchase

In Pro-Calc, the nozzles can be moved below and above the baseline size. Other FI values can be determined to hold the air to fuel ratio constant. That study with necessary adjustments in the fuel pump size and jetting allows us to move the flatlining pressure anywhere between 50 and 100 psi.  That study can be done in virtual reality before buying and testing parts.

That study becomes a significant tuning guideline to parts purchase before car or boat testing.  That exercise is a great prerequesite to FI flow bench testing and dynamometer testing as well. Trial-and-error selection of nozzle size instead would cost a lot of nozzle sets.  What usually happens without the metrics is a best first guess in nozzle size that the racer is stuck with.  With the absence of metrics, your guess could put you at well below 50 psi flatlining pressure for lazy performance or too high for reduced fuel pump life.  That assumes a flatlining fuel curve can be figured out from test runs before running out of money, parts, or time.  Instead the metrics provide a good guide to the setup needed for the air to fuel ratio target necessary for the fuel mixture. Metrics also provide the ability to reach a fuel pressure target.

Air Density Correction

A 95% air density value was illustrated in this example for a straight methanol baseline.  That is the result of one combination of temperature, humidity, and air pressure.  The reference, Motorsports Standard & Atmospheric Corrections …, manual as well as our Nitro book provided information on the effects of each of the following on fuel system nozzles & jetting needs from:

  • temperature effect on air density >> jetting
  • barometer effect on air density >> jetting
  • humidity effect on air density >> jetting.

This vital reference also provides information on differing effects of each of the following on racer acceleration and top speed from:

  • temperature effect on horsepower
  • barometer effect on horsepower
  • humidity effect on horsepower.

Each of these 6 effects is different:

  • jetting is affected in one manner by air density
  • horsepower is affected in a different manner by air density.

The effect of cooling the fuel is described. The effect from inlet vaporization on HP increase is described as well. Metrics from the Motorsports … reference are used throughout our publications and Pro-Calc.

Bragging Rights

I recall, over the years, meeting many race engine builders and tuners who bragged about how they did it all themselves without any help.  Those who made it paid their dues and burned up a lot of parts and an awful lot of time.  The tools illustrated above are a huge short cut to the dues paying alternative.  Nothing is achieved burning up time from ‘doing it yourself, without any help’ when short cut tools like this are available.