More Info on Pro-Calcs

Pro-Calcs, our on-line FI jetting calculator, was expanded for nitro methanol fuel mixtures.  The % of nitro mixture with methanol can be added.  The nitro mixture % can be entered by weight or by volumePro-Calcs will determine the correct amount for the air to fuel ratio.  Our Nitro book explains the difference between the two mixing ratios.  That knowledge is vital to running nitro, especially low percentages.

EXAMPLE: To illustrate a nitro addition, a blown Big Block racing engine is analyzed.  This engine on methanol makes 1,300 HP with the following setup:

  • new 6-71 blower
  • 40% blower overdrive
  • 8 hat nozzles @ 0.033 in. dia.
  • 8 port nozzles @ 0.033 in. dia.
  • 0.080 in main bypass.
  • 7 GPM fuel pump.

Note: Other values such as spark plug advance and compression ratio are not specified above. Those values will affect the outcome. However, that is beyond the scope of this example. Examples of those values and their effects are provided in our Nitro book however.

The addition of 10% nitro by weight is made.  Our Nitro book indicates a bump to over 1,450 HP (at the same engine speed) as long as a correct increase in fuel amount is made.  Since nozzle changes can upset fuel split and fuel distribution in a proven tune-up, an effective goal is to use the same nozzles if possible.


With the same nozzle layout, a reduction in only the main bypass jet would be a reasonable choice to bump the fuel.  Without knowing the amount of fuel to be bumped and the main bypass jet size reduction to reach that bump, the choice is to reduce the 0.080 in. MBP size.  The reduction may be somewhere from 0.075 in. down to 0.040 in.  Test runs at various main bypass jet sizes would be necessary to determine the necessary reduction.


We examined this setup using info in our Nitro book and our calculator, Pro-Calcs.

EXAMPLE — reduce MBP size (although not enough):

First, the necessary change in air to fuel ratio was determined from our Nitro book.  That value was the target value.  Then, using the same nozzle layout in Pro-Calcs, a simple reduction in the main bypass jet size was tried out.  Pro-Calcs indicated the fuel pressure increase with the main bypass jet reduction.  The main bypass size of 0.080 in. dia. for straight methanol fuel was reduced to several smaller sizes, one at a time, such as those that would be tried out in the alternative, trial and error, guestimate testing (without our book & calculator).  Those sizes were 0.075, 0.070, and then 0.065.  None of those changes put enough fuel to the engine to get to the target air to fuel ratio necessary for the 10% nitro addition.  If any of them were tried out in the engine as an alternative trial & error guestimate, it is likely that engine damage would occur.

EXAMPLE — reduce MBP size (enough):

A value of 0.060 in. dia. in Pro-Calcs was determined to reach the target air to fuel ratio.  Fuel pressure went from a little over 100 psi for the methanol setup to a little over 130 psi for the low percentage of nitro.  That was considered still reasonable for fuel pump life.

EXAMPLE — reduce MBP size (too much):

A further reduction in the main bypass size was checked out to investigate smaller main bypass jets.  Those sizes could also be what may be tried out in alternative, trial and error, guestimate testing.  Values of 0.055, 0.050, & 040 were tried out.  The air to fuel ratio was too rich.  In addition, fuel pressure skyrocketed to levels that would damage the fuel pump.

EXAMPLE — changing nozzle sizes in addition to the main bypass size:

Different hat nozzle sizes were tried out in Pro-Calcs.  Target air to fuel ratio and fuel pressure values could be reached with several combinations.  Unfortunately, most other combinations that would be guestimated were off in either the air to fuel ratio, the fuel pressure, or both.  It was apparent that the alternative trial & error guestimated nozzle and jet sizes without these computations was a shot in the dark since there are way more damaging combinations than good ones.  It is way more likely to guestimate a damaging combination before accidentally finding the right combination.  With nitro, broken parts more often become the indicator when you are wrong.


To illustrate, there are two ratios that are necessary to manage: nitro-to-methanol mixture ratio by weight and the adjusted-air-to-fuel ratio that is absolutely necessary for that mixture ratio.  There is a specific relationship between both of those ratios. Those relationships are identified in our Nitro book.  Pro-Calcs provides computations to manage those relationships quite easily with fuel pump, engine nozzle, and bypass jet sizes.  It is set up to handle both normally aspirated and blown engine configurations.  Pro-Calcs can handle one to 8 cylinders, up to 4 nozzles per cylinder, and most any temp., humidity, & pressure combination.

In addition, if you are mixing by volume, you can enter that percentage in Pro-Calcs, and it will determine the correct air to fuel ratio by weight.  It took me about 5 minutes on Pro-Calcs to adjust to the proper jet size in the previous example.  That new bypass jet size produced both:

  • the right air to fuel ratio indicated by the Nitro book for the fuel addition
  • a reasonable fuel pressure.