Tuning with methanol vs nitro
The basics of engine tuning involve finding the right ratio of fuel mixed with air to get the right amount of power out of your engine. When you have a familiar engine setup and always race with methanol, that task can become tried and true. When you add percentages of nitro, things suddenly get more complex. Following are some of the differences between nitro and methanol.
Nitromethane weighs more than methanol
Nitro weighs more than methanol. As more nitromethane is added to a mixture, the fuel weight per gallon increases. For the same volume, a heavier amount of fuel mixture is needed - that is represented by a smaller numerical air/fuel ratio. This means you will need more fuel per run than you did with straight methanol.
Nitro has more oxygen in it
Nitromethane has oxygen in it. The oxygen in nitro displaces the fuel in nitro. To make up for that, more volume is needed to bring in enough fuel. This is necessary for combustion with the oxygen in the nitromethane as well as the oxygen in the air. Additionally, nitro methanol mixtures burn hotter than straight methanol. As a result, the mixture is run rich for more cooling. The increased enrichment lowers the air/fuel ratio number even more.
The combustion of oxygen in nitromethane methanol mixture with the fuel is complex. As combustion progresses, fuel dissociation occurs in multiple steps. Instantaneous intermediate chemicals are formed as combustion goes from preliminary ignition to the final, flame spewing fire out the exhaust.
Air fuel ratios for nitro methanol mixtures
A maxed out, blown alcohol engine runs an air/fuel ratio around 3.1 to 1. That is 3.1 pounds of air to one pound of methanol. In contrast, a maxed out, blown top fuel engine running 90% nitromethane methanol runs an air/fuel ratio around 1 to 1. That is one pound of air to one pound of fuel. The number of pounds of air going through both engines is about the same. However, the number of pounds of fuel in the blown top fuel engine are much greater.
A real world example
In our low compression blown Hemi, a good AFR for racing methanol was 3.8 to 1. We used ProCalc to determine a 0.130 inch main bypass for that AFR with the jetting, fuel pump, and weather combination that we were running in a recent test program. That AFR produced a very good spark plug color. When we changed to 6% nitro, we changed the AFR to 3.3 to 1. A smaller 0.115 inch diameter main bypass jet in the barrel valve was determined in ProCalc for 6% nitro by weight. The first test run with that change netted a power increase close to 10%. The quarter mile speed went up 8 mph. The quarter mile ET went down 0.2 seconds. The difference in air/fuel ratio was a result of the heavier nitro with more oxygen in the nitro molecule.
Weather has many effects on engine tuning
Top fuel tuners have a data base of previous runs in similar conditions that initiate the approximate tune-up for a subsequent run. One of those inputs is the weather. The effects of weather on dissociation during combustion can be significant. Teams with more extensive databases have an advantage to selecting a tune-up. Even at that, major engine failures are often unavoidable from a slight change in conditions such as weather that affect combustion.
Tracking changes in weather become very important when dealing with nitro methanol mixtures. Air density changes have complex effects on a methanol mixture. Temperature, humidity, and pressure each have separate effects on nitromethane methanol mixtures. ProCalc can make tuning with nitromethane and methanol a simpler task.