Methanol has been in the news lately as a toxic chemical in hand sanitizers. While it is bad for that, it is an excellent fuel that is widely used in the racing world. It is relatively inexpensive and available worldwide. In addition, methanol does not fowl spark plugs. Increases in methanol enrichment can be done to offset detonation in high compression / boost applications. The result is more power for racing and at a reasonable cost.
By monitoring fuel volume relative to air density changes, methanol can be an easy fuel to tune. It has a wide tuning window compared to other racing fuels, giving it a comfortable margin of error. The best burn, or stoichiometric air/fuel ratio, for methanol is about 6.5 to 1. Normally aspirated racing engines use an air/fuel ratio of about 5 to 1. Racing engines on methanol are usually run on the rich side for engine response, a strong cooling influence, and good detonation resistance.
Weather considerations for methanol
For common racing fuel systems such as mechanical fuel injection as well as carburetors, fuel volume needs to be adjusted for air density changes. For mechanical fuel injection, that can be as simple as a main bypass jet change. For carburetors, that is also done with a jet change. For minor changes to air density, sometimes other changes can be done such as changing fuel pressure or air bleeds in the carburetor(s).
In EFI equipped engines in the “open loop” mode, programming changes to the fuel curve are needed to compensate for air density changes. In closed loop EFI, when sensors are properly utilized, the fuel system will automatically compensate for air density changes. However, the closed loop mode can be difficult to set up in some racing engine applications that must be run fuel rich. Extra fuel in the exhaust can interfere with sensors that monitor proper fuel system control.
Increases in air density such as from low altitude race tracks or from cool weather need an increase in fuel volume proportionate to the air density increase. If the air density goes up by 5% for example, the fuel volume needs to be increased by 5% to maintain an optimum fuel mixture. Air density tracking is a popular practice in racing, and that is why.
In general, anything that decreases the amount of oxygen in the air will also decrease engine power. When temperature goes up or humidity goes up, the amount of oxygen in the air goes down. Air density and engine power go down accordingly.
Different racing conditions necessitate different volumes of methanol. See the chart below of different engines and their recommended air/fuel ratios:
Source: 5000 Horsepower on Methanol
Tuning considerations from weather changes
When making tuning adjustments at the track, the simple rule is increase fuel when air density increases and decrease fuel when air density decreases. Knowing your engine’s optimum air/fuel ratio is the starting point. From there, maintaining that ratio with changing air densities becomes the challenge.
Here are some tuning considerations we have noted in our research.
- A common problem in methanol engines is excess enrichment. At a certain point the inlet mixture will remain too cool from the excess fuel. Liquid fuel pours through the engine with only a lean vapor combusting. This can cause an overly lean engine to backfire or burn a piston.
- Excess leanness from not enough fuel can also be misleading. If it is too lean beyond a certain point, the engine will not build heat. Spark plugs will not show any color, falsely indicating a rich condition. Maintaining a good air/fuel ratio will avoid this problem.
- For mechanical fuel injection, main bypass jet changes can be done up to a point. Main bypass changes also modify fuel pressure to the engine. For high altitude tracks, the nozzles need to be made smaller to bring up the fuel pressure. A proper combination of nozzles and main bypass are needed to maintain both the proper air/fuel ratio for power and fuel pressure for engine response. A combination of fuel injection nozzles and main bypass may provide a good air/fuel ratio but not a high enough pressure. The right combination of nozzles will maintain both.
- Ram air entering a forward facing inlet will influence the air/fuel ratio when the car is moving at high speeds. Generally the fuel needs enrichment with speed to compensate for more air entering as the car moves faster. However, forward facing air scoops that are near the hood may not get a good flow of inlet air as speed increases because of boundary layer limitations.
- Closed loop electronic fuel injection with oxygen sensors need an air-tight exhaust system, even as it gets hotter. In addition, if enrichment gets too high to retard detonation, afterburning in the exhaust will result in oxygen sensor errors and misleading fuel mixture adjustments.
- For methanol engines with exhaust temperature thermocouples, excess enrichment can cause excess heating in the exhaust from afterburning and lead to misleading readings. If the exhaust air leaks, outside air can affect sensor readings and provide incorrect information.
Methanol vs nitro… or methanol with nitro
The best stoichiometric air/fuel ratio for straight 100% nitromethane is about 1.7 to 1. That seems like a lot of fuel. However, it balances out when factoring in how much oxygen is already in the fuel. Nitro is harder to ignite, but once ignited, combustion characteristics can become unpredictable with all the oxygen present in the fuel.
As a result, nitro is usually diluted with methanol. As more methanol is added to the fuel mixture, the stoichiometric ratios go up, meaning a lower volume of fuel goes to the engine. A mixture of approximately 87% nitro and 13% methanol is reported to provide a best burn ratio in the popular nostalgia drag race classes. Because of a balanced mixture ratio, any volume of that mixture will burn completely. It will be neither oxidizing nor reducing. In this case, chemical reducing means an unburnt fuel rich condition in the exhaust.
For oxidizing mixtures, such as with 90% nitromethane or higher, an increase in the fuel volume increases detonation sensitivity. For reducing mixtures below 87%, an increase in the fuel volume decreases detonation sensitivity.
See common air/fuel ratios for different methanol/nitro mixtures below.
Source: 5000 Horsepower on Methanol
This illustrates how much different the volume of nitro methanol is for different applications. Weather or altitude changes necessitate adjustment of fuel volume in this case as well to maintain the air/fuel ratio specific to the application.
Tuning with nitro/methanol combinations can be more complicated than with straight methanol. In addition to combustion challenges, atmospheric humidity changes affect nitro powered racing engines more than straight methanol. For example, assume two racing locations coincidentally are at the same air density but one has a higher humidity. Nitromethane-powered race cars at the higher humidity location will be down on power compared to those running at the lower humidity location even though the air density is the same.
How is it possible to have the same air density at two locations with one of them at a higher humidity? Air density is affected by a combination of weather values, temperature, humidity, and air pressure. The air density effect from each is multiplied together to form the overall air density. All kinds of combinations can have similar net air density percentages. However, different combinations have different effects on power.
For all their challenges, nitro/methanol fuel mixtures have a fairly wide tuning window. Unfortunately, in open-ended competition, these engines are usually run at the limit of their capabilities to extract every ounce of power. The result is usually a lot of breakage.
Methanol vs racing gas
The best stoichiometric air/fuel ratio for gas is about 14.7 to 1, by weight. At that ratio, complete combustion occurs and there is, essentially, no excess air or excess fuel going out the exhaust. Only water and carbon dioxide are left over. While that would be a good highway air/fuel ratio for best mileage, racing engines on gasoline are usually run fuel rich. Normally aspirated engines run about 12.5 to 1 air/fuel ratio. Supercharged engines run even richer. The rich mixture produces better engine response. A bit of cooling influence and a bit of detonation resistance does occur from rich gasoline mixtures, but not much enrichment can be done without carbon fowling spark plugs.
Methanol uses a higher volume of fuel over gasoline, therefore a greater fuel volume adjustment is necessary when tuning for air density. However, because of the wider tuning window, methanol is easier than gasoline to ‘guestimate’.
Methanol vs E85
Tuning with E85 gets more complicated in comparison to methanol. Variability in ethanol to gas ratios is prevalent from many fuel suppliers. Mixture ratios of ethanol to gas varying from 70-30 to 90-10 are common. Tuning for these fluctuations is an added step when tuning for air density.
The best burn, or stoichiometric air/fuel ratio, for E85 is about 9 to 1 when it is at 85-15 mixture ratio. However, similar to methanol, racing engines using E85 are usually run on the rich side. As a result, normally aspirated racing engines using E85 run about an 8.2 to 1 air/fuel ratio. Since a racing mixture of E85 uses a lower volume of fuel over methanol, it is a popular choice in any kind of endurance racing where a lot of fuel has to be carried to finish a race.
‘Guestimate’ tune-ups for E85 are not quite as easy as methanol. E85 has a tuning window almost as wide as methanol, making tuning approximations almost as forgiving. However, the ethanol to gasoline ratio must be dealt with. Air/fuel adjustments for an 85-15 ratio will not work if the fuel is at a different ratio. Even if the exact ratio is known, the adjustments for fuel and air density changes can be complex. Taking this into consideration, extra care needs to be taken in order to ensure the best air/fuel ratio is being used for the current mixture of fuel.
Maintaining good air/fuel ratios ensures is important regardless of the type of fuel you are running. The trick is to know what that air/fuel ratio is and maintain it under changing atmospheric conditions. Good record keeping and good access to weather data make that possible.
Ref. 1 — Szabo, Bob, Blown Nitro Racing on a Budget, Szabo Publishing, 2013
Ref. 2 — Szabo, Bob, High Horsepower Tuning For Mechanical Fuel Injection, Szabo Publishing, 2015