Model Airplane News - RC Airplane News | CHANGING THE COMPOSITION OF FUEL

By Dave Gierke

Occasionally, you may want to change the component percentage of an existing fuel, such as increasing its castor oil content. This would be advantageous for the break-in and normal operation of certain engine types (e.g., ringless iron/steel pistons/cylinders; plain-bearing crankshaft support). What are the requirements? You must know the existing oil percentage, and the quantity of the fuel to be changed.



If 1 gallon (128 ounces) of the existing fuel blend contains 18% castor oil and you want to increase it to 22%, the oil content needs to be increased by 4%. \Here’s a formula that tells you exactly how much castor oil to add:


                            (F – I) x A
Ounces To Add = ————
                             100 – F

F is the final percentage of oil desired
I is the initial percentage of oil already in the fuel
A is the number of ounces you are treating.


If you have 1 gallon (128 oz.) of 18% synthetic oil fuel, and you
want to add castor oil to bring it up to 22%, then find the following:
F = 22; I = 18; F – I = 4
In the numerator portion of the formula, because there are 128 oz. in a U.S. gallon, multiply 4 x 128 = 512
In the denominator portion of the formula, 100 – 22 =78
Finally, —— = 6.6 oz. (195.2 ml)


There might not be enough room in a gallon can to accept the 6.6 ounces of additional castor oil. You may have to mix everything between two 1-gallon jugs. You can also use this formula for increasing a fuel’s nitromethane or methanol content. Of course, when you increase the percentage of these chemicals, the total volume of the fuel may change significantly; so recalculate the lubricant percentage based on the new total volume and modify as necessary.



Many technically minded modelers may want to consult my book, “Power: Beyond the Basics (available at the for additional information and techniques concerning glow fuel; some of the topics addressed include:

·         Determining an existing fuel’s lubrication content

·         Mixing your own fuel (getting the chemicals, obtaining the hardware, calculating the volumes, transferring the fuel blend, fuel use tips)

Updated: November 9, 2017 — 9:25 AM
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