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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