Formulas and Data

Formulas and Data
*This information posted with permission from Carlos Andrade (Shark Racing)


Celsius >Fahrenheit:
((C x 9)/5)) + 32

((F – 32)/9)) x 5

Sound speed::
on air – 1700 fps
on water – 1000 fps

1 inch=25.4 mm
1 foot=12 inches=30.48 cm
1 mile=1.609 km

Boat velocity according to propeller rotation:
In miles/h
(RPM x P x S)/1056
where RPM = engine rotation, P = propeller pitch and S = slippage factor (varies from 0.8 to 0.9)
In kilometers/h
(RPM x P x S x 1.609)/1056

1 gallon = 128 oz
1 gallon = 3.78 L
1 L = 33.86 oz
29.53 cm3 = 1 oz

Gas/oil ratio:
203.16 = 6 oz + 1 gallon = Mixture 22:1
206.71 cm3 = 7 oz + 1 gallon = Mixture 19:1
25:1 = 961.5 gas + 38.5 oil
20:1 = 952.4 gas + 47.6 oil

Specific Power

Length of a tuned pipe
Lt=(Eo x Vs)/N
Lt=length, in inches
Eo=exhaust duration, in degrees
Vs=sound speed in feet/second (1700 at sea level)
N=crankshaft speed, in RPM
Header length
L=R x .01745 x Z
R=curve radius
Z=curve angle
Divergent cone:
D2=root(D1² x 6.25)
D2=cone exit diameter
D1=cone entrance diameter
6.25 =exit/entrance ratio (constant)
Convergent cone
Lr= medium point of reflection on convergent cone
Le=length of convergent cone

Exhaust duration:
Determines the speed range of the engine.
Reduce the exhaust duration to increase power at low RPM and v.versa.
More exhaust duration means higher rotation range of the engine

Carburetor diameter:
D=K x root (C x N)
D=diameter, in mm
K=constant (aprox. 0.65 to 0.9, derived from the carb diameter)
C=displacement for the cylinder, in liters
N=RPM at maximum HP

Conversion site: (converts any metric numbers to American standard and v.v.) Thanks Don Betz

Gas Mixing Ratios

Mixture ratio:

There are two ways of establishing the gas/oil proportion on the fuel mixture used on R/C boats engines: the American system and the metric system, used in Brazil.
On the American system, volumes are measured in gallons of gas and ounces (oz) of oil. On the metric system, gas and oil are measured in cubic centimeters (cc).
Besides this obvious difference, there is another, not so obvious but equally important. On the American system we calculate the volume in ounces (oz) of oil to be added to 1 gallon (128 oz) of gas. This way, a mixture of 6 oz results in 134 oz of fuel: 128 oz of gas + 6 oz of oil. Usually, this is the way we refer to this mixture: 4 oz/gallon, 6 oz/gallon, etc. In the metric system, we calculate the volume, in cubic centimeters (cc) of oil to be added to gas to reach 1 liter of fuel (gas + oil) and this mixture is indicated as a proportion: 22:1, 25:1, etc., this meaning that, for instance, in 26 parts of fuel, 25 are gas and 1 is oil.
What system is better? Is there a better system?
Of course there is a better system. IT IS THE SYSTEM YOU ARE USED TO. Use the one you get used to. Paragraph.
To make the understanding more easy, the bellow table shows the equivalence between the two systems, being understood that we rounded the figures.


American system (oz) Metric system (cc)
128 4 132 970 30 1000 33:1
128 5 133 964 36 1000 27:1
128 5.5 133.5 960 40 1000 24:1
128 6 134 957 43 1000 22:1
128 7 135 950 50 1000 19:1
128 8 136 944 56 1000 17:1


Copyright © 2000 - Carlos Andrade



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