SPECIFIC GRAVITY
Specific Gravity is the comparison of the weight of gasoline in relation to the weight of water. Water is given a value of 1 and most leaded racing gasolines have a specific gravity of .740 or lower, while unleaded gasolines will generally read above .750. This is important because the specific gravity of a gasoline affects the mass of gasoline that can pass through a carburetor jet orifice. A low density gasoline has less energy potential than a equal amount of higher density gasoline. This means that a gasoline with a higher specific gravity will produce more energy when burned, since more energy producing hydrocarbons are introduced into the engine.
FUEL / MIXTURE
Another variable that affects the performance and octane requirements of your racing engine is humidity and barometric pressure. Low humidity or dry air will lean out the mixture, requiring a higher octane gasoline to prevent detonation. While high humidity will lower the octane needed to prevent detonation because the added moisture in the inlet air will cool the mixture making it more dense or richer. The ideal air/gasoline ratio at sea level is 15:1 at 60°F - 50% humidity and a barometer reading of 29.92 inches of Hg. This is referred to as the Stoichiometric air-fuel ratio which is the exact ratio required to completely combust a fuel to water and carbon dioxide. Fluids are directly affected by pressure and temperature, and air is a fluid just like gasoline. Therefore temperature, humidity, and barometric pressure all influence octane requirements.
OCTANE
This is probably the most misunderstood aspect of gasoline. A higher octane rating does not alone mean more power. A gasoline with a higher octane rating will produce less BTU¹s when burned than a lower octane gasoline. Higher octane ratings allow racers to run higher compression ratios in their engines without detonation or pre-ignition to create more heat energy, which means more horsepower. Octane is the numerical measurement of a gasolines ability to resist detonation and pre-ignition. Octane in racing gasoline is most easily raised by using Tetraethyl or Tetramethyl lead.
The federal government has placed a limit of 4.23 grams of lead per gallon. Any increase in lead would not raise the octane level by a significant amount, since the octane ceiling of gasoline 120. Lead substitutes such as Toluene and Xylene are now being used more often for environmental reasons with exceptional results. In unleaded gasoline octane is obtained through extensive refining but this reduces the yield and increases costs.
Detonation is described as "knock" or "ping" and is the reaction or explosion of the heavy ends of the fuel/air mixture in the combustion chamber after the plug has fired. Pre-ignition is caused when the fuel is ignited by a "hot spot" in the cylinder and the fuel/air mixture burns before the plug fires and the piston reaches TDC. This created tremendous pressures in the combustion chamber due to the burning gases being compressed further by the compression cycle of the engine. These are both very destructive situations and if left unchecked, will result in extensive engine damage.
Octane is rated in two different methods, Research and Motor Octane and often they are combined and averaged for a third rating we are all familiar with from the gas pump, the (R+M)/2 method. Octane is measured using a CFR (Cooperative Fuel Research) engine. This is a single-cylinder, overhead valve, variable compression engine.
Research Octane is measured on the CFR engine running at 600 RPM with the inlet air temperature at 100°F. A baseline gasoline with a known octane value is used to reference the octane level of the gasoline being tested. With the engine running on the test gasoline, the compression ratio of the CFR engine is gradually increased by decreasing the length of the cylinder. This is continued until the point of detonation and then decreased until detonation ceases. Then the octane value of the test gasoline is determined based on the compression ratio of the CFR engine just before detonation.
Motor Octane is tested in a similar fashion on the CFR engine with a few exceptions. The RPM¹s are increased to 900 and the inlet air temperature is raised to 300°F. Under the severe conditions of racing, the MON (Motor Octane Number) is more important than the RON (Research Octane Number).
The MON is a good indicator to high horsepower, high RPM performance and the gasoline¹s ability to resist detonation. RON on the other hand is of more value in lower RPM, torque applications and in eliminating pre-ignition. The RON values are generally 8 to 10 numbers higher than the MON, due to the higher engine inlet air temperatures of the MON test. There are unlimited ways to arrive at the pump octane or (R+M)/2 method, due to the averaging of the RON and MON values. Because of this, your engine may run well on one brand of 100 octane (R+M)/2 and poorly on a different brand of 100 octane (R+M)/2. The key is to match the compression ratio of your engine to the octane level of racing gasoline.
FYI
We run Sunoco Purple 110 mixed with 1.4 gallons of AMOCO 92 Pump Gasoline. It gives a good even burn with a bit more low end power in the CR 500 Motor. One might experiment on their own a little with the mix. Some folks use 100 Oct Aviation Fuel mixed with pump automotive gasoline. Not much on this program myself. As with any “Low Lead or No Lead” Fuel, They tend to not have the right anount of protection built in. Lead has a great lubrication property. Factory Racing fuels are formulated with the maximum amount of lead for performance, protection and engine part failure prevention aditives. NOT SUBSTITUTES.
Also I wanted to mention something about the Briggs fuel delivery system. When filling your Briggs tank which is mounted to the engine (Stock setup) dont fill it to the brim. Fill it to the ½ way mark instead. It seems to block off the vent when you go into the turns making the engine starve a little when you are in the middle to out point of a turn.
| Octane Mixing Chart | |||||||||||
| Gallons of 113 Octane Racing Gasoline | |||||||||||
 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
|
|
1 |
102.5 | 106.00 | 107.75 | 108.80 | 109.50 | 110.00 | 110.38 | 110.67 | 110.90 | 111.19 | |
|
2 |
99.00 | 102.50 | 104.60 | 106.00 | 107.00 | 107.75 | 108.33 | 108.80 | 109.18 | 109.50 | |
|
3 |
97.25 | 100.40 | 102.50 | 104.00 | 105.13 | 106.00 | 106.70 | 107.27 | 107.75 | 108.15 | |
|
4 |
96.20 | 99.00 | 101.00 | 102.50 | 103.67 | 104.60 | 105.36 | 106.00 | 106.54 | 107.00 | |
|
5 |
95.50 | 98.00 | 99.88 | 101.33 | 102.50 | 103.45 | 104.25 | 104.92 | 105.50 | 106.00 | |
|
6 |
95.00 | 97.25 | 99.00 | 100.40 | 101.55 | 102.50 | 103.31 | 104.00 | 104.60 | 105.13 | |
|
7 |
94.63 | 96.67 | 98.30 | 99.64 | 100.75 | 101.69 | 102.50 | 103.20 | 103.81 | 104.36 | |
|
8 |
94.33 | 96.20 | 97.73 | 99.00 | 100.08 | 101.00 | 101.80 | 102.50 | 103.12 | 103.67 | |
|
9 |
94.10 | 95.82 | 97.25 | 98.46 | 99.50 | 100.40 | 101.19 | 101.88 | 102.50 | 103.05 | |
|
10 |
93.91 | 95.50 | 96.82 | 98.00 | 99.00 | 99.88 | 100.65 | 101.33 | 101.95 | 102.50 | |
| Octane Mixing Chart | |||||||||||
| Gallons of 110 Octane Racing Gasoline | |||||||||||
 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
|
| 1 |
101.0 |
102.3 |
103.0 |
103.4 |
103.7 |
103.9 |
104.0 |
104.1 |
104.2 |
104.3 |
|
| 2 |
98.01 |
99.31 |
100.11 |
100.61 |
101.01 |
101.31 |
101.51 |
101.81 |
102.01 |
102.11 |
|
| 3 |
96.5 |
97.7 |
98.5 |
99.1 |
99.5 |
99.8 |
100.10 |
100.30 |
100.50 |
101.30 |
|
| 4 |
95.6 |
96.7 |
97.4 |
98.0 |
98.4 |
98.8 |
99.1 |
99.3 |
99.5 |
99.7 |
|
| 5 |
94.99 |
95.99 |
96.69 |
97.29 |
97.69 |
98.09 |
98.39 |
98.59 |
98.79 |
98.99 |
|
| 6 |
94.56 |
95.46 |
96.16 |
96.66 |
97.06 |
97.46 |
97.76 |
98.06 |
98.26 |
98.46 |
|
| 7 |
94.25 |
95.05 |
95.65 |
96.15 |
96.55 |
96.95 |
97.25 |
97.55 |
97.75 |
97.95 |
|
| 8 |
94.00 |
94.7 |
95.3 |
95.8 |
96.1 |
96.5 |
96.8 |
97.1 |
97.3 |
97.5 |
|
| 9 |
93.8 |
94.5 |
95.00 |
95.5 |
95.9 |
96.2 |
96.5 |
96.8 |
97.0 |
97.2 |
|
| 10 |
93.63 |
94.23 |
94.73 |
95.23 |
95.63 |
95.93 |
96.23 |
96.53 |
96.73 |
96.93 |
|
| Octane Mixing Chart | |||||||||||
| Gallons of 100 Octane Racing Gasoline | |||||||||||
 |
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | |
| 1 |
96.0 |
97.3 |
98.0 |
98.4 |
98.7 |
98.9 |
99.0 |
99.1 |
99.2 |
99.3 |
|
| 2 |
94.7 |
96.0 |
96.8 |
97.3 |
97.7 |
98.0 |
98.2 |
98.5 |
98.7 |
98.8 |
|
| 3 |
94.0 |
95.2 |
96.0 |
96.6 |
97.0 |
97.3 |
97.6 |
97.8 |
98.0 |
98.2 |
|
| 4 |
93.6 |
94.7 |
95.4 |
96.0 |
96.4 |
96.8 |
97.1 |
97.3 |
97.5 |
97.7 |
|
| 5 |
93.3 |
94.3 |
95.0 |
95.6 |
96.0 |
96.4 |
96.7 |
96.9 |
97.1 |
97.3 |
|
| 6 |
93.1 |
94.0 |
94.7 |
95.2 |
95.6 |
96.0 |
96.3 |
96.6 |
96.8 |
97.0 |
|
| 7 |
93.0 |
93.8 |
94.4 |
94.9 |
95.3 |
95.7 |
96.0 |
96.3 |
96.5 |
96.7 |
|
| 8 |
92.9 |
93.6 |
94.2 |
94.7 |
95.1 |
95.4 |
95.7 |
96.0 |
96.2 |
96.4 |
|
| 9 |
92.8 |
93.5 |
94.0 |
94.5 |
94.9 |
95.2 |
95.5 |
95.8 |
96.0 |
96.2 |
|
| 10 |
92.7 |
93.3 |
93.8 |
94.3 |
94.7 |
95.0 |
95.3 |
95.6 |
95.8 |
96.0 |
|
| Recommended Octane Chart | ||||||||||||||
| (R+M)/2 Octane Recommendations | ||||||||||||||
 |
89 |
91 |
93 |
95 |
97 |
99 |
101 |
103 |
105 |
107 |
109 |
110 |
113 |
|
|
13.1 |
X | X | ||||||||||||
|
12.1 |
X | X | X | Â | ||||||||||
|
11.1 |
X | X | X | X | Â | Â | Â | |||||||
|
10.1 |
X | X | X | |||||||||||
|
9.1 |
X | X | X | |||||||||||
|
8.1 |
X | X | ||||||||||||
|
7.1 |
X | X | ||||||||||||
