With all the "Buzz" on the new Sunoco fuel I thought this might be a good topic. Two important parts of comparing fuels, aside from having sufficient octane/detonation and preignition resistance, are the Reid Vapor Pressure and the vaporization curve of the fuel. Most fuel companies post this info, and is worth checking out anytime you make a fuel change. First is a quote I found that describes Reid Vapor Pressure.
"Reid Vapor Pressure is the vapor pressure of a chilled sample of gasoline or other fuel as measured in a test bomb at 100 F. The Reid Vapor Pressure differs from the True Vapor Pressure of the sample due to sample vaporization and the presence of water vapor and air in the confined space resulting from the Reid Vapor Pressure test method. To presaturate the sample for the Reid Vapor Pressure test, the streams are flashed at 1 atm and 33 F, and the resulting liquid product is then combined with air at the rate of 4 parts air and 1 part liquid. Next the system is flashed at constant volume at 100 F. The resulting gauge pressure is the Reid Vapor Pressure.
This is partly responsible by the light hydrocarbons used when the fuel is mixed at the factory. This is one of the most important parts on how a fuel performs and how easy it starts and runs cold. With all the carbs I test I can always tell when I am using a fuel that vaporizes well, it will alway have better idle characteristics on a race engine.
The second part is the vaporization or distillation curve. It is measured by what percentage of fuel is vaporized at a given temps. You will see some indication in the RVP on how good the vaporization curve is, the higher the number the better the curve, but there are some fuels that are skewed a little. Some may have some good early vaporization curve that gives it a good RVP, but have some different compounds mixed in like Toluene for example that take higher temps to vaporize.
Below are three VP fuels relatively close in octane specs, but looking at them will show you some different values to compare.
C14
(TYPICAL VALUES)
Specific Gravity: 0.693 @ 60°F/15.6°C Color: Yellow
MON: 114
RON:119
R+M/2: 116.5
RVP: 5.16 psi/107.6 kpa Oxidation Stability (min.) 1440+
Distillation:
10% evap @ 168.1°F/75.6°C
50% evap @ 210°F/98.9°C
90% evap @ 217.2°F/102.9°C
E.P. @240.7°F/115.9°C
X16
(TYPICAL VALUES)
Specific Gravity: 0.713 @ 60°F/15.6°C Color: Red
MON: 114
RON: 118
R+M/2: 116
RVP: 6psi/41.4kpa
Oxidation Stability (min.) 1440+
Distillation:
10% evap @ 155.7°F/68.7°C
50% evap @ 219.4°F/104.1°C
90% evap @ 254.3°F/123.5°C
E.P. @ 384°F/195.6°C
C25
(TYPICAL VALUES)
Specific Gravity: 0.698@ 60°F/15.6°C Color: Yellow
MON: 113
RON:120+
R+M/2: 118
RVP: 9.66 psi/ 66.6 kpa Oxidation Stability (min.) 1440+
Distillation:
10% evap @ 120.8°F/49.3°C
50% evap @ 162.5°F/72.5°C
90% evap @ 207.2°F/97.3°C
E.P. @ 231.6°F/110.9°C
Now this is SR18, what you now have to run.
Octane (R+M)/2 118
Research Octane 120
Motor Octane 116
Specific Gravity 0.704
Weight (lbs/gallon) 5.9
Reid Vapor Pressure 6
Initial Boiling Point 99
10% Evaporation 166
50% Evaporation 209
90% Evaporation 213
Final Boiling Point 223
"Reid Vapor Pressure is the vapor pressure of a chilled sample of gasoline or other fuel as measured in a test bomb at 100 F. The Reid Vapor Pressure differs from the True Vapor Pressure of the sample due to sample vaporization and the presence of water vapor and air in the confined space resulting from the Reid Vapor Pressure test method. To presaturate the sample for the Reid Vapor Pressure test, the streams are flashed at 1 atm and 33 F, and the resulting liquid product is then combined with air at the rate of 4 parts air and 1 part liquid. Next the system is flashed at constant volume at 100 F. The resulting gauge pressure is the Reid Vapor Pressure.
This is partly responsible by the light hydrocarbons used when the fuel is mixed at the factory. This is one of the most important parts on how a fuel performs and how easy it starts and runs cold. With all the carbs I test I can always tell when I am using a fuel that vaporizes well, it will alway have better idle characteristics on a race engine.
The second part is the vaporization or distillation curve. It is measured by what percentage of fuel is vaporized at a given temps. You will see some indication in the RVP on how good the vaporization curve is, the higher the number the better the curve, but there are some fuels that are skewed a little. Some may have some good early vaporization curve that gives it a good RVP, but have some different compounds mixed in like Toluene for example that take higher temps to vaporize.
Below are three VP fuels relatively close in octane specs, but looking at them will show you some different values to compare.
C14
(TYPICAL VALUES)
Specific Gravity: 0.693 @ 60°F/15.6°C Color: Yellow
MON: 114
RON:119
R+M/2: 116.5
RVP: 5.16 psi/107.6 kpa Oxidation Stability (min.) 1440+
Distillation:
10% evap @ 168.1°F/75.6°C
50% evap @ 210°F/98.9°C
90% evap @ 217.2°F/102.9°C
E.P. @240.7°F/115.9°C
X16
(TYPICAL VALUES)
Specific Gravity: 0.713 @ 60°F/15.6°C Color: Red
MON: 114
RON: 118
R+M/2: 116
RVP: 6psi/41.4kpa
Oxidation Stability (min.) 1440+
Distillation:
10% evap @ 155.7°F/68.7°C
50% evap @ 219.4°F/104.1°C
90% evap @ 254.3°F/123.5°C
E.P. @ 384°F/195.6°C
C25
(TYPICAL VALUES)
Specific Gravity: 0.698@ 60°F/15.6°C Color: Yellow
MON: 113
RON:120+
R+M/2: 118
RVP: 9.66 psi/ 66.6 kpa Oxidation Stability (min.) 1440+
Distillation:
10% evap @ 120.8°F/49.3°C
50% evap @ 162.5°F/72.5°C
90% evap @ 207.2°F/97.3°C
E.P. @ 231.6°F/110.9°C
Now this is SR18, what you now have to run.
Octane (R+M)/2 118
Research Octane 120
Motor Octane 116
Specific Gravity 0.704
Weight (lbs/gallon) 5.9
Reid Vapor Pressure 6
Initial Boiling Point 99
10% Evaporation 166
50% Evaporation 209
90% Evaporation 213
Final Boiling Point 223