For example, my honda has a compression ration of 9.6:1 so its best that I use Octane 93 Petrol?
Not really, because the table shows that I should use Octane 95 but that was based on tests done in temperate climates, where ambient air temperature is around 25 ° (say). In Sri Lanka, where ambient air temperature reaches as high as 35°C, a correction has to be made.
With an increase of 10°C in charge air (air taken in to the engine cylinder), the recommended octane number should be 1 number lower (or 0.5 for every 5°C).
The radiator water temperature will also rise by about 10°C and therefore the recommended octane number should be 2 number lower (1-2 for every 10°C).
So I should be using petrol having an Octane number of 90
What is Octane Rating
The octane number is equal to the numerical value of the percentage by volume of iso-octane, C8H18 (2,2,4-trimethylpentane, an isomer of octane), in a mixture of iso-octane and n-heptane, C7H16, having the same knock characteristics as the fuel being tested.
"An 87-octane gasoline, for example, has the same octane rating as a mixture of 87 vol-% isooctane and 13 vol-% n-heptane. This does not mean, however, that the gasoline actually should contain these chemicals in these proportions. It simply means that it has the same autoignition resistance as the described mixture."
For more info on octane rating read this wikipedia article
Another factor is activation energy (the energy barrier to start a chemical reaction). Fuels with higher octane ratings have higher activation energies. So if you have installed any fuel saver upgrades, that is something you should watch out for. Because fuel saver upgrades usually try to bring the activation energy down. This allows better combustion at higher engine speeds - so they claim.
:)
The following table gives the octane ratings of various fuels (based on tests conducted in temperate climates):
| Compression Ratio | Octane Number |
| 8:1 | 92 |
| 9:1 | 96 |
| 10:1 | 100 |
| 11:1 | 104 |
| 12:1 | 108 |
sources: 1 | 2
Running a fuel with a lower octane number than recommended retards fuel ecconomy because combustion efficiency will drop.
| Fuel | Research Octane Number (RON) | Heating Value (MJ/liter) | Cost (Rs/liter) | Specific Energy Cost (Rs/MJ) |
| Petrol | 90 | 32.0 | 93 | 2.90 |
| Petrol | 95 | 32.0 | 95 | 2.97 |
| LPG | 115 | 22.16 | 48 | 2.17 |
If you are thinking of converting your car engine to run on LPG, try to increase the compression ratio by milling the engine head and using a thin gasket . Or get a new ECU with modified ignition and injection timings. Or turbo-charge the engine so that the compression pressure matches the fuel.
