Octane ratings from the beach to the mountaintop
If the octane rating is substantially lower than the compression, the performance of the vehicle could be affected.
What you need to know:
- Irrespective of octane ratings, engines do also lose power at higher altitudes.
- Kenya’s fuel supply target is an octane rating of 93, in most places most of the time.
Does altitude have any effect on the grade of fuel your car likes or needs? - Clementina.
Short answer: Yes. Your engine will prefer (and might need) a slightly higher octane rating at the coast than in Nairobi or Timau. But most cars will work well enough with the same fuel at both low and high altitude.
Irrespective of octane ratings, engines do also lose power at higher altitudes.
Longer answer: The octane rating your car needs is determined first of all by its engine’s compression ratio. You can look that up in your particular car’s handbook. It is likely to be somewhere between 8.5 and 10.0 in most petrol engines.
As a crude rule of thumb, at sea level (say, 0 to 1,000 metres) the optimal octane rating should be the same number… without the decimal point. Compression 9.5. Octane rating 95.
Kenya’s fuel supply target is an octane rating of 93, in most places most of the time. This can vary slightly at source, or during transit and storage, but is estimated to always be somewhere between 91 and 95. Engines have quite a lot of tolerance for such variations, and any difference is unlikely to be noticeable on most cars.
The global target is a minimum of 91. Most cars can run adequately (though not optimally) on that. Many countries offer a range of options in petrol stations, with the lowest price at 91 and the most expensive at 97 or 99. Avgas (an aviation petrol for light aircraft) is about 101 and is also warranted by a few very high performance sports cars.
Compression ratios
So if your compression ratio is less than 9.3, relax. If the octane rating is higher than the compression ratio, it will do no harm and not affect performance (up or down). It will simply be more expensive than strictly necessary for your engine to perform properly.
If the octane rating is substantially lower than the compression, performance could be affected…in both vroom-vroom and fuel economy. In extreme cases the engine might start to “knock” and acceleration will be muted or erratic (splutter or lurching). If there is no sign of that, keep on keeping on with peace of mind.
For a lot of complex atmospheric and techno-chemical reasons, at altitudes well above sea level your engine can run well on a slightly lower octane rating, because the air is “thinner” – less dense – which, though the actual compression “ratio” remains unchanged, has the same effect as a slightly lower compression ratio.
The octane rating your car needs is determined first of all by its engine’s compression ratio.
It will also lose some power, no matter what fuel you use. The rule-of-thumb is about 10% for every 1,000 metres of climb. Nairobi is about 1,700 metres above Mombasa.
The hilltops on either side of the Rift Valley are between 2,000 and 3,000 metres. At above 4,000 metres, your engine power will be seriously reduced and even a gentle slope will be a struggle.
At 5,000 metres it might still run, but with only just enough power to peel a banana. That’s why light aircraft start with very high compression ratios and octane ratings…and have limited operational “ceilings”. That’s the layman’s version, anyway.
Progressive loss of power
Remember, too, that not all of an engine’s power output reaches the wheels. Up to 20 per cent is absorbed by the power train between the engine and the wheels, , and charging the battery (running all the electricals), and other things. Air conditioners impose a major draw.
Bottom line, if you drive from Mombasa to Eldoret a progressive loss of power is inevitable, not a fuel quality problem. Carry on. If you drive the other way, power will progressively increase, but again the same fuel will do the job.
On single trips between the coast and the highlands, most engines will have plenty of built-in tolerance to cope with these changes.
But if you are moving from long-term residence in the highlands to long-term residence at the coast (or vice versa) it is worth having your ignition timing adjusted.
This doesn’t remove the effect of altitude change, but helps compensate for it. “Timing” is a precise but not an absolute setting. It can be “advanced” or “retarded” (fractionally) to optimise performance for different conditions and usage patterns.
Too advanced can cause overheating, and too retarded disrupts combustion and wastes unburnt fuel. In advanced engine management systems, some have computerised sensors and control units that can make the adjustments automatically.
