Octane differences
Petrol in the USA and UK have different octane ratings. My car has been remapped alongside a 3.8 engine build to run on Shell V-Power which has an octane rating of 99 RON, which is pretty much as high an octane available in the UK. Standard unleaded fuel is around 95 RON and ‘super’ unleaded around 97–98 RON.
As part of the preparation for driving my car in the USA, I started to become concerned about the availability of high octane fuel in the States, especially in more remote areas where I would be covering many hundreds of miles.
At first glance, all American fuel appears to have a far lower octane rating with the choices typically being 87, 89 and 91. I suspect if I were use such low grade fuel in the 993 I would burn an exhaust valve by the end of the trip. However the differences are not as much as first appears.
The Americans, like with so many things, use a different system. The Research Octane Number (RON), used in Europe and most of the world is replaced with a Motor Octane Number (MON) in the USA. They look similar but are in fact two different tests, with MON testing carried out under more difficult conditions, MON number is always lower than RON, but the octane broadly similar between regular and super.
It is not that simple. The US gasoline stations use a reference called Pump Octane Number (PON). PON is the average between RON and MON (RON+MON/2=PON). To make it more confusing, PON is sometimes referred to as AKI which stands for Anti Knock Index. The difference between PON and AKI is just the labeling system.
RON versus MON conversion table
If a fuel is marked 95 RON it will be around 87 MON which would then average out at 91 PON and could also be marked 91 PUMP at the gas station. The table below is an approximate guide.
| RON (UK and EU) | MON (US test) | PON (US pump) |
| 90 | 83 | 86.5 |
| 92 | 85 | 88.5 |
| 95 | 87 | 91 |
| 96 | 88 | 92 |
| 98 | 90 | 94 |
| 100 | 91.5 | 95.75 |
Having researched the fuels and taken the time to understand the differences it is less of a worry as shown in the table above. I now realise the low number I’d always seen marked on the pumps at gas stations in the States was misleading (to me) and the fuel is roughly the same octane as the ‘regular’ and ‘super’ I am used to in the UK.
What does this mean for the Porsche 993?
UK and EU Porsche 993s left the factory mapped to run on 97 RON. My car has been rebuilt from 3.6 litres to 3.8 litres by Tech 9 Motorsport in Liverpool and remapped under their guidance by Wayne Schofield of Chips Wizard to run specifically on Shell V-Power – the most widely available high octane fuel in the UK at 99 RON. And 99 RON, which equates to 95 PON is not something that will not be easy to come by everywhere in the States. Shell gas stations are commonplace on the Interstates and Freeways but will it be easy to find in the Badlands of South Dakota, or the few hundred miles between Marfa and Comstock in the Texas desert?
Going by experience of previous USA road trips, 91 PON may be the highest available gasoline. Taking photographs of gas pumps is not a pastime of mine but I did remember taking a photo at Uranus Gas, an alien themed gas station in the Arizona desert where you can also shoot guns and eat burgers. They also sell the most expensive Magnum ice cream on the planet at around $10 each. looking back through my iPhotos I could just make out 91 PON being the highest. Hardly rocket fuel.
In reality, the car should run fine on Super in the States as the live engine map has a some tolerance programmed into it. Not ideal, but not something that will cause damage to the engine. If the retardation becomes obvious anywhere there may be a need to intervene.
Octane booster
The solution will be to add an octane booster to each tank of lower grade fuel should engine spark knock or pinging become evident. Both the Porsche 964 and Porsche 993 have a ‘knock sensor’, and the detonation thresholds within these Porsche air cooled engines are not static; they change with outside air temperature and the DME has a range it can adjust timing to accommodate less than optimal fuel but some of the regions this road trip will cover will see high altitudes and high temperatures that will push the operating ranges closer to their edges as both.
Higher altitude equals less power
The reduced oxygen at high altitudes will make an internal combustion engine less efficient and reduce performance. As a rough guide, engines lose three percent of power for every 1,000 feet of altitude gained. This means you could lose as much as 15% horsepower compared to sea level driving Pikes Peak in the Colorado mountains (4,302 elevation).
Higher elevations mean engines need less octane in the fuel to run properly. So in high elevation areas such as Colorado, gas stations often sell fuel with an octane rating as low as 85. However, If you use lower octane fuel and return to lower elevations the car will behave as you’d normally expect on low octane.
Higher temperatures equal less power and worse fuel economy
Heat kills horsepower. Colder temperatures usually mean more power as colder air has denser oxygen which helps combustion. Scientifically hotter ambient temperatures improve fuel economy because the engine warms up to an efficient temperature faster and warm air causes less aerodynamic drag than cold air. But if it is that hot you’ll have the air conditioning turned up to the max which apparently increases fuel consumption by up to 25% or the windows down and sunroof open increasing the drag coefficient. Not that I care much, petrol is cheap in the States compared to the UK where the fuel duty tax is around 65% of the cost before the government add another 20% VAT (tax on a tax!).