Fuel Choice "LRP is OK for my Cossie or RS Turbo, isn’t it?" is the question our service department gets asked almost on a daily basis! The answer is “No!” LRP or Lead Replacement Petrol is, as Esso in their LRP leaflet states, is suitable for all cars except vehicles used for towing, vehicles that sustain constant high speed or vehicles that are driven under load (turbo-charged or modified). So, if you drive like the PJ crew, load and speed go hand in hand! All Cosworths YBB onwards and 95% of CVH engines can use Super-unleaded fuel with no modifications. Late 4x4 Sapphire Cosworth green top YBJ engines and all Escort Cosworths can run on normal 95 octane fuel, if a standard ‘chip’ is retained. If modified, Super-unleaded fuel is recommended to protect the engine from ‘knock’ or ‘detonation’. The higher the RON (Research Octane Number), the higher the resistance to knock. Knock occurs in the combustion chamber when, due to a weak fuel mixture, too high a compression ratio for engine map, or advanced ignition timing, the fuel injected ignites itself. When the fuel is ignited by the spark plug, the second explosion creates a second flame front and, as the explosions collide, a knocking sound is heard (we use sophisticated equipment to check for knock and detonation). This is unhealthy, as the engine is not running efficiently; selecting the correct fuel can alleviate this. Detonation or pre-ignition is more severe than knock, ad melted pistons are usually the end product. This is caused by the temperature in the cylinders increasing due to poor fuel mixture in conjunction with excess over-boost pressure, or too much ignition advance, causing the spark plug to overheat. Typical RON numbers as follows: - Normal unleaded: 95 octane - Super unleaded: 97 octane - Shell Optimax: 98 octane Fuel additives can also help prevent knock (see additive information)