MGB Register Forum

I’ve seen E10 in one of the leading supermarket station last month when I filled up my daily car. Although it was still the 95 octane.

Remy, these cars were originally tuned to Ron on 5 star leaded petrol (circa 99 octane) but most engines will have had several changes affecting fuel requirements since then.
My BGT has a 9:1 compression ratio (requiring a high octane fuel), but has hardened exhaust valve seats fitted to allow it to use unleaded petrol.
I always use Shell V Power which is 99 octane. It runs well with no pinking and no running on after switch off.

Pinking or detonation from using petrol of too low an octane number can cause damage to the engine.

Running unleaded fuel without hardened exhaust valve seats or without specific additives can cause valve seat recession as there is no lead present to protect the valve seats.

Ian F

To deal with Remy's original question - My Drivers Handbook (for GHN4 and 5 or GHD4 or 5 cars) gives the recommended Octane Rating as 98 octane or above, so 4* as a minimum and 5* ideally in those days. In the 1960s most garages sold 2*, 3*, 4* and 5* fuel - mostly by the pump mixing 2* and 5* in different quantities to get 3* and 4*. So everyone should have been running Super Unleaded for years. This is now being called the Protection Grade Petrol and is recommended that all vehicles should use this if made before 2008, as the Ethanol content is being increased in ordinary unleaded from 5% to 10%. I have noticed the labelling on the pumps has already changed, but it should be remembered that 5% or 10% is the MAXIMUM level of Ethanol that can be added to the fuel. Currently Esso claim their Supreme (Super) Unleaded contains 0% Ethanol in most places around the UK. This might change if the wording of the legislation changes to be MUST contain 5% (or 10%) instead of the current recommended maximum.

Since seeing this post I've been looking for an understandable explanation of Detonation, Pre-Ignition and Pinking! This has not been easy to find and in many cases I suspect all three terms are muddled. I have found and offer this explanation for Alan Allard's book on Turbocharging and Supercharging, which was published in 1982 so from the times of the MGB. The only caveat is of course, Alan might have been wrong as well!

Detonation is a condition where the progressive burn of the air/fuel mixture across the combustion chamber is interrupted by a small pocket of mixture, usually near the exhaust valve, reaches a critical temperature and ignites spontaneously. This causes a rapid increase in cylinder temperature with consequent overheating of pistons and valves, leading to failure of these components.

Pre-ignition is when hot combustion chamber surfaces cause the critical temperature to be reached prior to ignition from the spark plug. This can be caused by carbon deposits or sharp edges around the combustion chamber, or sharp edges left by fitting unleaded valve seat inserts

Both detonation and Pre-ignition will produce the audible sound known as 'Pinking' (by us in the UK) when the process is severe, but often high speed detonation cannot be heard without detonation amplifiers or vibration sensors. This is why 'Detonation' can result in piston of valve failure without any prior warning either with sound or water/oil temperature gauges indication a rise in temperature. On the other hand Pinking caused by pre-ignition will often be heard by the driver who can ease up on the throttle to retard the ignition or simply and quickly increase speed and drive through the condition. Both detonation and Pre-Ignition can be brought on increasingly by increases in compression ratio or over-advanced ignition, so keeping to sensible compression ratios for the use of the car (no more than 10.5:1 for road use and 12.5:1 for race use) and using higher octane fuels (super unleaded for road and racing fuel - which is now 110 octane and related to aviation petrol) and maintaining the correct setting for the ignition are key to stopping these conditions occurring.
Hope this helps
Pete

Thanks Pete,
Very instructive post, I can hear pinking when driving my car, well I think it is as I don’t know much about engines. I’ll start using higher octane fuel next time I fill up and have it looked at if it carries on.

Just to add to Peter's comprehensive explanations, my understanding is that detonation is the condition to really be concerned about.
Fuel is meant to burn in an engine, but detonation is exactly what it sounds like - an explosion! This sends a shock wave through the moving engine components and can rapidly lead to mechanical failure. This is particularly dangerous as the detonation may occur as the piston is rising up the bore - a reverse in direction is not very desirable!

Ian F

Hi Remy.
Quite frankly you are very lucky to have 4* petrol available locally.
Fill with 4* when home and top up with Super Unleaded when out and about.
Easy!

Have used Shell Vpower for years and runs great, use super unleaded E5 in the MG when the E10 comes in, it's the protection grade for our classics.
I now have an unleaded head engine as the bottom end was worn, but never worried too much about valve seat wear anyway on the old engine anyway.

On an original head, lead memory is usually sufficient to prevent valve seat recession, and this is something that can be checked through regular tappet adjustment - if regular re-adjustment to take up clearance is required then there is valve seat recession. Unless you regularly drive at >3500rpm for long periods, then the consensus of opinion is that the lead memory will remain intact and minimal damage will result, if any. Sufficient at least to last the remaining life of the engine whereupon a conversion might be undertaken as part of the rebuild.