QuestionHi Howard _ very interesting discussion on the overheating 80 MG and especially: Octane. You mentioned low compression engines run cooler on lower octane fuel. Can you explain about the relationship between octane level and heat, and what exactly is happening when one talks of engine 'pinging' or 'knocking'. Are they the same thing?... Finally what do you mean by the term 'surging'?
( my engine code is - 18V 837Z L462 - am I correct in the "L" standing for low compression? )
regards
Paul
AnswerHi Paul,
High octane gasoline burns much slower then Low octane gasoline so when you put High octane gas in a low compression engine it burns so slow that it is wet and burning while the piston travels down so it tends to burn the oil off of the cylinder walls which causes excess oil use and it over heats the cylinder walls and does not burn completely.
The flame front in a combustion chamber burns much faster under the higher compression thus the flame under higher compression burns across the combustion chamber more completely in a high compression engine.
Low octane gasoline burns much faster but not as smooth. Octane rating is not a quality rating but only a point of detonation (explosion). When low octane gas is used in a high compression engine the the faster burn and the accelerated speed of the high compression makes the gas start to burn across the combustion chamber but it compresses the fuel/air mixture ahead of it and that heated compressed fuel/air mixture that has not yet burned explodes. That explosion striking the piston and combustion chamber is the "Pinging" (wrongly called "Valve Rattle")under acceleration. This "Pinging" "Valve Rattle" "knocking" is very destructive to the engine.
This noise that you hear at low RPM is a sure sign of "Detonation" however, there is also a high RPM "Detonation" too that is very difficult to hear which is just as destructive if not more so.
If you hear this in an engine you must do something about it right away. Put high octane gas in and if you already have high octane you need to do several things. Retard the timing, richen the fuel mixture, check the heat range of the spark plugs, check the engine for oil burning (rings), check the cooling system to see that it is not overheating, check the combustion chamber for excess carbon build up. These are the most common causes of Detonation (Pinging).
Surging means that at a steady cruise speed when you have the throttle set at a steady speed you can feel the car smoothly and lightly accelerating and decelerating as you hold the throttle steady. Usually most noted at lower cruise speeds like 35 to 45 MPH. It is not usually noted at higher speeds because of the much higher load on the engine to push a car at higher speeds.
The (80) MGB was a low compression engine however unless you know that the head has never been resurfaced nor high compression pistons installed or the block decked you can not tell if it is too high to use regular gas. Plus the combustion chambers can be carboned up enough to raise the compression ratio. That is why I suggested you just run the test. If you can run on regular gas with the factory ignition timing and not get Pinging then you should use regular gas. Your car should have 8 to 1 compression ratio which is the low compression engine. The letter "L" in the engine number was used since the early 70s so I don't think it means Low compression. Run the test I out lined before and you will know what fuel to use. Let me know.
Howard
