Not to be a Cliff Claven or anything but:
The octane rating of a fuel is based on the formulation's performance in a
special one cylinder engine with variable compression ratio. The rating
system got it's name from iso octane which is a straight hydrocarbon with 8
carbons and 18 hydrogens. The performance of iso octane in the test was
given the rating of 100. To perform the test, the test fuel is put in one
tank and then two fuels of known octane are put in other tanks. The knock
threshold of the test fuel is found and then the other two are fed to the
engine at different ratios until the mixture performs the same as the test
fuel. The final ratio of standard fuels is use to calculate the octane
rating. For instance, if the standard fuels were 80 and 120 octane and the
final ratio was 50/50, the octane rating of the test fuel would be 100.
The two most common test methods are the Research method (ROM) and the
Motor method. In the US, the pump octane is ROM + Motor/2. Typically the
two methods give numbers about 4 apart and I don't remember which one is
higher. So 91 pump octane would be about 93 octane on one scale and 89
octane on the other.
An engine benefits from higher octane fuel only up to the point that it can
run optimal timing without detonation. As others have said, our MG's can
run about 9 to 1 on unleaded premium fuel.
Lead does not lubricate the valve per se. It coats the valve and seat.
When the area gets hot, the metals can stick together and you will tear
small chunks out of the softer part- valve or seat. The coating of lead
prevents this by not allowing the metals in such close proximity to bond
and also by aiding heat conduction and keeping the engine cool. Once
coated, the benefits are there for a long time so if you have an old engine
that has used leaded fuel for years and change to unleaded, you should have
no problem. If you rebuild an engine, it makes sense to use hardened seats
and stellite valves since there will be no lead coating and the
sticking/tearing/eroding would be pretty bad with no lead around and the
original soft material. On an older engine you should be OK although if
you run it hard for a long time such as long freeway trips you may want to
check the valve clearance more often. If the valve clearance keeps
disappearing, you may have used up your lead on the valves and a rebuild
would be in order.
Regards,
Bill Eastman
61 MGA with well leaded valves
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