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A simple question with a complex answer.
At the time of construction (1970's) the highest grade petrol/gasoline fuel in Japan, US and Europe was generally 95 RON (Research Octane Number) and possibly as low as 91 RON. MOST IMPORTANTLY it had a Lead additive. It was commonly known as Super Fuel (95 RON with Lead). This would be the ideal fuel for CB750s and most engines built before 1990. Unfortunately, it is no longer available.
Lead was added to fuel to allow higher compression ratios within the cylinder before self combustion occured. When petrol (all fuels) within a cylinder are compressed it will reach a point where it self ignites (pressure times volume divided by tempreture = pre ingnition) before the spark plug fires. Self-ingition is also known as pre-ignition. If this occurs in your CB750 you will like hear an exhasut 'pop' as the pressure within cylinder is released prematurely via the exhaust valve, and you will have a noticable reduction in engine power.
Lead in fuel also had the positive effect of hardening the relatively soft cast iron heads in engines of the time as well as hardening the valves seats. It seems obvious, but the engiens were desinged for the quality of fuel at the time.
During the 1970s, with energy crisis of the time, there was increased demand for improved fuel economy and as a consequence engine designers aimed to acheive higher compression ratios (therefore increased power). To acheive a higher compression ratios they needed to change the fuel quality to allow it to sustain a higher pressure within the cylyinder before self-ignition.
The lead additive allowed higher engine compression before self ignition.
Unfortunaley, the process of burning leaded fuel caused high levels of carcengens to be released, it has a harmful impact on the brain of developing children and in high densities can cause acid rain. As a consequence it was removed from fuel during the 1980's.
To overcome the reduction in performance of an engine due to the removal of lead from petrol, the quality of petrol was improved by increasing the octance (RON). Octane defines the fuels anti-knock quality (resistance to pre-ignition) of fuel. The higher the octane rating the greater resistance to pre-ignition therefore higher cylinder pressure therefore more engine power.
The removal of the lead aditive (early to mid 1980's) and the subsequent addition of other addititives that are not as carcenagenic as lead results in a slight engine 'pinging' noise on engines built without catalytic converters. I must admit that this is not noticable in CB750 engines, or bikes generally, but it is noticable in other engines of the era.
Therefore to answer your question the ideal fuel for your mates bike would be a 95 RON fuel with a fuel additive known as a valve seat recession additive which mimicics the impact of lead in fuel. You buy these from petrol stations. The bike was built to run on Super fuel (95 RON with Lead) and this be the closest you could get these days.
Alterantively, a 97/98 RON fuel will achice almost the same outcome. The only difference between 95 RON with a fuel additive, or 97/98 RON and the 1970s Super fuel (95 RON with Lead) is that Super had the effect of hardening the valves and pistons. If an engine, whcih was built for use with Super, was not initailly run using Super fuel then the life of the valves maybe shortened.
Frankly, Ive tried everthing from 91 RON to 10% ethanol and my 1972 CB 750 does not notive the difference. When I can find it I'm going to try the 80% ethanol fuel. I think you would need a very well set up bike and a race track to start picking the differences in quality of fuel.
Finally, 20 mls of 98 RON Petrol (about one cupped handful) has the same kinetic energy as a family sized car travelling at 100km/h running into a brick wall. This energy is only realised if it is burnt perfectly. other factors within your engine are much more likely to have a greater impact on the perfromacne of your bike than the RON.
Cheers
Tim
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