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An overhead valve engine has valves in the top of the combustion chamber, but has a camshaft mounted low down in the engine block. The valves are operated through pushrods and rockers.
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An overhead camshaft (OHC) engine has its camshaft mounted above the valves.
The camshaft is driven by a geartrain, chain or belt system. The valves may be operated directly through bucket tappets or indirectly using finger followers.
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Opposite lock is where the driver steers in the 'wrong' direction, away from a corner rather than into it, to deal with oversteer.
Opposite lock is also known as counter steering.
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Octane rating is the resistance of a fuel to knocking.
Some petrol engines can adjust the ignition timing to take advantage of higher octane fuels, and to prevent engine damage if low octane fuel is used. But in most petrol engines there is no advantage in using a higher octane fuel than the maker recommends.
There are two different methods of measuring octane number, and they give different results. The Research Octane Number (RON) is the more widely quoted, and is higher than the Motor Octane Number (MON).
European petrol is available in several grades, with different octane numbers:
- 91RON Unleaded (not sold in UK)
- 95RON Premium Unleaded
- 97RON Super-Unleaded
- 99RON eg Shell V-Power
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Oversteer is when a car turns in to a corner more sharply than the driver expected.
If oversteer is not countered by the driver – eg by using opposite lock – the car may spin.
In the USA a car which oversteers is sometimes described as being loose.
Compare understeer.
