Preventing Engine Detonation

Tetraethyl lead, or leaded fuel, was marketed in the early 1920s as an anti-fuel knock additive. In view of the health hazards presented by leaded fuel, however, some small engine modifications may be a better route today.
David Gilbert
June/July 1993
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The use of unleaded gasoline in older engines has been a popular topic among many antique engine enthusiasts. Various gasoline additives and refining processes have enhanced the performance characteristics of gasoline for many years. Ideally, the spark ignited engine should burn fuel smoothly and evenly in the combustion chamber. When fuel explodes uncontrollably in an engine, a sharp metallic noise or 'knock' can be heard from the engine.

An engine with a fuel knock problem is said to be suffering from engine detonation or pre-ignition. Besides being noisy, engine detonation or pre-ignition can cause a reduction in engine performance, over-heating, and damage to pistons, piston rings, and valves. Some detonation or pre-ignition factors can be: poor fuel quality, high compression ratios, improper ignition timing, carbon deposits, high combustion chamber temperatures, excessive engine loads, and lean carburetion.

Tetraethyl lead, or leaded fuel, was first marketed in the early 1920s as an anti-knock additive. One of the benefits of this additive was to raise the gasoline octane rating without additional refining processes. Fuels with higher octane ratings tend to burn slower in the combustion chamber, and thus help to control engine detonation problems. Another characteristic of lead additive is deposits that can form in the combustion chamber. Lead deposits on the surface of the valve face provide a cushioning effect as the valve closes, reducing wear on the valve and seat.

Unfortunately, lead deposits are not selective and also tend to form on spark plugs, pistons, combustion chambers, and exhaust system. Lead particulates from the exhaust are also quite harmful to human health, and have been under the watchful eye of the Environmental Protection Agency since the 1970s. The current leaded fuel phase-out has public health as the primary interest.

Unleaded fuel is not new, you may know it as 'white gas.' Current unleaded gasoline production uses other less harmful additives and additional refining processes to control engine detonation. The only area of real concern is the valve wear problem on older engines. It should be noted that valve wear is aggravated by high engine speeds and loads, for extended periods of operation. Most old engines are not subjected to this type of use at the typical shows.

If you are currently restoring an engine for display, and would like to modernize the valves, you have two choices: replacement or modification. Replacement of the old valves and seats with hardened valves and seats will require some additional cost and machining. Second best is modification of the existing valves and seats by increasing the valve to seat contact area. For the latter, have your machinist add approximately 1/3 more seat contact area during the valve and seat refacing operation. The extra contact area increases the wear surface area and allows better heat transfer from the valve.

Do not be afraid to use unleaded gasoline in the old hit-and-miss engines. These engines are typically low compression ratio engines, slow speed (that's why I like them), and seldom overheated or overloaded. Compared to leaded, unleaded fuel will not foul ignitors and spark plugs as quickly, will produce less combustion chamber deposits, and will result in a cleaner running engine for our environment. After all, no one will be around to enjoy our old engines if we don't protect our future!


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