Octane, compression & detonation - Pontiac GTO Forum
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Octane, compression & detonation

I grabbed several sources from both the internet and a few books to piece this together. The question of what octane should I use for my "X" compression engine seems to be an ongoing mystery. You can build your engine to any compression you want, but be aware of what octane you will need to run it and if you try to get by with an octane too low for a compression too high, know the consequences and the damage that can be done. There are always exceptions to the rule, but this should be a guide for the average Pontiac engine build.

Spark knock (detonation) is an erratic form of combustion that occurs when multiple flame fronts occur simultaneously inside a combustion chamber and these multiple flame fronts collide, creating shock waves that produce a sharp metallic pinging or knocking noise. Detonation occurs because fuel is subjected to either too much pressure, too much heat or both. It usually happens during acceleration when the engine is heavily loaded and cylinder pressures are at their peak.

Mild detonation can occur in almost any engine and will not cause damage if the detonation is short lived. Prolonged heavy detonation is dangerous and can crack pistons and rings, blow out head gaskets, damage spark plugs and valves, and flatten rod bearings.

Detonation can be caused by a number of factors:

Too Much Compression for the octane rating of the gas being used or too low of an octane rating in an attempt to save money on the lower priced gas.

Lean Fuel Mixtures. Rich fuel mixtures resist detonation while lean ones do not. Air leaks in vacuum lines, intake manifold gaskets, carburetor gaskets can all allow extra air into the engine and lean out the fuel mixture. Lean mixtures can also be caused by a restricted fuel delivery, clogged carburetor jets, a restricted/dirty fuel filter, or a weak fuel pump. Air/fuel ratio can also be affected by changes in altitude.

Overadvanced Ignition Timing may create too much spark advance which causes cylinder pressure to rise too rapidly. If resetting the timing to stock specifications does not help, retarding timing a couple of degrees may be necessary to eliminate knock. Too much retard will also cause the engine to run hot.

Engine Temperatures can contribute to engine knock. A hot, or overheated, engine is more likely to suffer spark knock than one which runs at normal temperature - with normal temperature sometimes being a personal choice over the factory specified temps. Overheating can be caused by a clogged or too small a radiator, slipping belts, collapsed radiator hose, low coolant, a defective fan clutch, too hot a thermostat or a defective one, a bad water pump, etc.

Overheated Intake Air. With cars having thermostatically controlled air cleaners that provide the carburetor with hot air to aid fuel vaporization during engine warm-up, the air control door can stick shut so that the carburetor continues to receive heated air after the engine is warm, detonation may occur, especially during hot weather. Some cars have the hot air "stove" that directs heated air from off the exhaust manifold to the inlet on the air cleaner. A flexible tube/hose usually connects the two. It might be wise to remove the hose during summer operation.

Spark Plugs may be too hot. The wrong heat range plug can cause detonation as well as pre-ignition. Copper core plugs are less likely to cause detonation than standard spark plugs. You can experiment with heat ranges dropping down 1 or 2 colder steps. If too cold of a plug is used, the plug will experience fouling out as it isn't getting hot enough to burn off the deposits.

Failure of Exhaust Gas Recirculation (EGR) System. The EGR keeps combustion temperatures down, reducing the tendency to detonate. If the EGR valve is inoperative or someone has disconnected or plugged its vacuum hose, higher combustion temperatures can cause pinging.


Octane does not offer any better fuel mileage, increase engine horsepower, or make the engine start quicker. Octane rating is a measure of a fuel's ability to resist engine ‘knock’ during combustion. The octane requirement of an engine varies with compression ratio, geometrical and mechanical considerations, and operating conditions. Higher octane gas burns slower, it is more resistant to knock when and engine is subjected to higher RPM's and cylinder pressures. Compression ratios factor into cylinder pressures. Higher ratios cause higher cylinder pressures and therefore cause the engine to be more susceptible to pre-detonation or knock. The higher the octane number, the more compression the fuel can withstand before detonating.

Tetraethyl lead used to be added to gasoline to prevent engine pre-ignition due to higher cylinder pressures associated with higher compression ratios by allowing the gas to burn more slowly and smoothly and to increase the higher octane requirements of the higher compression engines.

In 1962, the average octanes of gasolines in the U.S. stood at 93 for regular, 99 for premium and 102 for the few super-premiums on the market. The octane numbers increased slightly by 1967 as they reached around 94 octane for regular, and 100 for premium with the super-premium Sunoco 260 still being offered at 102 octane and advertised as the "highest octane pump gas." There were also other blends that had less octane than Regular and other grades between Regular and Premium.

The Clean Air Act of 1970 stipulated the federal mandates for automakers in reducing the emissions of their engines. GM reduced compression ratios on all of its 1971 engines to permit the use of low lead, regular leaded or unleaded gasolines and soon other automakers followed. This move spelled the end of the “octane race.” By 1974, unleaded gasoline was being phased in at most U.S. service stations as the catalytic converters used on the 1975 cars required unleaded gas only.

The 93 octane of today is only slightly less than the octane of the Premium pump gas of the 60’s and early 70’s. In the past, 100 advertised octane was common and Sunoco 260 gasoline was 102 octane. Those octane ratings sound high compared to today’s octane ratings, but they were not that much higher than you might think. Advertised octane in the past was based on a different measurement than used today.

There are 2 ways to rate octane, the “Research” octane number and the “Motor” octane number. The research octane number is a higher number than the motor octane number, so oil companies used to advertise their gasoline with the more impressive research octane number. The motor octane number is about 10 points lower than the research number.

In the early 70’s, the government decided to require oil companies to post an average of the 2 different octane ratings, which lowered the octane rating numbers for the same fuel that had had a higher number before. When you now look on a gas pump, you’ll see a sticker for the octane rating that says “R+M/2.” This stands for Research octane plus Motor octane divided by 2, which is the average of both octane numbers.

So the old Premium from the 1960’s that had a Research octane of 100 also had a Motor octane number of about 90. That same 100 octane Premium from the past would now be posted as the average of the 100 Research octane plus the 90 Motor octane, divided by 2 giving you a 95 octane rating. It is still a little higher than the 93 octane of today’s best Premium, but not as big a difference as most people think. The Sunoco 260 using this formula would have a 97 octane rating, which is quite higher than the best 93 octane Premium and higher still than the more common and readily available 91 octane found at most pumps.

So if you still have a factory engine with its 10.5 or 10.75 compression ratio, you can see how the pump 91 or 93 octane will be a problem and lead to detonation. It'll be almost impossible to tune this out when the solution is to use an octane gas or octane supplement to get the gasoline octane up between 97-100 octane.

In Jim Hand's Pontiac engine building book are a number of engine builds from an assortment of builders. In it they provide a compression ratio used by their engine build and a recommended octane needed to run the engine. Many factors do come into play that can effect a required octane number, but this gives you an idea. Here are a few of them:

Iron Head, 9.0:1, 91 octane minimum, 93 preferred
Edelbock Head, 9.5:1, 92 octane
Iron Head, 9.5:1, 93 octane
Iron Head, 9.3:1, 93 octane
Edelbrock Head, 10.5:1, 93 octane
Edelbrock Head, 9.7:1, 93 octane
KRE Head, 9.8:1, 93 octane
KRE Head, 10.0:1, 93 octane
Iron Head, 9.25:1, 92 octane
Iron Head, 9.4:1, 92 octane
Iron head, 9.5:1, 92 octane
Edelbrock Head, 10.25:1, 92 octane
Iron Head, 9.0:1, 93 octane
Iron Head, 9.5:1, 94 octane
Iron Head, 9.8:1, 94 octane

Three problems Ethanol can cause:

It is a Water Magnet - It will pull water out of the air right through your gas tank vent hose. In our high-humidity climate this can be especially harmful to your engine. As moisture is absorbed, the molecules of alcohol/water become heavier than the gasoline in the tank, and they settle at the bottom. This process is called "phase separation".

Unstable Octane Levels - Due to ethanol's high octane rating, if enough water is present in a fuel tank the ethanol can migrate to the water layer, dropping the octane level. This will leave you with lower octane fuel in the tank, and if the separated mixture becomes great enough to reach the fuel pickup tube, it will be sucked up into the engine, where it can clog filters, carburetor jets and fuel injectors.

Solvent Properties - If there is any gum or varnish build up in your old tank or fuel system, the alcohol will dissolve these deposits, and they will be sucked into the engine fuel system. If that weren't bad enough, the alcohol and water combination mentioned in #2 may also encourage the growth of bacteria in the fuel tank which turns into sludge and damages the fuel system.
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