A carmaker-written detergent standard, the lead additive that let engines run harder, and the replacements that came after.
Top Tier is a detergent standard written by carmakers in 2004 after the EPA's 1995 minimum proved too low. It requires more detergent and no metallic additives. Separately, anti-knock agents such as tetraethyllead raised octane until lead was phased out and replaced by MTBE, ethanol and other additives.
When the United States set a federal minimum for detergent in petrol in 1995, the fuel did not get cleaner. Most of the marketers who had been dosing their petrol more generously than the new rule demanded promptly dropped to the legal floor. Carmakers watched injectors clog anyway, and within a decade they wrote a tougher fuel standard of their own.
That standard is called Top Tier, and it is only half of what separates one tank of petrol from another. The other half is the chemistry that stops an engine knocking: the anti-knock agents, of which lead was the most famous and the most costly. Together they explain a lot about what comes out of the pump.
What is Top Tier gasoline?
Top Tier Detergent Gasoline is a performance specification and a trademark, drawn up and backed by carmakers rather than by any government. Fuel carrying the label has to hold detergent additives at levels believed to keep an engine cleaner than the baseline laid down by the US Environmental Protection Agency (EPA). It also has to be free of metallic additives, which can harm the emission control system and add pollutants of their own.
The scheme has grown. As of 2026, the petrol version is sold by 105 licensed retail brands, and a diesel version by 6. The diesel programme was launched in October 2017. As of 2018, the petrol standard was supported by BMW, General Motors, Fiat Chrysler Automobiles, Ford, Honda (with Acura), Toyota, Volkswagen, Mercedes-Benz, Navistar, Audi and Volvo. For diesel, the backers were General Motors, Volkswagen, Detroit Diesel and Navistar.
One rule is easy to miss. A company that signs up must meet the standard in every grade it sells, from the cheapest low-octane petrol to the premium pump. It cannot treat the expensive grade and leave the economy one alone. Premium may carry still more detergent, but nobody gets a lower floor for choosing the cheaper pump.
The federal detergent rule was a floor, and most retailers who had been above it sank down to meet it.
Why did carmakers write their own fuel standard?
The trouble began in the late 1980s, when fuel injection spread through new cars. Injectors have tiny passages, and they clogged in cars running on petrol with too little detergent. Carmakers started recommending particular brands to their customers, yet some fuel marketers were still using no detergent at all. With industry support, the federal government stepped in, and the EPA introduced a minimum detergent standard in 1995.
It backfired. The EPA levels sat below what a few major marketers were already using, so those marketers cut their own dosing to match the rule. The federal figures had been designed to meet emissions standards, not to keep an engine healthy for a long life. Carmakers reported persistent clogged injectors and contaminated combustion chambers, which meant higher emissions and poorer fuel economy.
By 2002, repair records led carmakers to conclude that the federal level was simply too low. They asked the EPA to raise it and found the agency unresponsive. Their worry grew with plans for a new generation of cars built to the EPA's Tier Two emissions standards, which need more detergent to avoid losing performance. Engines with gasoline direct injection (GDI) are particularly prone to carbon buildup, and their makers recommend fuels with higher detergent levels. GDI began in high-end cars but now appears in mid-range and economy models, for instance in the Hyundai Sonata, the Ford Focus and the Hyundai Accent.
Four carmakers, BMW, General Motors, Honda and Toyota, sat down together in 2004 to define what a good fuel is. Working from the Worldwide Fuel Charter, a global body of carmakers and engine builders, they produced a proprietary standard calling for more detergent and tighter limits on metal content. Volkswagen and Audi joined in 2007.
What does engine gunk do to a car?
Think of a shower head with half its holes blocked by limescale. The water still comes out, but in uneven jets, some too strong and some barely there. A fuel injector with deposits behaves the same way, spraying unevenly and creating pockets with too much fuel beside pockets with too little.
Where the mixture around the spark plug is starved of fuel, the combustion that pushes the piston down is weakened and the cylinder may misfire. Once misfires reach a certain frequency, the on-board computer lights the check engine warning. Gunk collects inside injectors and across intake valves, and when it gets severe it can cut fuel efficiency, acceleration and power. Left alone, it can also bring rough idling, a tendency to stall, higher emissions and bigger repair bills.
The cure depends on how bad things are. In mild cases a mechanic may simply add a bottle of injector cleaner to the tank. Sometimes the injectors have to be replaced. Deposits on intake valves can be removed by walnut shell blasting, and in severe cases a costly cylinder-head rebuild may be needed.
General Motors fuels engineer Andrew Buczynsky says that nobody has identified the exact molecule in petrol that causes the buildup, though he still suggests Top Tier fuel to keep an engine cleaner. Automotive journalist Craig Cole puts the difficulty down to the fuel itself: an impure product refined from very impure crude oil, then mixed with extras from ethanol to octane enhancers.
Sulfur is a separate nuisance. Forms such as mercaptans and hydrogen sulfide can be left in finished petrol by refiners or pipelines, and they can contaminate fuel sending units and send the fuel gauge haywire. That problem has faded since 2006, because makers began building those units from alloys less affected by sulfur.
How does a retailer earn the Top Tier licence?
A fuel has to pass performance tests covering deposits on intake valves, deposits in combustion chambers, fouling of fuel injectors and sticking of intake valves. Additive manufacturers pay for the testing, and the cost varies from year to year. Petrol companies instead pay a yearly fee that scales with how many stations they run. Every station selling a brand must meet the standard before that brand qualifies.
Typically, Top Tier petrol holds two to three times the detergent the EPA currently requires. The extra additive is put at under a cent a gallon. For a marketer, the licence is also a way to stand out from competitors.
Ten distributors adopted the standard in 2004. Chevron and QuikTrip went first. Later that year came 76 Stations, Conoco, Phillips 66, Road Ranger, Kwik Trip/Star, Shell and MFA Oil Company. Many more have qualified since, and Top Tier petrol turns up across the United States and Canada, as well as with select brands in Mexico, Panama, Puerto Rico, El Salvador, Guatemala and Honduras. Kwik Trip/Star has since stopped meeting the standard.
Is Top Tier gas worth it?
According to the industry's own research and to motoring journalists, every vehicle benefits from Top Tier fuel over petrol that only meets the basic EPA standard. New cars are said to stay clean and run at their best, while older ones may gain performance and a longer working life. A Honda spokesman says regular use improves engine life, although the company does not require owners to buy it. Honda's Chris Martin says it backs the standard because it has seen benefits for customers over the long run.
Not everyone agrees on how much it matters. A 2007 USA Today feature gave space to three critics who saw little or no benefit and to three endorsers. Tom Magliozzi, co-host of the radio show Car Talk, held that Top Tier petrol is critical only for high-end vehicles. For the rest, he and another critic suggested a concentrated engine cleaner every 100,000 miles would often clear carbon buildup.
Mechanics instructor Jim Kerr pushes back. With some brands of petrol, he says, deposits can settle on the intake valves before 10,000 kilometres (6,200 mi) have passed, and he argues that not all petrol is equal. Ray Magliozzi settled somewhere in between, advising drivers to use Top Tier fuel at least most of the time if they want to keep injectors and valves free of buildup.
Bottled cleaners from the parts shop come with their own warning. Buczynsky says some work but most do not, and some can contaminate the catalytic converter. Use too much and the additive may coat the valve stems and leave valves hanging open.
What does an anti-knock agent do?
An anti-knock agent, or knock inhibitor, is a petrol additive that raises the octane rating by lifting the temperature and pressure at which the fuel ignites by itself. In a high-compression engine, ordinary petrol tends to knock, a sound also called pinging or pinking, or to ignite early before the spark fires at the correct moment. That early ignition is called pre-ignition. Picture a sprinter leaving the blocks before the starter's pistol: the timing the whole sequence depends on is thrown out.
How did tetraethyllead become the octane booster of its day?
The groundwork came from A. H. Gibson and Harry Ricardo in England, and from Thomas Midgley Jr. working with Thomas Boyd in America. They found that lead compounds changed how fuel burned, and by the 1920s adding lead was widespread. That made more powerful, higher-compression engines practical. The most popular additive was tetraethyllead, blended into American petrol, mainly to raise octane, from the early 1920s.
The price was heavy. Derek Bryce-Smith and Clair Cameron Patterson are credited with exposing the environmental and health damage caused by lead. Lead was also incompatible with the catalytic converters fitted to virtually all US cars since 1975. The United States first set standards to phase it out in 1973, and by 1995 leaded fuel made up only 0.6% of petrol sales, at less than 2,000 tons of lead a year. The Clean Air Act then banned leaded fuel for use on the road from 1 January 1996, and a driver caught using it in an ordinary road car now faces a fine of up to US$10,000. Leaded fuel may still be sold for off-road uses: aircraft, racing cars, farm equipment and boats.
The United Nations Environment Programme has put the payoff of the phase-out since the 1970s at $2.4 trillion in annual benefits, along with 1.2 million fewer premature deaths, higher overall intelligence and 58 million fewer crimes. A few experts go further and speculate that leaded petrol drove a worldwide wave of crime in the late 1980s and early 1990s. Blood lead levels fell sharply where bans were introduced.
Lead needed company in the tank. Scavengers such as tricresyl phosphate were added alongside tetraethyllead to prevent lead deposits inside the engine.
What replaced lead in petrol?
As lead faded, refiners had to make up the octane shortfall. Around 70% of the gap was closed at the refinery itself, by cracking other hydrocarbons into fuels that blend closer to the required octane. Most of the remainder needed chemical additives that do not come from refining.
Methyl tert-butyl ether (MTBE) took over from lead in the United States starting in 1979. It proved to be a toxic water pollutant, and a string of groundwater contamination scandals from the 1990s led the EPA to begin phasing it out in 2000. The Energy Policy Act of 2005 fixed a nine-year phase-out and designated ethanol as the successor. Politics helped: Congress saw ethanol as a backstop against any attempt to limit American petrol supply, and as a reward for Iowa corn farmers. What began as an additive used as needed became a fixed 5% blend and then 10%, today's most common US fuel.
Ethanol has drawbacks. It is hydrophilic, drawing water vapour out of moist air, and it raises the fuel's free oxygen. In traditionally built engines, both effects cause residue and corrosion, worsening as the ethanol share rises. Petrol that has aged may simply evaporate, whereas an aged ethanol blend sitting in an engine can do severe damage. Cars coped through ethanol-tolerant metals and seals and electronic fuel injection that can adjust combustion, and because a car's tank usually turns over within weeks. In small carburettor engines such as generators and lawnmowers, ethanol damage became the leading cause of failure.
| Additive | Where and when | Main point |
|---|---|---|
| Tetraethyllead | US from the early 1920s; on-road ban from 1 January 1996 | Toxic lead; protected valve seats |
| MTBE | US from 1979; phase-out began 2000 | Water pollutant |
| Ethanol | US, fixed 5% then 10% blends | Hydrophilic; hard on old engines and small carburettor motors |
| MMT | Canada, recently Australia; limited in Europe | Manganese compound; lets old cars run unleaded |
| Ferrocene | Added to petrol; vintage-car additive solutions | Safer than lead; can coat spark plugs |
| Iron pentacarbonyl | Once used in Europe | Replaced lead there for a time |
What about MMT, ferrocene and iron pentacarbonyl?
Methylcyclopentadienyl manganese tricarbonyl, mercifully shortened to MMT, has long been used in Canada, and more recently in Australia, to raise octane. It also lets old cars designed for leaded fuel run on unleaded petrol without a separate additive to prevent valve-seat erosion. Its safety is contested. A Canadian study in 2002, funded by carmakers who oppose the additive, concluded that MMT impairs emission controls and increases pollution. A later Canadian government study found that no Notice of Defect was potentially caused by MMT. A 2013 risk assessment by ARCADIS Consulting, following a European Commission method and verified by an independent panel, found no significant human health or environmental concern at levels up to 18 mg Mn/L, and Health Canada has said its combustion products pose no added health risk to Canadians.
Europe took a stricter line. After banning lead additives, it effectively banned other metallic fuel additives in 2004, setting an MMT ceiling of 6 mg/l from 2011 and tightening it to 2 mg/l in 2014. No MMT use has been reported in recent annual monitoring under the fuel quality directive. North American refineries had already stopped voluntarily by the end of the 1990s, since MTBE and ethanol were on hand.
Ferrocene and its derivatives are anti-knock agents regarded as safer than the banned tetraethyllead. Ferrocene-based additive solutions can go into unleaded petrol so that vintage cars built for leaded fuel can run on it. The iron-containing deposits it forms can build a conductive coating on spark plug surfaces. Iron pentacarbonyl, a straw-coloured liquid with a pungent odour, was once used in Europe as an anti-knock agent in place of tetraethyllead. All three are metal-based compounds, the category Top Tier fuel rules out.
What did lead's departure mean for classic cars?
Lead did one useful thing besides stopping knock: it protected valve seats from erosion. Once leaded fuel became unavailable, many classic engines needed modification to cope with lead-free petrol. Lead substitute products are also made and can sometimes be found at parts shops. Ordinary pump petrol carries other additives as well, to reduce internal carbon buildup, improve combustion and make starting easier in cold climates.
Where this goes next
The direction of travel is set by what has already happened. Top Tier started with ten distributors in 2004 and counts 105 licensed petrol brands in 2026, with a diesel branch added in 2017. Direct injection keeps moving into cheaper cars, which is why carmakers keep recommending higher detergent levels.
Leaded fuel is still in use in some of South America, Asia and the Middle East, whereas sub-Saharan Africa began phasing it out on 1 January 2006. More countries are drawing up plans to ban it soon.
