30 April 1999



How will the coming leaded

petrol ban affect farming?

Andrew Pearce lays out

some answers

EVEN if the Millennium Bug leaves us with any electricity to run the pumps, come next New Years Day you still wont be able to trundle down and fill up with four-star.

From the end of 1999, Britain joins much of the rest of Europe in banning leaded fuel. But "not in use" hoods will be appearing on pumps from this autumn, as retailers run tanks dry, and it is all in the interest of cleaner air.

While farmings heavy metal guzzles diesel, the smaller fry – ATVs, chainsaws, portable generators, pumps, elevators, post hole augers, strimmers, concrete mixers and such – run on petrol, as do a veritable heap of Land Rovers, vans and garden machinery. Which of this lot will be affected by the leaded phase-out, and if so, how?

The answers are not necessarily clear-cut, but with most engines there will be no problem. For although leaded four-star is to go, not many engines actually need it. And for those that do there will be alternatives, which well come to later. First, though, take a look at the box Why Lead? (overleaf) and its companion High Octane Stuff.

&#42 Which engines might have trouble?

Engines potentially at risk from low octane unleaded fuel are:

&#8226 Those with "soft" valve seats. For why, see box The Deepening Recession (overleaf).

&#8226 High compression units which need leaded petrols anti-knock ability.

The obvious place to start an inquisition on any given engine is with the handbook or dealer, either of which should reveal its fuel requirement. If both have disappeared or cant help, engine age might, though roughly.

In 1975 the USA ushered in catalyst-equipped cars and the unleaded petrol they needed. The size of that market prompted makers around the world to produce engines from lawnmower-size upwards that would run unleaded, even if a catalytic converter would not be used. So the mid-70s marks a watershed, around which many small engines (and a lot of bigger ones) started to accept low-lead fuel. Running these on leaded four-star will harm only your wallet, not the motor.

It is pretty safe to say that most small, post-1975 petrol engines in farming will be completely happy on basic premium-grade unleaded. This includes units from Briggs & Stratton and similar, plus all industrial motors from the Japanese big four of Honda, Kawasaki, Suzuki and Yamaha. ATV owners too can relax (all ATVs from the big four built after 1975 are designed for unleaded), as can livestock chasers on post- 75 Japanese four-stroke motorcycles.

Two-strokes in saws, strimmers and so forth have no valves, so seat protection is not an issue. And as very few have ever been fussed about octane rating, 95 octane premium unleaded is fine. There are a couple of things to note when you switch a high revving two-stroke from leaded fuel, though.

&#8226 Unleaded burns hotter and cleaner than four star. Old carbon deposits dislodged by the new regime can wear the bore and rings, so decoke the motor before swapping fuels.

&#8226 Be quite sure that carburettor mixture settings are spot-on. A marginally weak mix that gives no trouble with leaded burns hotter still with unleaded.

&#42 Give over, Rover

Whats the score with 4 x 4s, vans and light trucks? Japanese stuff just about universally takes ordinary (ie premium) unleaded, though check with an agent about anything pre-1975. As with small engines, filling with four-star when it is not needed simply means you are burning money unnecessarily.

Since Apr 1, 1991, all new UK cars had to have catalysts, and thus run only on unleaded petrol. But even before then plenty could use it without trouble. The rule with vans and light commercials is to check with a main dealer. It may be that the motors valve seats are hard, but its compression ratio is high enough to prompt detonation on unleaded: in such instances, retarding the ignition timing (usually by about 3deg) delays the start of burn and avoids the problem. For more on this, see box The Big Bang.

With Land Rovers the waters get muddier. At the time of writing, the factory had not issued a definitive listing of which of its engines are safe with low-octane unleaded. Unofficial opinion reckons that no 2.25 and 2.5 four-cylinder motor can take unleaded without seat trouble. With the V8s, some say that all UK high compression engines have hard valve seats and can handle unleaded, while others who know are adamant that 3.5 V8s will suffer. The consensus on 3.9 litre motors and bigger is that all are OK. Generally, wear in older Land Rover power plants (plus their originally modest compression) will mean no ignition timing adjustment is needed.

&#42 What happens if you cant use leaded?

Where diligent research or inspired guesswork confirms that tender valve seats prevent an engine (big or small) from switching directly to unleaded, there are several options – all validated by experience in other countries.

&#8226 Change the exhaust valve seats. If the motor is likely to be around for a while, the most satisfactory way forward is to swap soft exhaust valve seats for hard ones and then run on ordinary unleaded, maybe with a small ignition timing adjustment. Susceptible Land Rover engines are prime candidates for this, and subsequent power drop is minimal.

Specialists like Discovery Engineering (01622-687079) do the job for about £20 a seat, plus an optional £7-£8 for a bronze valve guide which will better hold lubricant: say £80-£110 for a four-cylinder head, to which add garage time for the on/off bit if you do not DIY.

An alternative from Land Rover dealers is a complete replacement head to suit 2.25 and 2.5 litre engines at £445.12 inc VAT, but plus fitting. If seat swapping is not an option through practicability or cost, look to the following pair of alternatives.

&#8226 Use Lead Replacement Petrol (LRP) as it turns up on forecourts. This will happen increasingly before four-star disappears, though not all garages will sell it. A British Standard has not been finalised though the outline is clear. LRP will probably use a compound of potassium or sodium rather than lead to cushion and protect valve seats. With a likely octane rating of 98, will suit engines of any compression ratio. Excise duty on lead replacement petrol is to be finalised, but it is likely to be more expensive than unleaded. Important note: Where engine use is extensive or hard (see below) valve seat conversion offers better ultimate reliability and can be more cost-effective.

&#8226 Use an anti-wear additive. Several firms sell a tip-in-the-tank product which takes over leads job of protecting valve seats. Independent testing has shown that these work within limits. For peace of mind stick with big-name brands. Add the required quantity of goo to either premium unleaded (for soft-seat engines that can run 95 octane fuel, after ignition timing adjustment if necessary) or to super unleaded (for soft-seat engines which must have 97/98 octane juice). For the latter, you can spice up regular unleaded with both an anti-wear additive and an octane booster to arrive at something approaching the dear departing four-star, though farming has fewer motors that need it.

Two notes apply here: Using both an additive and lead replacement petrol will be expensive and pointless, and high load/speed caution applies as with lead replacement petrol.

&#8226 Devices which fit in the fuel line claim to offer valve seat protection. Independent tests have so far failed to prove that they do.

&#42 The twilight zones

There are other possibilities where engine use pattern allows. Rather than splash out on harder valve seats, lead replacement fuel or additives, it can be enough to use a soft-seat engine gently. Practical tests show that given mixed use and no caning, such motors can survive the unleaded experience very well. The keynote here is light operation; easy work plus periods of idling. Definitely no towing, hill climbing or continuous high-speed running on unleaded, all of which will very quickly disappear susceptible valve seats.

Another small lifeline exists in the "lead memory" effect. While a motor burns leaded fuel, a thin protective film of lead compounds is laid down on the exhaust valve seats. On switching to unleaded this layer hangs around for a while, lasting for up to 12,000 miles if use is gentle but disappearing progressively faster with increasing thrashing. So a previously-leaded engine used now and again (and then only lightly) can linger for some while on memory alone.

One final big caution. Neither lead replacement petrol nor anti-wear additives are a complete substitute for leaded four-star. Although both will look after valve seats under normal speeds and loads, engines with particularly soft seats or poor local cooling can still suffer if worked really hard by heavy towing, foot-down cruising or, for a static unit, lengthy wide throttle openings. In such cases the only complete fix is to change the valve seats.


In the UK, leaded four-star is rated at 97 octane. Ordinary premium unleaded is 95 octane, so cant stand as much engine compression before potentially damaging detonation sets in. Alongside these staples, some outlets supply "Super" unleaded. Rated at 98 octane, this gets its detonation resistance from components other than lead and is safe in any engine requiring the octane value of four-star.

Bore new friends at parties with this – a fuels detonation resistance is judged on a scale of 1-100, corresponding to the percentage of the chemical iso-octane in an iso-octane/heptane mix. Knock point is found by using a variable-compression engine.

The big bang

IF a petrol engine is fed with fuel of too low octane rating, detonation (also called knocking, pinking or pinging) is likely. Imagine a piston rising on the compression stroke. On cue a spark starts the fuel/air mixture burning. If all is well a flame front moves steadily through the combustion chamber, burning the mix progressively and generating a controlled pressure rise in the chamber.

In detonation (above), both the advancing flame front and compression from the rising piston squeeze and heat unburnt fuel to the point that it explodes. Low octane fuel cant stand as much heat as stuff with a higher octane rating, so is more likely to detonate. Steep pressure rise produces the rattle of pinking, puts temperatures up, drags power down and gives the mechanical bits a very hard time. When this happens often, damage follows.

If the engines compression ratio is not so high that heat rise from compression is the sole cause of detonation (and if combustion chamber design is good) then retarding ignition timing can help. Holding back the spark until the piston is higher in the cylinder sets the mixture burning later, leaving less time for detonation. Slight mixture richening also helps.


Lead in four-star does two jobs:

&#8226 Boosts the fuels octane number. Higher octane rating fuels can stand more compression before detonating – burning in an uncontrolled way that can damage pistons, rings and valves. See box The Big Bang.

&#8226 Coats the exhaust valve seats with a cushioning layer. So it follows that the only engines that actually need four-star are those with a fairly high compression ratio (not very common in farming) and/or those with "soft" valve seats that need leads lubricating cushion.

The deepening


Exhaust valves run much hotter than inlets, as the latter are cooled by incoming charge. In an engine running on leaded petrol (A), a thin layer of lead oxides and sulphates is continually laid down on and eroded from the exhaust valve seats (1). This layer acts as a barrier between seat (2) and valve (3) so the two never actually touch.

The very low levels of lead in unleaded fuel dont give this protection. If the seat is soft – and valve inserts come in varying degrees of hardness from really soft upwards – the higher temperatures that come from increased load and speed eventually produce localised micro-welding. Small chunks of metal torn from the seat then embed in the valve, where they oxidise and harden. Every time the valve thwacks shut, these tiny hammers peck away at the seat and erode it more.

The result is valve seat recession (B). In the worst cases it destroys seats in a few thousand miles, though usually the process is slower – youll see it as tappet gaps closing up between services. Not only will the cylinder head turn into junk, but power drops as the valves ability to shift gas declines.

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