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"Widespread concern about the possible adverse effect of using unleaded petrol in vehicles having engines with cast iron cylinder blocks (side valve) or heads (OHV), has resulted in the execution of a detailed study of the scientific and engineering literature published in the USA and in Britain over the last 70 years or so. The study has covered all aspects of the phenomenon known as valve seat recession, which is said to occur when an engine valve 'sinks', under the pressure exerted by its own return spring, into the cylinder head or block. Much investigative work was carried out in 1970 and 1971 in America, when the introduction of exhaust emissions legislation first resulted in the use of exhaust catalyst systems. These devices are incompatible with leaded petrol and so unleaded petrol was re-introduced into the market in America at this time.
There is much evidence from reported scientific literature that valve seat recession occurs in cast iron engines operated continuously on unleaded petrol. Engine speeds and loads do not necessarily have to be high for recession to occur, although seat wear rates increase rapidly with engine speed. Wear rates of over 5 thousands of an inch per hour of operation have been reported at an engine speed of 4000rpm. When leaded petrol is burnt in an engine, lead compounds, mainly lead sulphate and lead oxide, are deposited in the combustion chamber. These compounds, laid down in the valve seat area, act as a solid lubricant, preventing metal to metal contact between valve and seat. As a result, serious valve seat wear with leaded petrol is almost unknown. By contrast, continuous use of unleaded pertol in cast iron engines allows metal to metal contact, with consequent adhesion between valve and seat. Particles of the seat material, removed by the valve as it opens, are oxidised by hot exhaust gases. Hard lumps of iron oxide form on the surface of the valve face; valve rotation grinds away at the valve seat, producing recession or 'shrinkage'.
Valve seat recession with unleaded petrol increase rapidly with engine speed. At sustained speed below 2000rpm, the risk of wear is low, while above 3500rpm, catastrophic wear can result. Valve rotation speed is closely linked with engine speed, and is seen as the primary factor influencing wear. Reduced wear rates are associated with a 30o valve seat angle, and lower compressed valve spring load. Sodium cooled valves may help in reducing recession. Hardfacing valve seats with Nimonic (80% nickel, 20% chromium) will reduce recession providing the composition of the hardfacing material is closely controlled.
There is ample evidence that exhaust valve seat recession was a problem in America during the nineteen twenties, being associated with truck and coach operation on the improved main highways, which developed rapidly during this period. Corrosion and wear resistant valve seat inserts were fitted by many American motor manufacturers as original equipment from about 1930 onwards. A contemporary record (1931) indicated that valve seat recession was much rarer in the twenties and thirties, because of inadequate road development which did not permit sustained high speed, and the predominance of small low powered cars of relatively poor performance. Manufacturers of heavy duty engines and quality cars frequently fitted valve seat inserts as original equipment.
Soluble fuel additives, usually based on organometallic compounds, can reduce valve seat recession in cast iron engines. At low treat rates compatible with exhaust catalyst operation, valve seat recession can still be a problem. Higher treat rates will give improved protection but may lead to unwanted side effects.
Phosphorus based fuel additives have been shown to offer satisfactory protection to valve seat recession in a range of engines and under most operating conditions. An additive of this type is in widespread use in New Zealand as an aftermarket additive for use at the petrol station forecourt, at the time of refuelling.
Treatment rates of between 20 and 50 parts per million minimum of lead in a soluble form in petrol, are necessary to protect exhaust valve seats from wear. Metallic pellets introduced into fuel tanks or lines are practically insoluble in petrol, and thus the negligible metal content of the petrol, which results from temporary contact with the pellets is too low to protect valve seats from wear. Permanent magnets attached to the outside of fuel lines through which petrol passes have no significant effect because hydrocarbon fuels are non-polar.
It is now known that from the first of January 2000, normal sales of leaded
petrol will cease in most of Europe, including the UK, as the result of the
EEC directive. Some European countries will apply for a concession to be
allowed to sell leaded fuel for a further 4 or 5 years, but the UK
Government is thought unlikely to apply to the EEC for such an extension. A
concession has been offered by the EEC to allow sales of leaded petrol for
minority interest groups. This concession would allow sales up to a maximum
of 0.5% of total fuel sales. However, it is not known how this small amount
of leaded petrol would be made available or distributed. Overall, there will
in practice be almost no leaded petrol available for road vehicles after the
end of 1999. Leaded petrol will probably be harder to find from about the
middle of 1999, as petrol station pumps, tanks and other dispensing items
are progressively prepared for the supply of other fuels.
A British Standard specification is being drawn up for a lead replacement petrol containing valve seat recession additives, but there are concerns about the level of protection which this product will offer. Also, it is not known to what extent this product will be available.
The best solution to the problem if valve seat recession is to fit hardened exhaust valve seat inserts. Millions of vehicles world-wide have successfully used such inserts to permit satisfactory continuous operation with unleaded petrol. Insert materials must possess good hardness and corrosion resistance to operate satisfactorily. Inserts made of materials rich in cobalt, tungsten and chromium have given way in recent years to lower alloy content iron-based sintered powder metallurgy materials. There are of lower cost, and the sintered matrix can be infiltrated with copper to give improved heat transfer and to provide a solid lubricant effect.
Valve seat inserts can be fitted to older cars to prevent recession with unleaded petrol. Fitting inserts is a skilled engineering job, but there are companies who specialise in this type of work, and who can demonstrate a successful record of valve seat modifications. The work demands engine dismantling, and in some engine types, the combination of long stroke, small cylinder bore, and fixed head design may prevent the fitting of valve seat inserts.
From January 2000, owners unable or unwilling to fit valve seat inserts to vulnerable engines are likely to consider the use of a fuel additive for protection against valve seat recession. Additives have been successfully used in several countries. Work is currently in hand to establish the level of protection provided by various additives or devices.
By 1999, additive solutions offering a known level or protection will be available. Steps are being taken to ensure that suitable products, to be added during refuelling, will be on sale at petrol stations, when leaded petrol is no longer widely available."
I urge Octagon members to get their 'heads (cylinder) looked at' very soon,
before the estimated 270,000 people concerned with running older vehicles
suddenly want machining work carried out. The demand could well outstrip
the supply of experienced engineering companies, causing higher prices and
lower quality of work. I am not going to trust to additives or catalysts for
my XPAG. My head is being seen to with a slightly higher compression ratio,
nice hard (and big) new valves and seats, a bit of porting and matching to
the inlet manifold and the waterways bead blasted. Should be OK and give the
TC a bit more 'oomph'.
Steve Taylor, Sutton Coldfield.