Technical Supplement No BKTSA001 - Unleaded Fuel

The Rover P4 Drivers Guild make every effort to ensure that the information and advice published is reliable and make proper reference to safety procedures. We are unable to accept responsibility for injury, damage or loss resulting from omission or error.

Workshop Manual Reference

Section A Rover 60,75,90 & 105 models

Section A-1 Rover 80 model

Section A-2 Rover 95,100 & 110 models

Compiled: Barry Kensett

This revision 22 December 1999


The U.K. adopted European legislation to withdraw sales of leaded fuel with effect from 1st January 2000 although in practise it was disappearing from filling station forecourts from mid-1999. Small quantities of leaded fuel are theoretically available for motor sport and aviation use but in practise there will be no reliable accessibility for classic car owners and alternative solutions must be found to keep cars running without incurring engine damage. The removal of leaded fuel is prompted by the "green lobby" coupled with some medical views that leaded fuel can release health hazardous substances into the atmosphere. The is no published evidence that the chemicals in unleaded fuel or lead replacement additives are less harmful than lead.

This supplement examines the issues as they relate to the Rover P4 cars and summarises the current recommendations of the Rover P4 Drivers Guild.

The Technical Problem

Lead (or more correctly Tetra-Ethyl Lead) was introduced into fuel in the 1920's as a means of raising the octane rating of fuel to enable higher compression ratios, and hence higher power outputs, to be used prior to the onset of detonation (pinking). As engine performances and operating temperatures increased, a secondary benefit of lead was utilised in engine design; this was the coating properties which provided a form of lubrication to the hot elements of the exhaust valves, principally the seats but to a lesser extent the stems and guides.

With regard to the seats, the problem can be visualised by observing the welding techniques of the traditional blacksmith who heated iron to red heat and then hammered two pieces together which formed a weld - this was before the use of oxy-acetylene techniques which fuses molten metal. The exhaust valves can have a tendency to weld themselves to their seats under high temperatures present in the combustion chamber. When the valve is lifted it is the usually softer material of the cylinder block which is torn away and either embeds itself in the valve or gets burnt away and discharged with the exhaust. Continued running in this condition destroys the valve seats and causes them to sink into the block - a process known as valve seat recession. This recession is not uniform and the seal is progressively lost leading to a leakage of hot gas which then rapidly erodes the valve. The process is accelerated if the valve clearances are not relaxed as the seats retract which cause the valves to be held off of their seats which accelerates the erosion.

The process is dependent on temperature at the local seat areas of the valves and it is unlikely that inlet valves will suffer from this problem due to the cooling nature of the incoming fuel/air mixture.

When using leaded fuel, a coating of lead was deposited on the seats and valve heads which formed an effective non-stick coating although if the engine was operated at high loads this could still be burnt away and there have been particular models of cars which needed better valves and seats than the norm for the time - in the P4 range only the 110 model was susceptible and was fitted in production with a hard valve seating face.

What then can be done to continue running the cars ? There is a lot of hype, commercial marketing, bar talk and panic around the issue not helped by the lack of sound information and guidance from successive governments which enacted the legislation and caused the problem for the classic car owner.

The first advice is don't panic ! Because of poor control over the replacement of leaded petrol with unleaded, you are probably running on unleaded petrol already. Trading standards officials have been overwhelmed by complaints of fuel being sold under false descriptions. Fortunately the lead coating takes some time to erode and remains effective for a time which depends on the way the car is operated and maintained but could be a year or more. The first option then is to do nothing. The options then develop along the lines of reducing operating temperatures, adding chemicals to petrol to simulate lead or buy petrol with additives included, using different materials for valves, using different materials for seats, using different materials for guides, Whatever option is selected it is prudent to monitor the condition of the engine.

Referring specifically to the Rover P4 range, the problem can be broken down into two categories, the Rover 80 and the rest (the six cylinder cars). The primary differences are the overhead valve engines of the 80 range where the valves seat directly into the head casting which is an arrangement typical for most classic cars and will probably suffer similar problems and have available similar solutions. The six-cylinder cars have inlet valves closing onto hardened inserts in the aluminium cylinder head which should pose no problem at all but the exhaust valves close onto similar hardened seats in the block which is a better situation than the 80 but not as good as modern cars designed for unleaded fuel and which may, or may not be a problem.

In addition to valve seat recession, we must also review the position on octane ratings as standard unleaded fuel is 95 octane whereas 4 star leaded was recently 97 octane. High octane unleaded is available at 97 octane but the supply position in the future is indeterminate. Use of low octane fuel without engine adjustment or octane lifting additives can give rise to detonation in the combustion chamber rather than steady burning. this detonation is heard as pinking and can be very damaging to engines, particularly it can cause piston failure.

The solutions in more detail

Reducing operating temperatures

Reducing operating temperatures will reduce or delay the onset of valve seat damage. This can be achieved by reducing the combustion temperatures or improving the cooling around the valve seats. This means not driving the car at high power levels and making sure that the cooling system is well maintained. Fortunately not many P4 owners now use their maximum performance and keeping the power down is not a serious problem. It is recommended that prolonged use above 3000 rpm and large throttle openings at lower speeds (which could result from towing for instance) be avoided; if this is not possible then engine or fuel modification should be considered. The engine speed quoted gives different road speeds depending on gearing but is typically around 60 mph. Owners who have fitted overdrive retrospectively to high axle ratio cars will need to watch their throttle settings on motorway upgrades. The cooling system should be carefully cleaned out remembering that particularly on the six-cylinder cars there is a buildup of sludge around the rear cylinders due to the angle of the engine as installed. This is also the hot end of the engine due to the shielding of the front cylinders. To properly clean these engines it is recommended that the water pump and water distribution pipe is removed and wire poked into every available orifice to disturb and flush out the sludge (it could be helpful to remove the core plugs for access). Continued use of high concentrations of anti-freeze in accordance with the manuals will reduce the rate of corrosion and coupled with annual flushings using a mild agent should keep the waterways clear. The 80 block is less complex than the other cars but similar flushing should be adopted.

Adding chemicals to fuel

There are a number of additives on the market which claim to replace lead to avoid valve seat recession and/or lift octane rating. The two main types offered are pellets which you dangle in your petrol tank or put into the fuel lines and chemicals which you add to fuel. Faced with this proliferation and no independent testing to provide guidance to owners, the Federation of Historic British Vehicle Clubs (FHBVC) commissioned some tests at the Motor Industry Research Association (MIRA) using A-series BMC engines run for the equivalent of 5000 miles under fairly high operating conditions. The cylinder blocks were especially commissioned from Rover to replicate the grade of cast iron used in the 1960's as current production used a hardening technique around the seats to enable the use of unleaded. The suppliers of the products were invited to submit samples for test which would be at their expense. Some suppliers did not submit their products leaving us to draw our own conclusions. Of those submitted, only five products passed the initial tests although some are still being being submitted and tested and at the time of writing (November 1999) the following fuels have passed the criteria set for the test:

Lead substitutes

Carplan Nitrox 4-star (potassium based)

Castrol Valvemaster (phosphorus based)

Redline Lead Substitute (sodium based)

Superblend Zero Lead 2000 (potassium based)

Octane boosted additives

Castrol Valvemaster Plus (phosphorus based)

Millers VSP Plus (manganese based)

Carplan Nitrox 4-Lead (manganese based)

None of the fuel catalysts which you dangle in your fuel passed the tests. Neither did dangling my granny's false teeth in the tank although granny said that she kept the Spitfires going in the Battle of Britain that way. The manufacturers whose products were not submitted for test or which failed predictably try to rubbish the tests but they are the best we have got so far and we support the conclusions.

The significance of the base of the additives is that they cannot be randomly mixed to maintain engine protection. If you choose to use an additive you should select one and stick to it.

Lead Replacement Petrol

LRP has replaced 4-star leaded in the pumps and you might think there has been no change. Careful attention will reveal that the British Standard 4040 reference does not apply to LRP although many pumps have not had it removed hence the complaints to Trading Standards. LRP is supposedly the same base across all petrol suppliers (potassium) and if this remains so can be safely mixed. The dosage however is below that recommended by the expert chemists and that recommended by the additive manufacturers. It has not been submitted to independent testing and at the dosage levels supplied may not be effective. The octane rating is 97 which should avoid need for retuning for P4 cars. The manufacturers are supposed to provide information at the forecourts but this is rarely available. We have seen some of the literature and there is a caution with regard to high duty usage so we can presume that LRP is not as effective as lead. The attitude of the fuel companies has been very offhand so far but there may be an opportunity for a niche supplier to step in and provide a better service. There is a view that the petrol companies will phase out LRP and Super Unleaded in due course and perhaps we should concentrate on living with standard unleaded, with or without an additive.

Engine Modifications

As stated above, it may not be necessary to modify the six-cylinder cars. The exhaust valve seats are marginally hard enough if the temperatures are kept down. Many P4's are believed to be running on their original seats which should be carefully inspected for cracking, particularly if the has been an instance of valve failure. The seats are available and can be replaced, the technique is in the repair manual and it is hoped to provide the results of a more detailed workshop on the topic in the next few months. The valves were not all of the same material when the cars were built and unless the car is of particularly low mileage it is unlikely that the original valves are still fitted. We therefore cannot be too clear on what standard of valves are fitted to our cars. New valves in austenitic steel are available and should be fitted if there are signs of damage when using unleaded fuel. They should be considered for replacement if the head is removed for other maintenance as a precautionary measure. Harder valves were originally fitted to the 110 model although they may have been replaced subsequently and are still an unknown quantity.

On the 80 model which does not have inserts it is recommended that the cylinder head be modified to fit inserts and harder valves, the parts are readily available.

The timing of modifications, if needed is not necessarily critical. If valve seat recession is taking place or the valve head is being eroded, the valve clearances will tighten up. If this condition is ignored it will cause early failure of the valve as it will be held off the seat. It is recommended, therefore, that the valve clearances be subjected to a six month cycle of checking. If they are not closing up then recession is not occurring. If the clearances are continually closing then engine modification should be planned into the maintenance cycle.

If it is decided to run on lower octane fuel (i.e. standard unleaded or additive without octane booster) then it will be necessary to retard the ignition to eliminate pinking and the consequent engine damage. We do not yet have the results of extended experience but it would appear that retarding ignition by two degrees should be adequate. Lowering the compression ratio could be considered but is unlikely to be necessary unless octane ratings drop still further. (Some Rover P6 models which were designed to run on 100 octane have a bigger problem).

There are a couple of other potential snags with unleaded fuel. The lubrication provided by lead to to hot end of the valve guides is removed and there could be problems of wear or sticking. We believe that this is not a major issue for the P4. There is also a potential problem with the chemistry of fuel attacking nitrile rubber seals in the fuel system and this should be monitored. There is also evidence of owners using unleaded for the first time getting fuel system problems due to dirt - perhaps unleaded is a bit more aggressive in flushing out the dirt of ages.

To summarise, there are a number of options to address the unleaded fuel issue and owners will choose those which give them the best peace of mind. Our recommendation is not to panic, to use unleaded fuel with the ignition retarded as necessary. Monitor the exhaust valve clearances and if damage occurs on six cylinder cars to remove the head, check that the seats are not cracked and replace if necessary, fit harder valves and replace guides if necessary. Keep speeds and loads down and ensure cooling system is clean. On 80 models, monitor clearances as above and if damage is occurring fit hard seats and valves.

A number of Guild members are already running on unleaded fuel and some have been doing so for a considerable time. This page will be updated if we have anything new to report on the topic and we recommend you check regularly - note the revision date at the head of this document.








ęB.Kensett 2014