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A guide to unleaded additives

Running the Risk Published: 18th May 2011 - 0 Comments - Be the first, contribute now!

A guide to unleaded additives
A guide to unleaded additives Lead is dead - and so will your engine be if you don’t use an additive urge the experts. But more of us are disregarding such advice
A guide to unleaded additives Pour in additives in decline as motorists take a chance to save money - but is it a risk worth taking if it can cause piston meltdown?
A guide to unleaded additives Millers produces varied range; VSL can up a fuel’s octane rating to racing quality
A guide to unleaded additives Fitting hardened valve seats doesn’t automatically mean that you can use straight unleaded - but many think it does
A guide to unleaded additives There’s a big choice; from lead replacements and octane boosters to fuel system cleaners and now cat cleaners
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Are you one of the increasing tribe of classic car owners taking a chance and not using a lead replacement additive anymore? Then even if an unleaded cylinder head has been fitted, you are in danger of an expensive engine meltdown!

If you’ve gone to the expense and trouble of having your classic fitted with a modified cylinder head so it will safely run on unleaded, then don’t feel too smug about it: you probably still need to use a fuel additive to avoid an expensive engine blow up! Yes that’s right, just because you have hardened valve seats fitted it is not automatically okay to use unleaded petrol. Ever since leaded fuel was phased out, owners of older cars were worried how they could safely run their cars and were faced with two options: either have the engine converted to accept lead free petrol or use a dedicated additive which acts as a lead substitute. Trouble is, carrying out the former has given many classic car owners a false sense of security – worse still some enthusiasts still had their modified engines expensively fail on them. Why?

Lead or dead?

Well, according to one fuel expert, Millers Oils, classic car owners were mainly concerned with the problems of valve seat recession where the lack of lead caused the valves to hammer themselves into their seating. However Millers says this has actually has been far less a problem than the industry initially predicted, possibly due to the relatively low mileages covered by the average classic car user.But this preoccupation with valve recession led many to overlook the danger of burning a fuel with an inferior octane rating to that which your car was designed for, and so promotes potentially fatal detonation as a result. Lead - in the form of Tetra Ethyl Lead (TEL) - was initially developed for use in aircraft engines as a means of gaining large increases in power output without destroying the enginesthrough detonation or pre-ignition (pinking).When the leaded fuel was made available for road use, it was soon found that vehicles using Ethyl petrol did not need their valve clearances adjusting as much as those running onnormal petrol did. That is why many motorists today believe that lead’s only use in petrolwas for such valve seat lubrication, and why they assume that putting hardened seats in means you can safely run unleaded petrol. This is not so, particularly on higher higher-powered engines that are not so tolerant of low grade petrol such as Lotus Elans, Rover 2000 TCs,Triumph Dolomite Sprints and many late 80s turbocharged engines such as the Ford RS Turbo CVH unit. Here’s another myth exploded. Over the past 20 years octane ratings have gradually been eroded; remember good old Five Star had an octane rating of 101 - and you can’t beat a dollop of lead! Contrary to popular opinion,a good number of older engines will not run as well on 98 octane unleaded (and even the new 99 octane as introduced by supermarket giant Tesco) as they will on 98 octane leaded, even though it supposedly has the same octanenumber rating, claims Millers Oils. This is due to the differing efficiency of burn by the shape of the combustion chamber, which is controlled in part by the design of the cylinder head. Simply put, unleaded fuel burns less smoothly than leaded and the toluene and benzene, which is added to the fuel to act as a lead substitute, burns to form what are called peroxides. Unfortunately peroxides are inherently unstable and explode, forming shock waves that cause detonation. This explains why engines designed to run on unleaded from the outset feature sophisticated knock sensors, which retard the ignition setting at the onset of harmful detonation. Older engines did not have the luxury of such knock sensors and relied on one of the properties of lead to prevent pre-ignition and subsequent detonation - as lead burns, it forms oxides and these slow down the rate of burn and prevent detonation, oxides also lubricate the valves.Hence the ‘burn’ of leaded fuel is much less aggressive than unleaded and the risk of detonation is considerably reduced.

What should you do?

With petrol costing £1 a litre last year, it’s understandable that many classic car owners counted their pennies and refrained from using an additive, which typically cost between £5-12 per tankful according to product used. Also with the additive market on the wane, their words of caution have to be tempered with their vested interests, especially considering many marquespecialists don’t heartily recommend using lead replacement additives anymore. The real problems lie with the public’s education; the majority of older car owners believing the real worry of the lack of lead was potential valve seat smash up rather than piston burn out. Detonation damage is due to a variety of factors and not simply due to low-grade fuel. Heat and load are large contributory factors as well and classic car owners should always ensure that their car’s cooling system is in tip-top shape to keep the engine as cool as possible. As for load, continuous unsympathetic hard driving will cause harm (to any car!), but we wonder how many classic car lovers thrash their collectibles mercilessly.

Tune talk

Of course, if pinking is a problem (and remember sometimes preignition can go unheard and so undetected), then retarding the ignition is an effective cure – albeit with drawbacks. For one thing, performance will suffer and for another, too much retardation will cause hotter running which is itself a pre-ignition promoter. What you should do is ensure that the distributor isn’t worn. They were never the most sophisticated bit of equipment at best and when wear occurs, it can lead to timing scatter and compound any pinking or running on problems. It also means you’ll never be able to time the engine properly. One answer could be to have a modified cylinder head made up to your requirements. Apart from fitting hardened valve seat inserts, you could also specify that the combustion chambers are gasflowed at the same time, which usually involves removing some of the metal around the valve seats. This reduces the compression ratio making the engine more tolerant to lower octane, yet because the head is more efficient overall performance won’t suffer either. It’s an expensive way to go about things however. Given that valve seat recession isn’t the engine killer everybody expected it to be, where does this leave ‘unleaded’ cylinder head conversions, given that youprobably still need to use an additive anyway? Well, if the engine is being rebuilt, then as part of the cylinder head overhaul it still makes sense to have hardened inserts installed - for no other reason that it’s another worry off your mind if you run out of an additive at some point. But if your engine is in good shape, then the benefits of deliberately carrying out a head job are not so clear-cut anymore. The choice is yours, butcertainly the argument for using fuel additives are as strong now as they’ve ever been.

Classic car expert Kim Henson give his views…

I was seriously concerned about the large-scale disappearance of ‘proper’ leaded petrol from our forecourts, as I have a number of classics which are used regularly and in total cover a fairly high annual mileage. I chose to run with an FBHVC-approved additive, although I also had several ‘spare’ cylinder heads modified for my cars for use with unleaded petrol, to fit as and when any valve problems arose. To date since then, I have not experienced ANY valve/seat problems, and the spare heads are still ready and waiting to be fitted - some time! In fact the only head I have changed is on my Austin A60 Cambridge; this was due to a head gasket weep, not to valve seat problems. All the same, I checked the valve clearances and compression readings just before I removed the standard (unmodified) head, and found them to be the same as readings I had taken some years previously, just before the general demise of leaded/Four Star fuel. This was heartening, as I have used my A60 for long, fast trips, often heavily laden, including runs to the hotter parts of France, for example. These are the circumstances in which valve seat recession is most likely. When I removed the head to install a new gasket, I took the opportunity to fit an unleaded head at the same time. This has also been running perfectly ever since. I reckon the best long-term solution is to have the engine modified with more resilient exhaust valves, exhaust valve seats and valve guides. For pre-war cars with side-valve engines, the cost of valve/seat modification work can be huge, so for my own pre-war cars, I shall continue to use Superblend, of which I am particularly impressed with.

Additive answers

Lead replacement, more power, lower emissions, improved economy, longer engine life and more - they are all there in a bottle to be simply poured into your fuel tank! The fuel additive market is massive and while sales of straight lead replacement concentrates are falling, sales of other enhancers are bucking the trend thanks to increasing interest from the hardcore enthusiast sector. For example, apart from normal lead substitute and octane booster additives, there are also motorsport-based alternatives, which are more heavy duty to cope with hotter running temperatures and increased loads. Millers CVL range complies with MSA and FIA motorsport regulations and - in the case ofCVL Turbo octane booster - increases the rating by a claimed four octane (40 points) and can make the likes of a top quality road fuel such as Shell Optimax (largely regarded in the industry as the top road brew around, incidentally) as potent as race pump petrol and ideal for sport or track day use. Fuel system purge/cleaner additives largely came about as a switch to petrol injection became universal. Due to their finer working tolerances, their inner workings are more prone to becoming gummed up and tarnished. By and large they all work to varying degrees and it’s well worth giving an EFIengine an occasional birthday - and a wonder additive may just ensure that your car passes the annual MoT emissions test at the first go.

Accept These Substitutes

The following additives have been tested as being effective against valve seat recession, and are approved for use by the Federation of British Historic Vehicle Clubs

Tetrosyl Automotive

Carlube Lead Substitute (supersedes potassium-based Nitrox 4 Star Lead Treatment and manganese-based 4 Lead Substitute and Octane improver). This additive is said to prevent engine knocking and pinking, in addition to preventing power loss (due to valve seat recession) in older vehicles. Tetrosyl Automotive, Bevis Green,Walmersley, Bury, Lancashire, BL9 6RE. Tel. 0161 764 5981.Website:


Castrol Valvemaster and Valvemaster Plus Incorporating phosphorus. Castrol’s Valvemaster is claimed to protect engines under all driving conditions, to help keep the fuel system clean and to provide excellent corrosion protection. In addition, Valvemaster Plus incorporates an octane booster for engines running with relatively high compression ratios. Castrol,Wakefield House, Cambridge, CB4 5QZ. Tel. 01954 231668.Website:

G. T. Autochemilube

GTA Power Plus Formula 2000. A potassium-based additive. GTA is now supplier of Castrol products. G. T. Autochemilube Ltd., Unit 4, Riverside Business Park, Tramway Road, Banbury, Oxfordshire, OX16 5TE. Tel. 01295 272799.Website:

Millers Oils

Millers produces a wide variety of petrol additives for road and track use. Millers VSPPlus is a manganese-based additive, which benefits from the inclusion of an octane booster, raising the octane number of the fuel by two (so, for example, 95 octane fuel is effectively rendered 97 octane by the use of VSP Plus). New launches include CVL and CVL Turbo; lead replacement and octane booster additives aimed at the enthusiast for fast road, track day and motorsport use which, when combined with Shell Optimax, are claimed to have the same values as racing fuel. Millers Oils Ltd., Brighouse,West Yorkshire, HD6 3DP. Tel. 01484 713201. Website:

Delta Oil

Red Line Lead Substitute Employing sodium to deliver the goods, Red Line Lead Substitute is said to clean intake systems, carburettors and injectors in addition to preventing valve seat recession. It was developed in the U.S.A. Delta Oil Ltd., Unit F1, Market Overton Industrial Estate, Market Overton, Rutland, LE15 7PP. Tel. 01572 768311.Website:


Superblend Zero Lead 2000 relies on potassium to prevent valve seat recession. The dilution rate of the latest version (in terms of the ratio of Superblend to petrol) is lower than its predecessor, meaning that less of the product is required for a given volume of fuel; in other words, it is now cheaper to use! Superblend Lubricants Ltd., Unit 2, Feldspare Close, The Warrens Industrial Park, Enderby, Leicester, LE9 5SD. Tel. 0116 286 1001.Website: In addition to the products tested and approved by the FBVHC, there are many other products available which claim to reduce/eliminate valve seat recession. Just one is the manganese-based Redex Lead Replacement Octane Booster. Check out STP products too

Tesco Tops the Lot

Tesco has recently introduced a new, higher octane (99 RON) Super Unleaded fuel, typically two octane numbers higher than the usual ‘Super Unleaded’ petrol (97 Ron), and higher than Shell’s Optimax (98 Ron). It is important to appreciate that this is NOT a lead substitute petrol, and is not intended to tackle the issue of valve seat recession. Rather, it may be more suitable for cars (including classics) with engines running with relatively high compression ratios, which can struggle on standard (95 RON) unleaded petrol; ‘pinking’/detonation and possible piston damage can be a worry with such motors running on standard 95 RON unleaded. Initial assessment by enthusiasts has indicated good results with the new Tesco fuel it is claimed (There is an interesting review of the fuel and a forum relating to it on

How does your fuel rate? Fuel Ron Rating

Premium 95: 95.0
Premium 95 + Octane Plus: 97.1
Premium 95 + Competitive: 97.1
Shell Optimax®: 98.3
Shell Optimax® + Octane Plus: 99.4
Shell Optimax® + CVL: 100.8
Shell Optimax + CVL Turbo: 101.4

The FBHVC says

Jim Whyman, secretary for the organisation, told Classic Cars For Sale the FBHVC has no specific policy on this matter as each enthusiast will have different circumstances which have to be taken into account. However, he added that this is a serious matter as the volume of sales of leaded fuel is dwindling drastically. This could eventually have implications in terms of supply… He went on: “Those who use engines that can be fitted with hardened valves and seats at reasonable cost might just as well save the cost of additives, or leaded petrol, and put what they save towards the cost of converting the engine in due course when the existing valves eventually give out. But there is no point in converting the engine unless it becomes necessary - and that may never happen if the vehicle concerned is not used much, or only ever on a light throttle when the valves may last for years. On the other hand, those with engines that either cannot be fitted with hardened valves/seats at all, or cannot be converted economically, would be wise to spend the extra on additives to extend the life of their engines.”

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