Home Introduction Buyers Guide Dreams Latest Fame For Sale Capri Forum History Jobs My Capris News Archive Pictures Q & A Rant & Rave 2016 Restoration Service Technical Unleaded
The Ford Capri Laser Page

A word from the Automobile Association...

I thought the following details released by the AA may be of interest to other Capri owners (written circa 1999 but the information is up to date as of Jan 2021):-


by The AA Technical Information Centre.

So leaded petrol is a thing of the past. Most people agree that on balance, this is for the general good, but what are the possible problems?

It was in the 1920's that a lateral thinking named Midgley discovered the benefits of lead compounds as fuel additive. Engine efficiency was severely limited by the fuels then available as they tended to burn too rapidly in the cylinder. The treatment allowed the development of higher 'octane number' fuel, up from 60 to perhaps 110 in the end. At the time, there was no consideration given to the affect of the lead compounds on the engine valve seats, it was only found later that lead had the property of protecting them.

There are therefore two possible problems with pre-1988 cars, designed to run on leaded petrol, if they are to be run on Premium Unleaded.

Firstly, there will be a loss of about two 'octane numbers' from Four-Star Leaded. With most cars this will barely be noticed: usually the ignition timing can be re-set to avoid any tendency to pre-ignition or pinking, and there are engine tuning books that show the correct settings. A check of the ignition timing is part of the routine service schedule, so the extra cost of re-setting should not be great. Clear instructions should be given to the garage when booking in for service.

Secondly, in the case of an engine with the valve seats cut directly in a cast iron cylinder block (as used to be the normal practice), the loss of the protective affect of the lead compounds means that under conditions of hard, high speed use, erosion of the seats can lead to valve seat recession. If the seats recede by more than pre-set valve clearance, the valves will overheat and severe damage may result.

The use of 'Super Unleaded' petrol can overcome problems due to the reduced octane value of Premium Unleaded, in the case of those cars that cannot easily be re-tuned to suit. However, it does not address the problem of valve seat recession.

The long-term answer to valve seat recession is to have hard alloy valve seat inserts fitted into the cylinder head. If the car is to be kept for a long time, as a classic or as an old family friend, sooner or later the valve seats will need attention anyway, so bringing this work forward will be worthwhile. It may even save money, as you can use the cheaper fuel. In the case of many popular cars, an exchange head may be available, or a specialist machine shop will be able to do the work on most engines: one-make car clubs might be able to advise where best to go. For the short term, owners of a 'leaded only' engine have the choice of:

(a) Dosing the fuel with proprietary branded additive, or

(b) Seeing what happens on unleaded.

Of those options, (a) is probably the favourite. It is really not possible to say that valve seat recession cannot happen with these additives, the problem being that there's an enormous range of engines out there, and they can be in any condition from pristine to near collapse. However, the use of fuels with the additive blended by the fuel supplier is probably more reliable than adding it yourself, as mixing and dose rates can be quite difficult to get right in small amounts. Option (b) is not completely daft, in that driven gently for limited mileage, the valve clearances will not be taken up between services, so with reasonable care no harm should result provided you know what to look for. If compression is lost on a cylinder, or the valve clearances disappear, then something will have to be done.

These comments will cover probably 95% of the older cars on (or temporarily off) the roads in the UK. There are unfortunately going to be some exceptions though, which may need special thought. For a start there are some specialist high performance cars that need high octane fuel. These vehicles were made in the days of 100 octane pump fuels, but are getting by at present on 97 octane leaded. Going to 95 octane unleaded just could cause trouble, even with the ignition retarded. In such cases, the long term option is to lower the compression ratio, at some power loss, but with benefit to engine life. Usually a new set of pistons can achieve this, again, engine specialists can advise. There are some proprietary 'octane boosting' fuel additives, but the affect of these on different fuels can be unpredictable.

Some engines will present particular problems in fitting hardened valve seat inserts, either because the seats are very close together. If inserts cannot be installed, you are back to option (a) or (b) above.

There are advertised fuel-line or fuel tank devices that claim to allow use of' unleaded petrol in otherwise unconvertible cars. We have not been able to establish the science behind these claims, and hence do not recommend their use.


At present we are aware of a couple of possible sources of proprietary lead-replacement additives for 'DIY' dosing of unleaded petrol. This sort of product has been in use in Germany, Austria and Switzerland for some years with no reports of serious problems, though there could be reservations about the effect on turbo-charged cars. Other sources will become available before leaded petrol disappears.

1) Castrol Valvemaster additive:  this has been used by several Capri owners who report that it works extremely well. Treats 250 litres of fuel at a cost of approx. 7p per litre. (2021 figures).  Note that there are two types: Valvemaster & Valvemaster Plus.  The later provides an octane boost to unleaded petrol to counter pinking and knocking and costs approx. 15p per litre (2021 figures)

2) Millers VSP Fuel Treatment.  Treats 40 litres of fuel at a cost of approx 62p per litre (2021 figures)

3) Redex 4-star Substitute.  This is the additive that I use in my Capri as it is cheap and readily available from petrol stations and Amazon. Treats 160 litres of fuel at a cost of approx. 4p per litre (2021 figures)

Previous Up Next

This web page is owned

and operated by Mark Swetnam

Last updated 26/10/21
©1995 to 2021

E-mail: Mark@Swetnam.co.uk

Keeping 'The Legend Alive' on the World Wide Web since 1995!