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This a modified and updated article originally appeared in Issue 123 (June 1999), part of our Unleaded Survival Special; our advice on coping with life after unleaded...


The Pre-War Car

IT'S OFTEN SAID, with some justification, that pre-war cars weren't designed for leaded petrol so they don't need it. Tetra-ethyl lead was only blended into selected fuels in those days (marketed as Ethyl or 'knockless' grades), and only high performance cars used it. So the quality of the materials chosen for pre-war engines was not compromised in the knowledge that lead would provide protection. (However, that doesn't mean that they will survive as well today on unleaded as they have done on 4-star.) Specially hardened valve seats were rarely fitted, though old head castings do tend to be pretty tough.

Owners of pre-war and early post-war cars, who drive them gently and infrequently, can probably survive the lead-ban without taking any special precautions at all. As far as octane levels are concerned, the problem may be that ordinary Premium Unleaded actually has too high a rating, and burns too hot for some low-compression, long-stroke engines. The addition of a little paraffin (kerosene) can help. Its use as an octane depressant has been agreed by HM Customs & Excise, and you should start experimenting with around one pint to four gallons. The only possible worry is that excess, unvaporised paraffin might wash down the bores and dilute the oil - so don't overdo it, and keep checking the oil's viscosity on the dip-stick.

Older performance cars - or any others which may get driven in a enthusiastic manner, over longer distances - would benefit from an anti-wear additive.

The Fifties Car

By the mid-fifties, fuel quality was improving and compression ratios were increased accordingly. But even so, the vast majority of these classics can run on 95-octane without any adjustments. Valve seats are unhardened, though, and an anti-wear additive will be needed. Valve clearances should be checked regularly if the car is driven at more than 3,000rpm for long periods - once they start tightening up it's time to think about seat inserts.

Sixties/Seventies Cars

Classics of the sixties and seventies have the most to lose with the banning of 4-star. The 'get-away' sixties was when octane ratings really started to climb - culminating in the 101-octane Super blends. Most of these cars, therefore, will have compression ratios of at least 8.5:1 and require a minimum of 97-octane fuel. (Many, indeed, were designed for 100-octane, but these will already have been detuned.) They also have the softest valve seats.

Looking at octane requirements first, the choice is to detune or not to detune. Detuning (by retarding the ignition) will allow the use of 95-octane, but will reduce performance and may cause overheating (thereby reducing the effectiveness of an AWA) unless the compression ratio is also reduced. Not detuning will mean that 97-octane is still needed. This will be the rating of LRP, but using LRP means that you are letting the petrol company choose the additive type for you. Or you could use Super Unleaded with or without an additive - assuming that it's still available. (There are rumours that some companies will drop it in favour of LRP, which would be a truly devastating blow.)


Octane boosters

The rating of 95-octane Premium Unleaded can be raised by using a manganese additive or, as our own testing appears to show, by fitting a fuel catalyst. In both cases, the makers claim an octane boost of around two points. The advantage of the fuel catalyst approach is that you can choose whichever AWA you like. (Note: CMM strongly recommends the use of an AWA, in addition to a fuel catalyst, in engines with soft valve seats - our advice is that you do NOT use a catalyst alone to prevent VSR. You must bear in mind that, other than anecdotal evidence, there is absolutely no proof that catalysts prevent valve seat recession on their own. There is evidence, though, that they can raise octane ratings, reduce noxious emissions and, to a lesser degree, improve mpg.)

Valve Seats

Classics that are driven fast, regularly, will eventually need hardened exhaust seats - as will those that pull heavy trailers. But the job can be delayed until other engine work is required or until valve trouble becomes evident. Meanwhile, use LRP or a good AWA - and check cylinder compressions and/or valve clearances every few thousand miles.

If your engine was professionally rebuilt within the last few years, it may already have hardened seats - ask the reconditioner. Every alloy head has inserted valve seats, but these will not necessarily be tough enough for unleaded. Rolls-Royce have always used hard seat materials - and that's true of engines they supplied to BMC for the Princess 4-litre R.

Different Strokes

Engines with side valves or an 'inlet-over-exhaust' arrangement (early Land-Rovers, for instance) have their exhaust valves in the block - fitting inserts involves more dismantling, plus the transportation of a much heavier casting.

Rolls-Royce formerly used IOE engines, but the company has always fitted hardened valve seats - and that's true of the engines they supplied to BMC for the Princess 4-litre R. Obviously, engines without poppet-valves can't suffer recession - so sleeve-valved engines, rotaries and 2-strokes are in the clear. Rotaries or high performance 2-strokes that are tuned for 4-star can use LRP purely for its octane value.

Turbocharged classics form a more worrying category. The SAAB 99-Turbo was the first, series production, European turbo and it has soft valve seats - as do early 900-Turbos. Hardened inserts can be fitted though, and SAAB advises that this be done (you'd be crazy not to.) But, like certain Volvo turbos, these engines need 97/98-octane and can't be detuned for Premium Unleaded. Ideally, if you can get it, use Super Unleaded. If you are forced to use LRP, only buy from a major fuel company and always stick to the same brand - the turbocharger itself may otherwise be at risk.

The use of a manganese substitute, to boost the octane-level of Premium Unleaded, can't yet be recommended for turbo engines as no test information is available. (Although a paper by Howard Stott, of the Corrosion & Protection Centre, UMIST, Manchester, suggests that manganese is unlikely to damage them.)

If you do own a turbocharged classic, it would be wise to check the exact position both with the car's manufacturer and the relevant LRP/additive supplier.

David Landers

E-mail CMM!What is YOUR opinion on this controversial subject? E-mail your views to us at Or you can contact The UK Department of the Environment Transport and the Regions direct - from their website you can email your views to them. For other unleaded fuel information in the archives, click here and here. For the official Government news, take a look at this page. Check out for loads of interesting stuff, including a marque guide. The BBC's 'Watchdog' programme covered the issue in October 1999; click here. Bayford Thrust has started selling leaded 4-star again (February 2000), click here for forecourt locations. And many thanks to Mike Millen for providing us with this very interesting link on the subject; take a look at 'The Lies of Unleaded Petrol Pts. 1-3' on this page:

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Most recent revision 31 January 2000 13:24:37 GMT - Copyright © 1996 - 2003 CMM Publications. Illustrations by ©Dave Iddon. All Rights Reserved.