Rolls-Royce Owners' Club
of Australia

Unleaded Fuels in Rolls-Royce and Bentley Engines

Tony Ward
Technical Officer, Rolls-Royce Owners Club (NSW Branch)

To elaborate a little on what Mr.Lee has written I discussed the subject of valve seat recession with a number of motor engineers. I was told that the principal causes are sustained high r.p.m., i.e. 4,500 and over, high compression with steep camshaft profiles and heavy valve springs causing valves to snap shut quickly with heavy impact. Lead not only raised the octane rating but cushioned the impact of the valve upon the seat, which is particularly valuable at the higher temperatures associated with exhaust valves.

Now, let's look at how these factors affect Rolls-Royce and Bentley cars. To my mind, not one model from the earliest Silver Ghosts to the end of the six-cylinder engine in the Silver Cloud I and Bentley S1 series meets these conditions in any way! Rolls-Royce engines were robustly built from the best materials available in their period. They were under-stressed, low in r.p.m. and low in compression ratio.

It is true that though the Silver Cloud started off at 6.6:1 compression ratio it was raised to 8:1 on the later series of that model. However, Rolls-Royce paid particular attention to valve and valve seat material quality and at 25 m.p.h per 1,000 r.p.m. in top gear you would only be doing 3,000 r.p.m. at 75 m.p.h. (120 km/h), which is above the maximum speed limit! In a Bentley Mk.VI at 22 m.p.h per 1,000 r.p.m. you would only be running your engine at 3,400 r.p.m. at 75 m.p.h!

The other key factor in the debate is mileage. Just how many miles per annum are we likely to do in these cars? Not much in the case of pre-war cars, I would think, but perhaps somewhat more for the post-war six-cylinder models.

However, a couple of experienced technical members of the R.R.E.C. sounded a note of caution in conversation with me at Althorp last year, saying that they would like to see a test carried out in which a Rolls-Royce model was run for a large mileage on unleaded petrol. The valve seats would need to be checked before and after the test to see if any recession had occurred.

It is interesting to note that lead was not originally added as a valve seat lubricant, but rather for the reasons explained by Mr.Lee. The lubricating property was an added bonus.

In discussions with an Ampol engineer I was told that the octane ratings of petrol in Australia at the present time are as follows:

Petrol RON
Super (leaded) 96
Regular (unleaded) 91
Premium unleaded 95

Shell have recently introduced a reduced-lead fuel which they call "Half Lead". However, the octane rating has been maintained at 97 by changes in the production process and *not* by adding benzene. It is ironic that the lethal qualities of benzene, which is a carcinogen, are now causing concern. It is currently present in all grades of petrol marketed in Australia and may well prove, as research progresses, to be even more of a health danger than lead.

In the USA, a lead substitute for valve seat lubrication purposes, called Valve Master, was developed by DuPont. This is poured into the tank with every fill of unleaded petrol. [Information about commercial source removed for posting. ChrisG] An item in a recent R.R.E.C. Bulletin states that tests are being conducted in Britain on an anti-valve seat recession additive which has been used extensively in the USA

Although some of the valve seats in our older cars may not be of the hardest material, eg. the bronze seats in the Phantom II alloy head, the key factor I think is usage. Are we really going to do the large mileages for any deterioration to be noticed? In any case, an additive as a substitute for lead may well be the answer to our problems - if indeed there are any problems!

This article is intended as information only and should not be taken as a recommendation by the author or the Club of any particular course of action.

(Reproduced with permission)

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