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Classic Engines, Modern Fuel

Comments on Topic: Why is petrol volatility important?

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Submitted by The Author

Petrol or gasoline is not a single substance. It is a mixture of over 300 different hydrocarbons and additives. This mixture in modern petrol is very different from that of classic petrol. More than just the removal of the tetraethyl lead.

The different mixtures change petrol’s physical properties and the way it behaves in an engine. Chapter 2 of the book describes why  and how these changes affect the way the engine runs 

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Submitted by Ian

I have just read your book which I find fascinating. I have two old cars a 1914 Vauxhall and a 1924 Alvis 12/50. Both have fixed jet carburettors and magneto ignition with manual advance. I have been driving the Alvis since 1984 and the Vauxhall since 2009.

The Vauxhall is a L head side valve with the updraught carburettor on the opposite side of the block to the exhaust manifold. This suffers from hot start problems. If it doesn't start first swing on the handle I have to turn off the ignition, open the throttle wide and turn the engine over several times. I think this is to get a fresh cool mixture into the inlet system as the modern fuel vapourises too easily as you explain. The engine has a CR of about 4.5:1 and revs to a max of 2300 rpm. I have found once running that I run it on full advance, 35 deg BTDC for all occasions, retarding it for hills doesn't seem to have an effect. I have recently been running it on Super Unleaded but haven't really noticed any difference. From your comments perhaps I should add 5% paraffin.

The Alvis is OHV with an updraught Solex. It has a fabricated steel inlet manifold with no hot spot and an insulator between the carb and manifold, as a result I have never had hot start problems. The CR is about 7:1 and revs to 4500 rpm. Again once running at 42 deg BTDC retarding the ignition does not have any noticeable effect. I would like to try this on a tankful of Avgas as my impression is that it does not perform quite so well as it used to. Again I run it on Super Unleaded but not really noticed a difference.

Both cars are set up to run on the rich side. I plan to take them to a rolling road to optimise ignition timing and main jetting. I do not think there is much more I can do but am reassured that ethanol will not cause problems for me as all the fuel lines and car components are copper or bronze.
I would be interested in your comments,


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Submitted by The Author


I am glad you found the book fascinating.

Your comments about ignition advance are interesting. As the book describes, the tests showed that ignition advance appeared to have only a minor effect on power output. I have suggested the degree of Cyclic Variability in a side valve engine will be worse than that of the XPAG making the power output even less sensitive to ignition advance as you observe. You may wish to look under the Category “Combustion and Cyclic Variability” on this bulletin board to see my comments. There are also links to two videos showing the inside of a running side valve engine.

Even though power output is not sensitive to ignition advance, exhaust temperature will vary.

May I suggest you buy a meter with a thermocouple? By fastening the thermocouple to the exhaust, with for example a hose clip, you will be able to monitor your running exhaust temperatures and use these as a guide to setting advance.

As for fuel volatility. Avgas is expensive, you may find Sunoco Optima (links - UK and US) a better choice. It is only marginally more expensive than Avgas. The XPAG ran better on this fuel and its volatility profile is a better match to 1960’s petrol. An added benefit is that it does not “go off” and it is ethanol free. Ideal for storing your cars. It should resolve the Vauxhall’s hot restart problem.

I suggest it is worth trying kerosene. It will reduce petrol volatility and help with the Vauxhall’s hot restart problem. Start with a small volume of petrol and 5% kerosene. If that appears OK, I suggest increasing to 10% or even 15%. Remember if you do decide to use it regularly, apply for a certificate from HMRC.


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