Fuel additives

For up to date information on Fuel Additives for Bio-fuels in old engines please refer to the
Federation of British Historic Vehicles website www.fbhvc.co.uk > Legislation & Fuels > Fuel Information.

I’m a bit of a fan of fuel additives, and always put a diluted amount of Castrol Valvemaster in the tank when I fill up.  Why do I do this?  Well, to be honest, it’s a habit left over from an old LandRover that needed something to stop the valve seats regressing (wearing away).

Now, I know the valve seat issue doesn’t apply to our cars - the engine design predates the leaded fuel era anyway.  However, there are some Pros and Cons, some of which I am only just discovering.

First issue: Ignition timing.  It is a simple fact that advancing the ignition so that the full force of the expanding gasses occurs through the maximum stroke of the piston will give the most power.  So, at first, you’d think that more advance is always a good thing. Where this all breaks down is when the ignition is so far advanced that the fuel burns out of control.  Modern fuels have a much higher “flame speed” than those of the ‘thirties, and so burn much faster.

The familiar mechanical clattering, “pinking”, occurs when the fuel is detonating rather than burning.  In an Austin, this can be very serious—excessive advance and pinking puts unsustainable shock stresses on the crankshaft, which will eventually cause it to break. The effects of ignition timing were well known when unleaded fuel first hit the pumps—to prevent pinking, many moderns of the time had to have their ignition retarded to stop this happening.  The alternative cure was to raise the octane ratio of the fuel by means of an octane booster.

But at the time, it didn’t affect our Austins.  Why?  Simply because the compression ratio is so low.  More on this in a minute.  But the upshot is that the Bosch distributor which I am using is much more advanced than the old Lucas was capable of, but I still don’t get too much clatter.  This is a Pro point.

Second Issue: Vapour lock.  Petrol is more volatile than diesel oil, Jet-A or kerosene, not only because of the base constituents, but because of the additives that are put into it.

The desired volatility depends on the ambient temperature: in hotter climates, gasoline components of higher molecular weight and thus lower volatility are used.  In cold climates, too little volatility results in cars failing to start.  In hot climates, excessive volatility results in what is known as "vapour lock" where combustion fails to occur, because the liquid fuel has changed to a gaseous fuel in the fuel lines.

In Australia, the volatility limit changes every month and differs for each main distribution centre, but most countries simply have a summer, winter and perhaps intermediate limit. [Source: Wikipedia].  So, by using an additive, I get less problems with the fuel boiling off in the petrol pipe.  Another Pro point.

Third Issue: Cold starting. For exactly the same reasons, the lower volatility can make the engine harder to start from cold because the octane booster has made the fuel so much less volatile that it's hard to get it to ignite on the first couple of strokes.  Definitely a Con point.

Fourth issue: Fuel height in the float bowl.  Many of us have suffered from an apparent misfire when the engine has been running at full power and then the car driven up a steep hill.  The spluttering (in some cases) is caused by the fuel level being too high in the bowl, so the excess fuel pours into the choke without enough air to burn it.  The cure for this is to lower the float valve by adding a couple of washers.  The reason this is necessary is that unleaded petrol is less dense than leaded, so the float sinks lower making the petrol level higher.  If you inadvertently fill up with super unleaded, you have just made this situation worse.  By adding a bit of extra chemistry, I just raise the specific gravity a little bit; the float sits just a spot higher and the effect is less marked.  Again a Pro point, but one that wouldn’t apply if I sorted out my float valve.

But does octane booster actually give more power, (ignoring the effects of ignition timing)? I’ve taken the following from the excellent “Carbibles” website: www.carbides.com

It's a common misconception amongst car enthusiasts that higher octane equals more power. This is simply not true.  The myth arose because of sportier vehicles requiring higher octane fuels.  Without understanding why, a certain section of the car subculture decided that this was because higher octane petrol meant higher power.

The reality of the situation is a little different.  Power is limited by the maximum amount of fuel-air mixture that can be jammed into the combustion chamber.  Because high performance engines operate with high compression ratios they are more likely to suffer from detonation and so to compensate, they need a higher octane fuel to control the burn. So yes, sports cars do need high octane fuel, but it's not because the octane rating is somehow giving more power.  It's because it's required because the engine develops more power because of its design. There is a direct correlation between the compression ratio of an engine and its fuel octane requirements.

The table is a rough guide to octane values per engine compression ratio for a carburettor engine.

Compression ratio


5 : 1 72
6 : 1 81
7 : 1 87
8 : 1 92
9 : 1 96
10 : 1 100
11 : 1 104
12 : 1


Copyright ©1994 - 2007 Christopher J Longhurst.
All Rights Reserved


This article, written by Geoff Hardman, originally appeared in CA7C Seven Focus, July 2007 pp13-14..