Octane rating of modern petrol and will it affect your old engine? (Ver 2)


Will it or won't it?

In this article, Richard Notton, who makes a sub-hobby out of the technicalities of fuels and oils discusses the issues regarding this topic. What follows is a combination of several different email discussions all roled into one article with the most recent being a response after Tony Gull posted a query on the "Military Vehicles Mailing List" in February 2001.
The usual warnings and cautions apply to relying on any information in this article - do so at your own risk.

Article begins

Back in the 1930's Mr F R Banks of the Associated Ethyl Company brewed the horrendous fuel for the winning Schnieder S6 with its fledgling Merlin, this was some noisome stuff made of Romanian petrol, acetone and huge amounts of TEL, something approaching 1mg/gallon; I think current leaded petrol is something like 0.015mg/gallon. I have spoken to the chief scientist at, as it is now, the Associated Octel Co. about leaded fuel and got a lot of interesting data.

The Battle of Britain was fought on 80 octane stuff and there was always a need for more grunt especially with the advent of the FW 190, we knew of Yank 100 octane but it didn't perform that well since the level of aromatics are important; ultimately a common spec was agreed for 100 octane aviation spirit using Caribbean petrol and a gob of TEL, the first shipment arrived in the tanker Bunker Hill and a large reference quantity was removed for the chemical "library", this stuff was and still is, known as "Bunker Hill 100". The purpose of TEL and why reference samples are important comes later in this discussion.

The only way soluble lead, TEL (Tetra Ethyl Lead) gets into gasoline is by man adding it, any lead compound in current US or UK pump fuel would be below measurable limits and would destroy the standard CAT in short order having you fail a smog test and me the MoT.

TEL works by burning to lead oxide and forming a reflective heat shield around the burning mixture stopping the rest of the charge from simply going bang. TEL itself does nothing for the valve seats, in fact its a problem as it chokes the cylinders and shorts spark plugs but such are the gains that something had to be found to alleviate the problem. Initially ethylene dibromide was used, but proved expensive and subsequently a chlorine eliminator was almost universally adopted, after combustion the lead oxide reacts with the eliminator to make a white crystalline salt which for the bromide eliminator melts at 370 deg C and with chlorine 500 deg C, a lot lower than the melting point of the original lead oxide at some 900 deg C. Its this melted liquid salt that lubricates the valve seats and eats the stems away, it also appears as the grey/white deposit in exhaust pipes and the streaks down aircraft cowlings or over the wings although largely not seen now with regular cleaning and 100LL but very evident in WWII photographs. The other deduction is that as no problems with exhaust seats were apparent, no one went looking for a non-existent problem so specific engine powers climbed steadily and the lead salts went about their job unknown, that is, until the TEL was removed. . . . . . . . . . . . Note, soluble lead (TEL) is totally, utterly different from metallic lead, the exhaust lead salts (halides) are not absorbed by the human body, do not destroy brain cells and are water soluble. A significant point that politicians, greenies and other do-gooders would not or could not understand. Chemistry often does this of course, eg., NaCl, sodium chloride. Sodium is a quite nasty and poisionous stuff, so is chlorine; sodium chloride as a compound is of course table salt.

> Re the 70-80 Octane fuel recipe...I got replies but none were conclusive.
> Dr Deuce came closest by infering HO fuel will cause trouble. So what are
> people running in their WW2 engines?
> I dont want to detonate my excellent 270 by running 92 Octane.

You cannot damage the engine by using a higher octane than is needed, you just do not realise all the _potential_ power.

Octane ratings tell you very little about how the fuel actually performs, it is a realative measurement indicator of the performance in the single cylinder, variable compression, laboratory test engine (CFR engine) (that bears little or no resemblance to any automotive engine). This is a weird lab test engine with variable compression, its title is a "Co-operative Fuels Research engine" known simply as a CFR.

Using the sample fuel the engine is adjusted until knocking occurs, then a test fuel of iso-octane (cetane I think) which is highly knock resistant is used with n-heptane which has a rating of about 10 and can be considered the zero reference because it always knocks; by increasing the ratio of cetane to n-heptane the point of knocking is again reached and the rating for the fuel is then the ratio of the two, ie., 20% n-heptane to 80% iso-octane would be called 80 Octane. By this means you cannot get numbers above 100 so after this figure the last two digits represent the ratio of TEL added.

TEL was added in America initially to boost radial aero engines in the 20's for (successful) carrier take-offs by allowing a useful power boost of the then typical 75 oct AVGAS.

> Interesting since if you look at the original pre-war Piper Cub
> Aronca Chief, ect aircraft owners manuals they all state that if
> the 80 octane aviation fuel is not available then "tractor gas"
> read automotive pump gas was an acceptable alternative..

That might be so for the US at the time.

> So what happened between the BOB and latter in the war since the merlins
> that were stuffed into P-51 required 145 octane"purple"
> gas ? Both 145 purple and red 80 octane aviation gas were
> dropped somewhere around the mid 70's leaving only 100 Low lead
> blue left. Really upset the warbird crowd in that they all had to
> drop the power setting from 54 - 56 Hg on take off down to a max
> of about 44 Hg.. Lost lots of HP that way, but better to loose the
> HP than detonate the head off the motor ..

Britain at the time was introducing 100 oct AVGAS having found that simply dumping more TEL into the gas allowed higher boost pressures and therefore power, simply open the throttle more.

The problem was desperate shortage of the stuff so US imports were arranged, however clerks are not chemists and the simple octane statement falls far short of the full definition since the aromatics in gas have a marked effect on performance. The initial US 100 oct AVGAS performed no better than the 80 oct at the time until the chemists realised the very different hydrocarbon structures produced from the two different sources of original crude base stock.

Once this was defined equivalent fuel could be formulated with the initial batch arriving in the tanker "Bunker Hill" and reference samples still available to this day are known as Bunker Hill 100.

The 145 oct you refer to means a refined gasoline to 100 oct with some 45 milligrams of TEL per gallon added, enough to have an army of high-pressure greenies turn red with rage. . . . . . . . . .

Warbird racing is peculiar to the US where you have a more amenable FAA and the infrastructure/demand to provide new-manufacture approved spares, Merlins and Griffons are cosseted here to keep them flying by lightening the airframe of guns and armour together with operation at zero boost typically, thereby the effective sea level naturally aspirated power is maintained at altitude and the use of 100LL is not a drawback.

What it doesn't tell you is the make up of aromatics and lead (if used) that contribute to this octane rating, these things can have a drastic effect on the real engine.

Almost invariably horror stories of damaged engines are incorrectly attributed to octane ratings, high octane unleaded being used in an engine that must have a valve seat lubricator, without a current, proven lead substitute being added or, more obtusely and a documented WWII occurrence, high lead (so high octane) fuels being used in MVs causing corrosion of the valve stem and valve failure. In this instance the octane rating is always blamed but has nothing to do with it at all, the seats are fine getting high doses of melted lead salts but the stems, not being of a compatible steel alloy like 21-4N steels, just corrode away fast and loose their heads with drastic results.

I would advise against using paraffin (kerosene)(depending on continent) as an octane reducer, it will not volatilise and will simply wash the oil off the cylinder walls and then contaminate the lube oil, better to use a heptane fraction which is a cheap industrial solvent very similar to unleaded petrol (gas)
(Doug - I wonder if this is what we know as "Shellite" and "White Spirit" in Australia?)
and has an octane rating of about 10, one pint to a gallon of 95 octane fuel will reduce it to 85 octane but its not really needed except in special cases like pre-war, air cooled, side-valve (L head) motor cycle engines.

If you need a lead substitute additive for exhaust valve seat protection (note, up to a nominal 3000 rpm lead is never necessary) do choose carefully, many products are around that contain manganese. These are very effective at lubricating the valve seats but are unstable and form a filter blocking sludge. Soluble iron such as ferrocene is better than lead for seat protection but forms red iron oxide, otherwise known as Jeweller's Rouge which is decidedly unfriendly for the bores and pistons.

Bear in mind that unleaded of today, compared with early WWII unleaded, has the benefit of some 60 years of R&D, the lead-free octane improvers currently widely used in pump fuel and advanced refining processes now leave no combustion residues and so the valves have an easier life anyway.

It was commonplace to de-carbonise/valve grind engines pre and post war every few thousand miles both to remove the deposits built up owing to poor refining and catch seat recession before it became a problem, this is now unnecessary.

Many thanks to Richard.


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