PETROL

Having studied the situation and experimented with various fuels over the last 5 years, I think I can shed some light on the subject in simple terms.

Modem fuels have two main differences from the petrol our cars were designed to cope with. They are much more volatile and they have a much higher octane which, although it sounds contradictory means that the charge bums at a slow-er rate than low octane fuels.There is also the contentious issue of lead or no lead. Tetra-ethyl lead was used as an octane booster from about 1929 onwards, although cheaper grades of petrol were not universally leaded until after the war. The issues are:

1) Volatility. This is the "evaporativeness" of the petrol and is related to the amount of spirit (light grades or "aromatics") in the fuel. Our cars were designed to run on 20's and 30's petrol which was not so volatile as modem fuel and felt much greasier.

Problem: Highly volatile modem fuel can evaporate in the fuel pipes causing vapour locks. This in tum causes complete engine failure or at best rough run-ning. This problem is usually apparent on a hot day after the car is stopped with a hot engine. The petrol in my Chummy carb will actually boil on a hot day -the carb stops working when this happens!

Solutions: a) Lower the volatility of the fuel: under a special dispensation we may now add paraffin to petrol as long as it is for a car first used pre 1/1/47 and is less than 10%. I don't know what effect this has on the octane rating (see below).

b) Try to isolate the carb and fuel pipes from the heat. Route the pipes away from the exhaust. Ruby and Box rear tanks: try to isolate the main fuel pipe by wrapping it in heat proof tape as it runs down the chassis. If you can, make a heat shield for the carburettor.

c) Use the lowest possible octane fuel you can find. For reasons we'll see in a minute, this helps lower the underbonnet temperature. Advancing the ignition will also help get the temperature down - if necessary replace the rear main to get the engine advanced!

2) Octane Rating Modem fuel has a much higher octane rating than our cars expect or need. Remember the higher the octane rating the slower the fuel bums. This is to cope with the modem engines (relatively) huge valve overlap and higher compression ratio.

Problem: Using high octane petrol means that the fuel bums more slowly in  the cylinder. Our engines have the fuel in the cylinder and buming for a much shorter time than a modern. Net result? The charge is still buming when it goes out of the exhaust pipe! The exhaust gets very hot and the exhaust valves bum out in double quick time. (Mmm ever noticed how short a time Austin Seven exhaust valves last these days?). Also the underbonnet temperature sky-rockets and we make problem I above even worse.

Solutions

a) Again, use low octane petrol. You will make the problem worse by using super unleaded or 4 star. (Ever used an A7 on French "Essence" ? They run beautifully because Essence is only 89 octane or so.)

b) Again advance the ignition, the earlier you light the mixture the sooner it will go out!

c) The real answer of course is the wretched modem fuel bums for too long and needs speeding up. Maybe paraffin would do this but I haven't tried it yet. Perhaps we should import large quantities of French petrol

3) Leaded vs Unleaded. Having established that the Seven would thrive on a low octane fuel, why do we need to use leaded petrol when lead is used as an octane booster'? In the thirties both leaded and unleaded fuels were available. Economy cars like the Seven were able to run on leaded fuel but were designed for "straight" fuel. After the war almost all fuel was leaded and the motor man-ufacturers were able to take advantage of a side effect of the lead in that it acts as a lubricant to the valve gear. The manufacturers were able to use softer grades of cast - iron in blocks and heads and softer valve guides. By the 1950's and 60's, the average engine was fully dependant on tetra-ethyl lead and use of unleaded fuel in these engines will cause disaster.

Modem unleaded fuel has octane improvers other than lead (e.g. Benzene) and performs no upper cylinder lubrication at all. This has little consequence in its own right in a pre-war cheap car, I have found that my Sevens have covered many thousands of miles on premium unleaded with no valve problems at all (except that the wretched exhaust valves last all of five minutes - due to the mixture still being alight as it whistles by.)

The main problem is that even premium unleaded has too high an octane rat-ing and premium unleaded is lower octane than Super. The main point is don't use 4 star or supergreen. Its far too high octane and this will ultimately cause you problems in the overheating/vaporisation area and probably bum out your exhaust valves even faster.

Any one for some good 68 Octane from Khazakstan?

TIM REYNOLDS

SuperTip No 4

I think we have all heard, and frequently experienced, the problem of pre-evaporation on a hot summers day's in traffic (scuttle tank owners excepted) - the petrol vaporising in the pump and pipes before reaching the carburettor - and the advice to have a piece of towelling and a bottle of water avail-able for such an event. But, have you ever tried to soak the towelling without wasting a stack of valuable water in the gut-ter? HINT! Take the towelling in a polythene bag - pour the water into the polythene bag!!

SUPERTIP FROM ERIC JONES