PREMIUM UNLEADED PETROL

[Cameron 77C3]

This was in our club mag this month.  I thought it would be worth putting up on the forum for all our Corvettee buddies to read.

The article reads:-


"This reply was from the technical advisor at BP Fuels in relation to my query about “shelf life” of their Ultimate Premium Unleaded petrol.

My car is a small block 1968 vette and was using the Ultimate petrol + Nulon valve lubricator and experienced pinging and misses after the fuel was left in the tank for approx 3 weeks.  His reply is printed below and hopefully will benefit the other Vetter that previously used leaded or LRP petrols.

Regards Greg"


The reply reads:-


"Dear Sir

Thank you for your enquiry.  The shelf life of BP Ultimate in a sealed container is year.  In the tank of a vehicle which is exposed to the atmosphere it would be 4 to 6 weeks depending on temperature.

The main affect is a loss of light components which causes some reduction in Octane.  The fuel can also get heavier and this increases the fuel carbon content.  To counter this we always recommend filling with a portion of fresh fuel if the vehicle has not been used for two to three weeks, this will restore the light ends.

The octane of BP Ultimate is a guaranteed 98 minimum, lead replacement petrol is 96 minimum and Premium Unleaded is 95 minimum.

Potentially the problem you described could be linked to carbon deposition in the engine.  The engine you describe with V8 and Holly carburettor is a high user of fuel and if the fuel is heavy due to loss of light component then there is significant carbon deposit build up on the spark plugs and internal combustion chamber when using unleaded petrol, this will cause spark plug fouling and misfire or hot spots which will cause knock.  This is because under some conditions the fuel air mixture is rich and excess fuel is entering the combustion chamber and is not burnt quickly enough to stop carbon formation.

This did not happen with leaded fuel because the lead additive lowers the burn off temperature of carbon from approx 700 deg C to around 450 deg C.  This removes carbon deposits leaving the characteristic white deposit of lead oxides.  Leaded fuels were also lighter and vintage engines were designed for lighter fuels and tend to run rich.

BP lead replacement petrol also used a metal additive, in this case potassium as the valve seat protection additive at a high treat rate, this also lowered the burn off temperature of carbon.

I believe that Nulon contains potassium but I do not believe it is at a high enough treat rate to prevent carbon build up.  An alternative is to try using BP Ultimate with Flashlube additive which is based on sodium.

You can test for carbon build up by looking at the spark plugs when you experience misfire.  If you observe a grey/black graphite looking coating on the porcelain insulator then the graphite is short circuiting the spark producing misfire.

Hope this addresses the problem
Garry Whitfield
Technical Manager"


Cam

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I had also heard the Shell Premium had a short shelf life due to an "oxygen bomb" used to "activate" the fuel and increase the octane.
Only more reason to drive our cars more often....