does Tesco 99 go off more than vPower?

duncan · Post by **duncan** » Wed Dec 12, 2007 12:36 pm

Just something that I have noticed whilst mapping recently that a map done for Tesco 99 that is completely knock free will, if left in the car for a period of time unused will then start knocking a couple of degrees here or there.

I havent noticed the same with Shell vpower.

I know that fuel loses its octane gradually and that race fuel is particularly prone to this.

Gerry H · Post by **Gerry H** » Wed Dec 12, 2007 1:14 pm

That's worrying as my car sits for weeks at a time, especially at this time of year. I also carry 20L in a can, that may also be unused, including through the season.

Be keen to know your findings on this Dunk.

duncan · Post by **duncan** » Wed Dec 12, 2007 1:43 pm

I only spotted it the other day Gerry then looked back through logs.

When the map was done it was knock free (say on a saturday am)

But consistently logs from mid week (the week after) showed a couple of points where there was some knock. Not much.. but retardation all the same.

I wonder whether the ethanol in the Tesco has a shorter in tank life and the octane of the 99 reduces.

I will see if I can properly replicate the test with vpower.

[geek mode]
I have noticed that there is a 3% difference in closed loop fueling between the two as well which carries forward into open loop.
ie.
if mapped on 99 and it wants to take out 3% fuel if run on vpower.
if mapped on vpower - it wants to add 3% more fuel for tesco 99 because it considers it is running too lean.

I now have two maps - one vPower with injectors at 867cc and a tesco99 at 842cc to account for the stoich differences in fuel.

[/geek mode]

Post by **Ben** » Wed Dec 12, 2007 1:54 pm

:-D you are a geek .... could weather have an effect, outside temps etc?.

duncan · Post by **duncan** » Wed Dec 12, 2007 1:57 pm

it could do

both days were 5C external ambient temp and both as crisp and dry as st stevens pizza.

Gerry H · Post by **Gerry H** » Wed Dec 12, 2007 2:06 pm

But you would expect to see a small difference between the two with Tesco having 5% methanol although it should be less than 3%

duncan · Post by **duncan** » Wed Dec 12, 2007 2:32 pm

Tesco 99 Octane contains bioethanol

Gerry H · Post by **Gerry H** » Wed Dec 12, 2007 2:47 pm

[quote="dynamix"]Tesco 99 Octane contains bioethanol

duncan · Post by **duncan** » Wed Dec 12, 2007 3:03 pm

Indeed - I would have expected that difference

I didnt expect to see rapid deterioration of the fuel quality in the tank though. (if it was that and not my mapping before anyone says

)

Gerry H · Post by **Gerry H** » Wed Dec 12, 2007 3:12 pm

So, how will you dis/prove the deterioration theory?

You won't be able to conrol samples or conditions.