post from flying scotsman:
-------------------------------------------------------------------------------------------------Ive used Shell optimax since day one on my 172. With an 11:1 compression ratio the engine in the 172 will run "below par" on less than 98 octane. The knock sensors will probably have a de-tuning effect on the engine to reduce pre-ignition. The CR on smaller 16V engines is around 10:1 so are less likely to pre-ignite. Putting optimax in these engines will probably have negligable gains. My last Clio (1.6 16V Dyna+) also with a 10:1 CR would run ok on 95 and 98 RON fuels. However I reckon the Super Unleaded 97 RON gave a slightly better fuel consumption.
If the older members can remember when Shell launched their Formula Shell (circa 1988 ish). This used a sodium substitute for lead. My higher than average CR MKII 16 Golf then used to run sweet on this juice. It all crumbled when Vauxhall claimed that it screwed up their inlet valves?! Still think Vauxhall used this as a cover up for their sh*te engines.
Anyway, from all this, I reckon Shell are one of the pioneers in developing good forecourt petrol. The Asda/Tesco petrol may be cheap but will generally not give good performance / mpg figures. optimax is probably the best fuel available at the present time. If you can aford it and have a higher CR engine ....use it!
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