Toby Atwater wrote:
>
> I just realized, I have no idea what these cars run on.... I bought 3
> gallons of 97 "supreme" octane petrol, but I have no clue if that is
> necessary.
Probably unnecessary, unless you've had major head work/piston
replacement done with an eye toward improving performance.
> What spridegts were designed to run leaded fuel? any? all? as I
> understand it... the A series engine runs about 8.5 :1 compression, what
> octane should I run to prevent detonation?
Your '69 would have come *from Abingdon* with an 8.8:1 CR, which means
you should probably be able to use 89 octane on it. I've found that in
many cases, multiplying the compression ratio by 10 gives (as a VERY
rough rule of thumb) the octane requirements. So your 97 octane is
probably not necessary.
However... note those asterisks around "*from Abingdon*." That means
that when the Sprite was first bundled up for its transatlantic trip, it
would have had the 8.8:1 CR. In the intervening three decades, anything
may have happened. A previous owner may have put in high-compression
pistons, or overbored the engine (which raises the CR very slightly as
you're squeezing a larger cylinder into the same combustion chamber).
And of course, the effective cylinder pressure may be lower due to worn
rings/valves, etc. And an intermediate owner may have installed one of
the lower-compression engines from a later car...
...Now I'm spacing on exactly *when* the conversion to 8.0:1 CR was --
1972? This is for the U.S. market, to clarify...
> I could always just experiment and try 87 octane and see if it
> pings/pinks/detonates/knocks ...
Betting a net.nickel -- my guess is that 87 will ping slightly, 92
won't, and 89 will vary with brand/gas quality. My daily driver ('83
Audi CGT, inline 5) has an 8.2:1 CR but pings like mad on anything other
than Chevron 92 octane. It's *almost* worth finding out why, but not
quite; this car, while rewarding and pleasant to drive, was not designed
with the same easy access to maintenance items that we love about our
Spridgets. And don't get me started on the water pump (it's *behind*
the front engine cover, *behind* the timing belt...).
Note that you can use the vernier adjuster (if it still works :-) on
your distributor to microtune the advance to compensate for gas quality;
it's got arrows marked A for advance and R for retard, and if memory
serves it's 11 clicks per full degree of advance. On my '74, I just
used to advance the hell out of it, tweak the SUs a flat rich to cool
the charge and avoid lean-burn detonation, and run 92 octane; I mean,
given how little gas these cars use in relatively stock tune, why not
just buy the good stuff and adjust the engine to take advantage of it?
I never had detonation problems on the Midget, but then I probably had
the 8.0:1 CR, which means I would have had to use something like
kerosene mixed with cheap vodka to get detonation.
Enjoy the Sprite!
--Scott
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