Lower octane fuel generally has greater energy content than higher.
Higher octane allows higher compression ratios without detonation
("knock"), which allows more efficient combustion (more than offsetting
the lower energy content). Pure ethanol, for example, has an octane
rating of 110 (I think), but has only 2/3 the energy content of gasoline.
Theoretically, all else (wind, slope, air density, etc.) being equal--they
probably weren't--you'd get better mileage with a lower octane fuel
(especially if you throttled back to prevent knock).
Having the hood up is more aerodynamically efficient. IFF all else is equal,
I'd expect 1-2mpg more with the hood up (note modern convertibles have
lower EPA mileage ratings than their fixed-head equivalents).
bs
***************************************************************
Bob Spidell San Jose, CA bspidell@comcast.net
'67 Austin-Healey 3000 '56 Austin-Healey 100M
***************************************************************
> Hi,
>
> I've just come back from a longish trip in my MkII BT7, 500 miles approx. No
> real issues arising except how amazed I was at how much less fuel I used on
> the return trip with hood up and using higher octane fuel. BIG difference.
> Obviously a combination of RON & wind resistance, but does the panel think
> it would be more one than the other and if so which???
>
>
>
> Now, the main topic. .after the first 200 miles or at the first set of
> lights (when I could hear something), I noticed a loudish "Tick, tick" noise
> form the front/dash. I established that the rev counter was not working and,
> by holding the cable, I could feel it "pulse(?)" in time to the noise. So
> noise, cable & rev counting are linked.
>
>
>
> I surmise that the cable must still be rotating, hence the drive coming out
> of the distributor is OK.
>
> I presume that the instrument is possibly u.s. (Functioning OK before, no
> period of apparent decline).
>
> I suppose I'll have to take instrument out and start from there???
>
>
>
> So, are there any immediate fixes/issues that arise from these symptoms??
> (Like, for example, intermittent click from fuel pump and good wallop on
> bulkhead means pump's points are up the spout.)
>
>
>
> I haven't done anything yet except take out the cable to prevent it making
> things worse.
>
> Is a rev counter something that one can fix at home?
>
> Simon.
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