> So Tom,
>
> Is the higher-priced premium really needed? I always use it in my cars - a
> 1500, and a Miata with the timing advanced. And it "feels" better, but since
> San Francisco has the highest gas prices in the county, am I just padding
> Chevron's coffers?
>
> Leigh Brooks
> BADROC
>
> PS - it cost $22 to fill up the Miata the other day. There was a woman
> filling up a Range Rover - she was still pumping and the meter was over $60!
Hi Leigh-
The octane you use is dependent on a number of things, all of which add up
to a car's tendency to ping. High compression, high initial advance, sharp
spots in the combustion chambers, etc.
Set your car to its ideal settings, carb/timing/etc. Put some cheap gas in
it, see what happens. If there's no audible pinging, I think you're in the
clear. If it pings, retard your timing a couple degrees (within reason -
don't go below 5BTDC or so), and see if that helps. If it's still pinging,
go up to midgrade, and repeat.
I have found that compression ratio, coupled with cam radicality (is that a
word?!:-) is the single biggest indicator of required octane. A higher-end
cam bleeds off compression at lower RPMs, so you can run more timing and not
have to worry about pinging. OTOH, a high-compression engine with a low-end
cam (IOW, stock), will have very high compression at idle, and will need more
octane to prevent run-on.
All that said - here's what I run:
'71 PL521 - LZ22, 9.5+:1 compression, dual SUs, stock L20b cam, 92 octane
'69 2000 - U20, stock engine with non-smog distributor & pertronix, 92 octane
'72 240Z - L28, 8.3:1 compression, '73 cam, GM HEI, etc, 87 octane
Now that you;ve made it through all that - here's the short version, IMHO,
if you don't need the octane to prevent pinging/run-on, you're just throwing
your money away.
All IMHO, YMMV, standard disclaimers all around;-)
Kyle
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