At 07:46 AM 2/25/2002 -0600, you wrote:
>In a 58 Imp, it is my experienced contention, that 100 octane is only best
>if you enjoy wasting money.
Hugh's statement is correct, if there is no engine knock with the lower
grade fuel, using the higher octane is a waste of money, however....
>If you read the article you will find out they specifically advise such
>gas, stating conclusively that you will derive no benefit at all from
>using a higher octane, even if you do advance the timing. I have
>certainly found this to be the case.
Generally speaking, as the spark timing is progressively advanced, knock
will occur, and the higher the octane of the fuel, the more advance will be
tolerated before knock (especially with a 10:1 compression ratio engine
like the 58 hemi). How much advance will be tolerated depends on the
engine as well as the fuel, and the Hemi is definitely the best engine of
the period in tolerating knock, but knock WILL occur eventually (especially
with 89 octane). Also, as the timing is progressively advanced, the
performance of the engine (torque and efficiency) improves, and this
improvement is very desirable because its virtually FREE (no "performance"
gismos and the like!). Now, some engines (especially low compression 70's
engines) have such resistance to knock that knock will occur when the
timing is over-advanced so much that performance is trailing off. Its
possible that the late Hemi's were like that, but given the high
compression ratio, I think its unlikely. Either way, one can "cash" on the
engine's resistance to knock by either running cheaper lower grade fuel and
saving money, or by advancing the timing beyond the knock limits of the
lower grade fuel and enjoy better performance and efficiency (and cooler
exhaust) but paying more for the higher grade fuel. Hugh showed me once
how tough it is to reach the distributor of the 392. Given that, I don't
blame him for not readily trying to play with his timing till he finds the
"optimum". Even more frustrating for folks with 392's, as the ignition
points wear, the timing could eventually change a bit. So, the effort for
finding your optimum spark advance may be a never ending battle!
As a side note, later wedge big blocks also had fairly descent knock
resistance compared to other similar engines of the period, in spite of the
relatively large bore. The late 60's wedge heads for example were designed
so that plenty of turbulence was present accelerating flame
propagation. However, the knock resistance of the Hemi combustion chamber
can only be bettered by the pent-roof 4 valve per cylinder engine.
D^2
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