96 & 98 octane

Plausible but I'd have thought the opposite should be true, if the fuel
charge is taking measurably longer to combust then you are running the
risk of part of it being blown out the exhaust ports. Maybe it's cam
timing that means some engines(1) get a benefit and some don't ?

JL
(1) where "some engines" equals the remarkably similar specced
(superficially) engines referenced earlier in the thread

 

Sounds quite possible, wish I knew a chemist so we could find out the
real situation !

On that basis then, the variables for whether the engine is more or
less likely to gain a benefit from a higher octane juice would be
1 compression ratio
2 stroke length
3 cam timing
(or more likely a function of 2 and 3 combined)
4. relative wear on the engine (blow by)
5. ?? some sort of calorific measure per gram/cc/ w/v (whatever the
correct measure is) perhaps - is there a difference in energy value
between a litre of 91 ULP and 98 UP-ULP ?

Other parameters ?

JL

 

Well, a bit... I'd say you're clutching at straws there, then I'd think
about it for a bit...

I don't _think_ that's it...

The instantanous cylinder pressure is determined (ignoring complications
for now) by the volume of the burnt gasses, their temperature, and the
current volume enclosed by the cylinder/piston positioning (varying over
time).

I suspect that both low and high octane fuels end up with pretty much
the same oxidised gas mixture (they've both fundamentally got the same
number of Carbon and Hydrogen atoms to turn into CO2 and H2O -
everything else is pretty minimal by comparison I think), and with
pretty much the same volume (otherwise carb/FI tuning differences would
be much more prevalent).

There's a quite significant (in speed of combustion terms) period where
the leverage that the piston has on the crank is quite low - for a few
degrees either side of tdc I don't suppose the cylinder pressure makes
much difference at all. So long as the air/fuel mix is completely burnt
by the time you get to 10 degrees past tdc or so, I suspect the integral
over time of any potential cylinder pressure differences before then
would approach zero.

Since we know either sort of fuel burns fast enough to "work" at redline
rpm, it seems unlikely that the burn isn't fast enough to have pretty
much happened completely before the crank turns far enough for it to
make any difference how quickly the cylinder pressure builds, at least
at the sort of rpms used in typical commuting.

Nice try, but I'm _pretty_ sure it's not gonna fly...

big (oh, and whats your current email, IK?)

 

And here's another one: the XLV, when its CDI exciter coils start to
burn out, starts to get worse mileage and loses some midrange power,
even though it doesn't actually misfire? Both spark plugs per cylinder
are still firing, just weakly. Surely a fire is a fire?

It turns out combustion isn't as simple as all that and the spark energy
has an effect on how long the combustion spends 'smouldering' before it
really takes off and starts net releasing energy. So that's how.

Maybe its similar with this octane nonsense? The XLV seemed to get
better mileage on premium too ...

-----sharks

 

Is that definitely whats going on, or your current working hypothesis?
I'm thinking along the lines of "half burnt out coils = more resistance
= lower current = weaker magnetic field = longer time for collapsing
field to reach the spark gap potential = retarded timing = worst
mileage" as an alternative...

Admittedly, my previous argument I used against IK about the time and
leverage issues the piston has around TDC apply equally well to this new
argument of mine...

big

 

Working hypothesis. Comes from a fellow on a mailing list who alleges to
Know About Such Things, but it's untested.

.... 'cos that's a fairly credible explanation too. When I get the XLV
running again I might test it. Not sure how long the "collapsing field"
time is likely to be (for a CDI system) compared to a degree of rotation
though ...

And as a bonus, if higher octane fuels gave a lower spark gap potential,
that might advance a bit, and maybe explain why some bikes seem to run
better on premium and others seem to get their cases split!

-----sharks

 

Fouled plugs on my Blackbird twice in 100,000kms. Both times were the
only times I filled up with Optimax. Used it in my VTR with out a single
problem.