> Accurate or BS?
Well yes and no. Here is some of what I learned by
reading the Bosch Automotive Technology handbook.
> What is the difference between higher octane
and lower octane?
> Well, 87 octane will ignite at a lower
pressure than 93 octane.
Here he is wrong. We all know from our experiments
in igniting items that require burning, that
gasoline will ignite in open air without pressure
no matter what octane it is. So the real
explanation is that both low and high octane will
ignite at the same pressure but lower octane gas
will burn faster at a given pressure than higher
octane. This isn't normally noticed by our slow
senses but in the cylinder the flame front rate of
the burning will travel faster in low octane fuel
than in a higher octane. The higher the octane,
the slower the burn rate. If the pressure in the
cylinder is increased the molecules are squeezed
together more thus increasing the burn rate in any
octane fuel, till what results is a rather quick
and sudden explosion. Again at a lower pressure
for lower octane vs. higher octane. This results
in all the way from a slight ping of the valves
being shoved closed faster than the spring rate on
the valve springs were designed for, to a sudden
louder more violent knock of a piston slap and or
rod knock in more severe cases, from overpressure
rather than a controlled progressive burn as the
engineers intended. This has even been known to
break crankshafts or totally destroy engines as is
seen in drag racing all the time. Increasing the
octane reduces that burn rate giving time for the
piston to get over TDC. So in the end a slightly
earlier ignition timing or higher compression or
both can be used to get more power from the same
amount of gas or more gas and air can be inducted
and thus allowing even more power to be produced.
So, with 93 octane, you can compress the air (think turbo or supercharger) and
have the
> spark ignite sooner (timing advance) which
creates more power. With 87 octane, since it
change here to explodes or detonates not just
ignites at a lower pressure,
> high compression engines might ignite the gas
before the intake valve closes and you
> hear a noise (ping). Often, in old cars, you
will hear this pinging under load (moderate
> throttle going uphill). What this noise is, is
the detonation of the gas and air mixture
or exploding
in your cylinders before the intake valve is closed. The explosion is partially
sent into the intake
not..., some of the gas and air mixture might get
pushed back but no actual explosion or flame
because in that case you would get an immediate
backfire and flame out of the carburetor. Or in
the case of drag racing seeing your supercharger
get blown off the engine.
causing the noise. What a knock sensor does is detect this noise (the “pings”
or “knocks”)
and tells the computer to spark/ignite at a slightly later time.
Which allows the piston to travel over TDC and
reduce the pressure and speed of the burn. Though
usually the detected ping is so small before it is
corrected that most people don't even notice it.
*The only reason the manufacture tells you that you have to run premium fuel if
that the
vehicle was tested for gas mileage with premium. Running premium will get you
slightly
> more power per molecule of fuel, thus better
gas mileage. If you run 87 octane, you might
> get slightly (and I mean slightly)
Probably 1-2 MPG which can always be excused by
citing driving technique
less gas mileage out of your vehicle. The manufacture
> doesn’t want to get sued for advertising a
certain MPG and you getting slightly less. They
> have to tell you to run premium or risk
litigation with the EPS or the consumer.”
Accurate or BS?
AZBob
Sent from my iPhone
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