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Old 03-02-2012, 09:11 AM   #74
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Anyone hear that bell...?! School's in:

Originally Posted by BowerR64 View Post
When you get a tune it says premium fuel only and this is why because there IS a difference. It burns better, cleaner or something and I'm sold on the better gas now.
Why does the cheap fuel cause knock and what is it they put in it that does it?...
After that tank was done i filled up with my normal 92 premium from the BP near the airport i usually go to and i instantly felt the difference
I hate to say it, but, check the Top Tier list; BP hasn't been rated for good gas in a while!

As far as why cheap gas 'knocks', strap in for the answer: because heptane is cheap as shi*.

"Antiknock Quality
An octane number or rating was developed by the
petroleum industry so the antiknock quality of a gasoline
could be rated. The octane number is a measure
of the fuel’s tendency not to experience detonation in
the engine. The higher the octane rating, the less the
engine has of a tendency to knock.
Two methods are used for determining the octane
number of gasoline: the motor octane number
(MON) method and the research octane number
(RON) method. Both use a laboratory single-cylinder
engine equipped with a variable head and knock
meter to measure knock intensity. A test sample of
the fuel is used in the engine as the engine’s compression
ratio and air-fuel mixture are adjusted to develop
a specific knock intensity. There are two primary
standard reference fuels: isooctane and heptane. Isooctane
does not knock in an engine but is not used in
gasoline because of its expense. Heptane knocks
severely in an engine. Isooctane has an octane number
of 100. Heptane has an octane number of zero.

A fuel of unknown octane value is run in the test
engine equipped with a variable compression cylinder
head and a knock meter. The severity of knock is
measured. Various proportions of isooctane and
heptane are run in the engine to duplicate the severity
of the engine knock when the test fuel was run.
When the knock caused by the isooctane and heptane
mixture matches that caused by the fuel being
tested, the octane number is established by the percentage
of isooctane in the mixture. For example, if
85% isooctane and 15% heptane produced the same
knock severity as the tested fuel, that fuel would be
rated as having an octane rating of 85."
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