Intake air temperature
1 Octane Number per 13 degrees F
{ Jim Bell rule was 1 ON per 20 degrees F}
from this old post:
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Several days ago I posted Jim Bell of Kenne Bell Company's
'rules of thumb'
about octane, temperatures and pressure effects on engines.
Here are some quotes from a recent article on engine knock,
written by an engineer who just finished doing his Master's Thesis on Engine
Detonation at an Australian University.
I have converted the metric temperatures and pressures in the original article
to good old psi and degrees F. Words inside { } are my comments.
begin quotes
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"Causes Of Knock
As discussed, for a given engine the type of fuel is the most important factor
in promoting or preventing end-gas autoignition, also known as 'knock' or
pinging. The following guidelines show how various engine operating parameters
affect the fuel octane requirement of an engine. The higher the octane
requirement, the more likely an engine is to knock for a given fuel.
Effect On Octane Requirement
(ON = octane number)
Spark advance
1 ON per 1 degree Spark Advance
{ Jim Bell rule was 0.5 to 0.75 ON per 1 degree Spark Advance}
Intake air temperature
1 ON per 13 degrees F
{ Jim Bell rule was 1 ON per 20 degrees F}
Air/Fuel ratio
Chance of knocking Peaks at around 5% rich of stoichiometric {about 14 to 1}
Decrease of around 2 ON per A/F ratio around peak
(Jim Bell rule was also 2 ON per 1 A/F change)
{ so richening a Magnum engine from 14.7 at cruise,
to 11.7 at WOT like the factory PCM programing does,
has an effect like switching gasoline from 87 octane to
[14.7 - 11.7] x 2 + 87 = 93 octane. The extra rich mixture at WOT
is apparently an attempt by Dodge to reduce pinging. I wonder if the 'Reflash'
of the PCM computer by the Dodge dealer not only retards timing but also
richens the WOT mixture even more ? The even richer mixture would be felt as
a horsepower decrease like the so called 'Death Flash' is supposed give. }
Intake Pressure
3-4 ON per 1.4 psi
{ so if a KN filter/inlet system actually lets more air pressure get to the
intake manifold this would INCREASE the tendency to ping. Jim Bell rule was
1.4 ON = 1.4 psi increase. Since 14.7 psi is atmospheric pressure and is also
equal to 30 inches of mercury 'barometric presssure' or 400 inches of water
pressure, then if you have a vacuum/boost gauge fitted to engine you can judge
how intake mods have affected your engine's need for higher octane gasoline. }
Compression Ratio
5 ON per 1.0 Compression Ratio increase/decrease
{ Jim Bell rule was 3 ON per 1 CR increase. The Mopar Performance PCM
computer advances the ignition timing by something between 12 and 9 degrees.
A better way to get both hp and better MPG would have been to raise the
5.2/5.9 V8 compression ratio from 8.9 to about 9.9 with a head gasket change.
The 4.7V8 has a compression ratio of 9.0, but with two sparkplugs per cylinder
and knock sensors the 5.7Hemi was designed for 9.6}
{Aluminum cylinder heads leak more heat out of the combustion chamber but
surpress pinging compared to cast iron cylinder heads. The common rule of
thumb from Smokey Yunick was that "An aluminum cylinder head has to be run at
1 compression ratio more to make the same power as an iron cylinder head" The
old 5.2V8 iron cylinder head was 9.0 compression ratio and the new 5.7 Hemi
aluminum cylinder head is 9.6 }
Exhaust back pressure
1 ON per 4 psi
{ so a low restriction exhaust system may increase pinging very slightly, and
a quick fix to reduce pinging might be to put a big dent in the exhaust pipe.
A stock 1995 5.9V8 has about 7 psi of backpressure at full throttle at 4000
rpm.}
Coolant temperature
1 ON per 18 degrees F
{ Jim Bell rule was 1 ON per 10 degrees. The 180 degree thermostat change
that is so highly touted lowers the coolant temperature by about 15 degrees
from the stock 195, so its effect is small - like putting 88 octane in instead
of 87 octane.}
Similar ON effect for cylinder head and engine block temperature.
{ so removing the rubber strip from the firewall cowl on a Ram truck may
slightly reduce the tendency to knock too}
For naturally aspirated engines, the two main design and tuning parameters
given in the table are the compression ratio and the spark advance. For an
engine at wide open throttle (maximum inlet manifold pressure) and normal
operating temperature; these two parameters, if increased, are then
proportionally the most likely to produce knocking combustion.
Detection Of Knock
There are several ways of detecting when knock is occurring in an engine. Only
some methods, however, are capable of measuring the knock intensity.
1. Sound of Knock
Knock is audible to the average human ear when more than about 10% of total
engine cycles knock. The human ear is the most obvious and readily available
device to detect knock. The ear is surprisingly sensitive and routinely used
in determining the octane requirement of an engine. The sound of knock is best
described as a sharp metallic "ping". This sound is caused by the high
pressure and frequency waves impacting against the cylinder walls, giving rise
to a ringing knock as though they had been struck by a light hammer. The
ringing noise is worsened by the way in which the energy of the pressure wave
is not completely absorbed when it first contacts the chamber wall. Instead,
the wave is only slightly diminished and then reflected back across the
chamber, continuing this process until all the energy is lost to the engine
structure as vibration or dissipated through the gas.
2. Sight and Pressure of Knock
A high-intensity flash is observed in the cylinder when knock occurs. This
facilitates the use of optical probes and ionisation detectors to measure at
least the presence of knock if not its intensity. Much of what we know about
combustion today has come from the analysis of photographs taken using
high-speed cameras as first successfully developed in 1941. More recently, as
the technology has become widely available, direct measurement of cylinder
pressures during combustion has been used to study knock. Direct pressure
measurement allows the study of the actual intensity of knock. The
piezoelectric pressure transducer has been the standard sensor for some time.
Typical pressure traces from a cylinder experiencing knocking and non-knocking
operation were shown earlier.
A major obstacle in developing feasible combustion pressure sensors has been
to overcome sensor performance degradation caused by high combustion
temperatures and strong electromagnetic interference (EMI) from sparkplug
wires. So far, due to inherent material limitations, conventional
piezoelectric type combustion pressure sensors cannot operate over 200°C
without water-cooling. In addition, signal conditioning electronics must
usually be located near the sensor head to combat the strong EMI and stray
capacitance loading effects of spark-plug leads."
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end quote
