plynn41 said:
Rich, is this true or not?
High end tuners measure exhaust temperature as a way of determining fuel mixture requirements?
Not true for anyone that actually knows how to tune. Plug reading and wide band oxygen sensors are the preferred methods by "real tuners"
plynn41 said:
I.e., a hot exhaust indicates a lean mixture and a cool exhaust indicates richness.
Incorrect. Higher than "normal" exhaust gas temperatures (EGT) indicate late combustion in the cycle. Lean mixtures tend to burn slow and raise the exhaust gas temperatures, overly rich mixtures can burn even slower (to the point of some of the fuel burning in the pipe) and raise the EGT even higher. This rich effect is especially true at low rpm where the combustion tends to be inconsistent at the best of times. Want graphic proof? Leave the choke on longer than you should on a modern four-stroke and the pipe will glow bright enough to be seen from outer space. At WOT and high engine speeds rich mixtures tend to have less impact on EGT so it gets even more unreliable as a means to determine A/F ratio.
Retarding the ignition timing will also raise the EGT. This is a fundamental part of two-stroke tuning. Retarding the high speed ignition timing on a two-stroke will raise the EGT and some of the available combustion heat is manipulate the pipe wave timing and improve the scavenging/ram tuning. Less efficient combustion tends to raise exhaust temperatures, so they are effectively trading off some thermal/combustion efficiency to improve the scavenging and increase the delivery ratio of fresh charge.
So rich mixtures can potentially raise EGT, as can lean mixtures, and ignition advance which is variable at different rpm also has a huge influence on EGT. Don't forget that engine load can also influence it. So you have to ask yourself, when the EGT changed what was the root cause.
I think the critical part that is often times over looked when people try to measure tuning variables with things like EGT is they fail to grasp all of the interactions that contribute to the number they read.
plynn41 said:
Your post suggests that exhaust temperature can't indicate anything about the jetting because it could be rich or lean.
Given how little time is spent at WOT on an MX bike it's difficult to isolate temp changes in the exhaust based solely on A/F ratio. I really believe that the influence of exhaust timing, and ignition advance/retard make part throttle readings "pretty useless" especially given how slowly most affordable thermocouples respond. Considering the number of throttle transitions you'll see during an average lap around the local MX track it's hard to see a real value in tuning EGT. For karting use where WOT performance plays a greater role EGT seems to hold much greater value as a tuning aid but I still tend not to trust it as anything but a reference based on a previously established and confirmed baseline temperature.
Even in those situations you are just measuring a secondary effect of combustion efficiency with EGT not air/fuel ratio, so ignition timing, charge purity, and a host of other factors will influence the final temps. Wideband O2 sensors feeding a fast DAQ system is IMHO the only proper way to measure A/F ratio without HUGE resources, but it's still an expensive proposition so it's largely ignored by a lot of "tuners"
It's interesting to note that Honda with all it's resources uses a detonation counter system that detects knock intensity and frequency. They developed it as an affordable tool for racers to get optimum engine settings (jetting, ignition timing) without the need of an expert tuner that can read plugs.
I guess they don't think EGT is all it's cracked up to be either. ;)
Here's a little excerpt from a paper Honda published. The full paper has lots of the the pertinent details
From SAE 1999-01-3324 / JSAE 9938079
Detonation Counter for Production Racing Motorcycle
Hajime Soda and Tomonori Sato
Honda Racing Corp.
Copyright © 1999 Society of Automotive Engineers, Inc.
ABSTRACT
When the detonation caused by pre-ignition and post-ignition,
it is generally known that strong vibrations from
the combustion chamber develop from impact of that
combustion pressure. This will affect the engine. Methods
to accurately measure the strength of this detonation and
its usage are being considered and used from various
viewpoints. We measure this detonation quantitatively by
the detonation counter. This is done in order to bring out
optimum performance of the engine, specifically the
spots, carburetor settings, and ignition timing. Where
adjustments must be made and it is useful as a tool for
judging, conducting, and setting adjustment directions
and volume quickly and accurately. Furthermore, simplification
of this system has the enable it to be sold on the
market. As a result of this, it now has spread to and is
being used for racing base.
INTRODUCTION
Increasing the compression ratio, making air fuel ratio
(hereinafter referred to as A/F) lean are generally used
as methods to increase output, and increase fuel savings
for vehicles used for racing. However, these methods
usually increase detonation in the combustion chamber,
as a result this detonation hampers output performance.
The method, which measures strength and frequency of
detonation and assesses abnormal combustion quantitatively,
is being considered from various angles. As the
detonation of internal combustion engines are caused by
many factors, such as air mixture pressure, temperature,
A/F, mixing condition, octane content of fuel, engine
speed (NE), structure of combustion chamber, etc.
Although the detonation is judged accurately, it is not
easy to decide how to set the engine to control the above
noted detonation.
The case of engine for racing engines, the optimum set-ting
of A/F for carburetor, we must consider that the
weather conditions, rider’s engine speed range, throttle
opening range in each circuit, each race, each run. This
is necessary to bring out optimum engine performance.
In the past, the mechanic looked at and confirmed the
burning conditions of pistons and plug, the amount of car-
bon stuck, and damage to the piston head after circuit
running. Then they made a comprehensive judgment of
acceleration of engine speed, traction, burning noise
caused by detonation and information from the rider. This
largely depended on human intuition. As a result, there
are cases of engines not running at the optimum setting,
but rather at a low performance condition. Lap times did
not improve, and there were cases when the engine was
badly affected by frequent occurrence of detonation. This
means that the mechanic (or rider) needs to have a lot of
experience and considerable data, which has accumulated.
In recent years, gasoline used in racing has changed
from leaded to unleaded gasoline. This was due to environmental
protection and lowering octane content of gasoline.
As a result, the generation limit of detonation was
lowered and the number of reported cases of burn have
increased.
We want to reduce the load to the mechanic and to the
rider by making feedback of techniques, setting methods
of 500cc racing motorcycle on the market, and relating to
detonation developed by factory activities, as much as
possible. Detonation counter was developed as a tool to
set the engine at optimum condition that does not depend
on human intuition. The detonation counter indicates the
number of detonation. The mechanic set the engine condition
depends on the number.