In my last long, one sided post (diatribe? Santa brought Nancy a
dictionary so I really must look this word up) that once again proved the
technical superiority of the MGA, I stated that the two variables we could
control for engine efficiency were throttle position and rpm. This was
assuming a constant state of tune and, as we all know, that state of
affairs exists only in a theoretical world for our beloved LBC's and does
not sufficiently describe this side of Utopia.
When it comes to tuning, the two variables we control are spark and
mixture. For both of these, short of swapping parts, our control is
limited to a single adjustment so we, to a degree, must cast our fates to
the judgement of Abingdon and trust that once we optimize (wishful
thinking?) a single point for these settings, the rest of the points on the
curve will fall in line. Since theory is the topic of the day and someone
in a brilliant flash of foresight has chosen to limit the length of
individual posts, I will limit the topic here to fuel air mixture theory
and the occasional shot at Tri**phs and extolling the virtue of MGA's ;-)
When discussing the fuel air mixture, a graph is usually shown that has two
curves drawn on it. The ordinate of this graph (x axis, independent
variable, left to right scale) is the fuel air ratio and the abscissa (y
axis, dependant variables, up and down scale) is either bmep or bsfc
depending on which curve is drawn. For simplicity, both curves are usually
drawn on one scale and the stoichiometric (ideal) mixture is marked as a
vertical line. The fuel air ratio is usually expressed as a percentage of
the ideal mixture so stoichiometric would be 100%. 80% would be a very
lean mixture and 150% would be a very rich mixture. To fully characterize
an engine you would need a five dimensional graph (mixture, throttle
position, rpm, load, temperature) so these graphs are seldom generated but
they all have the same basic characteristics so a single 2d graph can be
used to discuss general trends.
The two curves that typically appear on a fuel-air graph are bsfc vs
mixture ratio and bmep vs fuel air ratio. they both start out low at lean
mixtures, increase in the middle, then drop off at rich mixtures. Unlike
torque and hp curves, however, these curves have specific cutoff points on
the lean end and the rich end. These points are called the lean limit ant
the rich limit. These are the point where the engine starts to misfire due
to too much or too little fuel. Both curves also (obviously) have a peak
but the position of this peak and the general shape between the two curves
(bsfc, bmep) are quite different. Fuel efficiency (bsfc) tends to have a
very steep slope from the lean limit up to a peak on the lean side of
stoichiometric and then tapers off gradually as the mixture gets richer.
Torque output (bmep) on the other hand has a more gradual rise until about
110% fuel and then it would remain fairly stable until around 130% (if
memory serves me correctly) at which point is would start tapering off
until it peters out at the rich limit.
So, if one were to look at the graph and identify the points of interest
from left (leanest) to right (richest mixture) they would be lean
combustion limit, best economy, stoichiometric, lean best power, rich best
power, rich limit. By my memory, the lean limit is typically 70-80% and
the rich limit is typically 140-150%. Since the engine tends to run OK by
ear (no misses) between these settings, there is a pretty big window for
tuning and still have still your engine will run OK.
For performance cars, two of the most popular carbs are the SU and the
Holley. You would think that this would be because of their superiority in
maintaining an accurate mixture but that is not the case. The SU and
Holley are popular because they are relatively simple to tune at a single
point and do a reasonable job of maintaining the mixture between the lean
and rich limits at other points. With either a home mechanic can get the
plugs to look good after a quarter mile run or get the color tune just
yellow from blue and afterwards be rewarded with reasonably miss-free
performance throughout the operating range. Strombergs, Rochester Q-jets
and Carter T-quads (a favorite of mine from my Mopar days) were scorned be
the performance crowd because they "couldn't put out the power." There is
nothing inherent in these carbs that limits their power output. In fact,
removing an 850 cfm T-quad and replacing is with a 750 cfm Holley was
actually a step backwards in ultimate power capacity. The real issue is
that these newer carbs were tuned leaner from the factory to control
emissions and they tended to be more complicated in an attempt to control
the fuel air mixture better. The home enthusiast did not need the extra
complexity so they got a bad rap. Also, due to the complexity, they do not
age gracefully (particularly the Stromberg) so once again people move back
to the more simple devices.
SU's are straight airflow devices. If the engine is gulping 2 lbs of air a
second, the SU will mix in X lbs of fuel regardless of other variables.
Also, the cold engine enrichment mechanism is fairly simple. Furthermore,
it is well known that a lean mixture runs hotter and can be more sensitive
to knocking. Because of these facts, the typically LBC is tuned towards
the rich side of the spectrum and tends to stay between lean best power and
the rich limit most of the time. This gives good power and throttle
response and since the rich limit tends to be "softer" than the lean limit-
an engine misfires or gets lazy on the rich side while it cuts out at the
lean limit- occasional excursions outside of the preferred ratios are
better tolerated by the operator. The car is also less "cold blooded."
What's the point of all of this? None, really. It is slow hear at work
today. I can't bother anyone else so the MG listers are the ones that
suffer. I will continue to tune the MGA to the rich side because it runs
better overall, it stays cooler, and the choke has to be used less after
startup. The cylinder lubrication does suffer a bit due to fuel washing
off the oil and the oil should be changed more often. Also, the top end
would probably carbon up if the car ran under low load conditions for a
long time but that is just another excuse to run it harder ;-) I could
improve the economy by tuning leaner but fuel is not exactly the largest
expense to owning an old LBC and besides, I would trade miles per gallon
for smiles per mile any day!
So, when extolling the virtues of SU carbs, remember try to be accurate to
their merits. These devices are not the most precise fuel mixers over all
operating conditions every created by man. In fact efficiency would have
benefited greatly by even a rudimentary attempt to adjust the fuel air
mixture for engine load. However, they are good looking, reasonably
robust, simple to adjust, and capable of keeping an LBC running well (if
not efficiently) for years. Although they can also supply fuel in
reasonable quantities, Strombergs and Webers belong on Tri**phs and Fiats
so just say no!
Regards,
Bill Eastman
61 MGA with two beautifully adequate H4's
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