I love a good technical discussion!
Concerning factual innacuraccies in my last post. Todd may be right that
best power is richer than 110-130 percent fuel. For sure he is right that
the percentage is the percent of fuel delivered compared to stoichiometric
(14.7:1). I don't look these numbers up and my last class on this was
almost 20 years ago. I could pull out my text book (written in 1947 if my
memory serves me correctly) but the actual numbers aren't that important.
What is important is that, for a given air flow (which translates to a
given dashpot level to achieve a constant velocity) the engine requires a
different strength mixture for optimum performance depending on whether it
is pulling or cruising.
Concerning the design ingenuity of the SU carb. I agree that anything that
does a better job is considerably more complex. The SU is a very elegant
solution to a very complex problem. Plus they look really cool when
compared to the more mundane mixing devices.
However, they still have no way to compensate for engine load. They simply
mix a specific amount of fuel for a specific velocity of air. You choose
the mixture you want- say 110% and that is the mixture that is delivered no
matter what (theoretically).
The signal that they use to position the needle is outside of the throttle
butterfly and, therefore, does not compensate for changes in manifold
vacuum. Manifold vacuum is a primary indicator of engine load. An engine
at low rpm with the throttle wide open (low vacuum, high load) and an
engine at high speed cruise (high vacuum, low load) could have the same
airflow (and its associated dashpot level to achieve constant velocity)
but have very different mixture needs. At low rpm under power, you want
the mixture at lean best power or richer (the best power mixture level is
actually a fairly wide band. I thought it was 110-130% but is could be
richer). At high rpm during cruise, you would want to lean the mixture to
achieve more efficiency. In modern cars, this is done by electronic
wizardry. Holleys use a "power valve" that opens at a preset _intake
manifold_ vacuum to enrich the mixture under power. Rochesters and Carters
use metering valve which are again controled by _manifold vacuum_ to
adjust for engine load. Look at emissions control ZS carbs and they have
all sorts of extra circuits including, I believe, a rudimentory power valve
(they changed as regulations became stricter). SU's do not use manifold
vacuum as an input signal so they _cannot_ adjust the mixture for engine
load.
SU's do provide an accelerator pump function with their dashpot dampers.
This provides more fuel during a transition between lower airflow and
higher airflow conditions. This is a relatively short term enrichment to
make up for the fuel that runs along the bottom of the manifold taking
longer to reach the engine so, without an extra boost, the engine would go
lean and stumble on tip in. This eliminates the hesitation often
associated with throttle response but it does _not_ enrich the mixture
under load.
Concerning LBC's getting poor fuel economy. A typical MG gets 20-25 mpg.
I have a modern car that weighs 50% more, has nearly double the power, and
gets 30-35 mpg. The main reason is _not_ aerodynamics. It may have a
better Cd but it also has near double the frontal area. It is the fact
that computer controll allows precise mixture adjustment and the associated
greater efficiency. You can tune an MG to get better mileage but is will
be gutless and you will risk putting a hole in a piston because the SU
can't provide a richer mixture under high load.
Many new cars use mass air flow to adjust the mixture. Just like an SU,
you may say. While it is true that the SU is a primarily an airflow
adjusted device, modern cars that use air flow sensors also use throttle
position and rpm to calculate fuel delivery. A low airflow with low rpm
and the throttle wide open would trigger a very rich mixture. Again, SU's
use only airflow.
Concerning the use of oxygen sensors. My understanding is that most oxygen
sensors are very sensitive right around stoichiometric but almost useless
in the more rich or lean conditions. In other words, an oxygen sensor
would help tune at cruise where you want to be near stoichiometric but it
would not help much under acceleration because best power settings are
outside of its sensitivity range. Maybe they have improved lately. Also,
I don't think that Oxygen sensors are DC devices so I don't know if you can
read one with a VOM but, again, don't bet the farm on this because this may
be poor memory or old information.
Am I suggesting that you dump your SU's? Not at all. The devices work
quite well for their simplicity. The amount of money and time needed to
replace them would never be retrieved in fuel savings. A good set of SU's
provide acceptable driveability and isn't that what its all about?
Regards,
Bill Eastman Can I go home now? My brain hurts!
61 MGA with beautiful, simple, elegant, perfectly acceptable twin SU's
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