I was thinking last night (a dangerous passtime, not to be attempted
without appropriate medication) about my long discourse concerning SU
carburators. It occured to me that some of the logic used, although
theoretically sound, does not fit the real world results.
For instance, if constant depression / velocity carburators were truly
independent of what goes on behind the throttle valve when mixing fuel with
air, then a carburator would require one setting and one setting only for
any application. Yet SU's require different needles for different engines.
Also, I know from personal experience that engine changes which improve
engine airflow efficiency- such as an exhaust header- will make the engine
run leaner. Again, theoretically this should not effect the air fuel
mixture ratio on an SU but, nonetheless, you have to install richer
needles. People often say that a modified engine needs a richer mixture
but that is not precisely true. The air fuel ratio for best power is the
same for an F1 car is the same as it is for your great aunt's Matador. (I
know people tune race cars richer than street cars but the car doesn't
benifit from an overly rich mixture any more than a street car). Somehow
the venturi is being effected by conditions far away from where it is
mixing fuel.
In my job, I work a lot with design engineers and technicians. Quite
often, the people with proven theoretical brilliance get in the most
trouble because they have trouble understanding that the real world didn't
read the textbook. Those that listen to the technicians, who have years of
experience, are usually succesful. Those that are too stubborn get
promoted into management where they can't do any harm. One memory I have
from college (most of the others center around the price of beer) is that,
whenenver you derived an equation in physics, the first thing you did was
choose to ignore or hold constant all of the variables that you couldn't
measure or calculate. The second thing you did was throw out all of the
second order effects because they were too small to make any difference.
This is why the only law that is universally obeyed is Murphy's law.
LBC content. The weather has sucked this week. Enough rain to discourage
top down driving but not enough to help the crops. I've only put 50 miles
on the A so far :-( I also haven't taken anything apart to fix it. Talk
about LBC withdrawal!
One last thing
Q: How can you make a small fortune on MG restoration?
A: Start with a large one.
Regards,
Bill Eastman
61 MGA wasting away in the garage
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