Skye, you may get a hundred replies to this question. You
may get only this one. Hard to gauge these kinds of threads.
I'll put my two-cents in embedded in your text.
> I've heard a lot about the usefulness of vac gauge testing
> and I'd like to give it a try.
It was THE accepted way of testing and tuning engines
before all the exhaust gas analyzers -- especially in the
British race, hill-climb and rally world of the 50's and 60's.
> Do I hook it up to the vac advance port on the manifold?
Yes or no, and I can't over-emphasize that enough. :-P
As the anti-polution-crazy 70's came and went, the British
car manufacturers jumped through many hoops trying to
keep their little cars viable for the US market. Much of
that effort came in the form of tweaking the timing under
vacuum control. Examples include vacuum advance AND
vacuum retard with some acting under full manifold vacuum
and others acting on ported vacuum from the carb. All
are different therefore, the first step is to know what
system you have on YOUR car. Here are the differences
that matter to you, the tuner:
If your distributor advance is connected to the carb,
it is 'probably' a ported vacuum advance. In this system,
the throttle plate (butterfly), when closed, just covers the
hole connected to the vacuum port. In this system, there
is little-to-no vacuum applied to the distributor's vacuum
advance capsule with the throttle closed (e.g., at idle).
That is when the manifold vacuum is at its highest. Once
the throttle cracks open, the engine gets a gulp of air, fuel
flows from the jet to balance it, the vacuum drops some
but the hole is also uncovered. The timing then <usually>
advances in anticipation of driving off. Once the driver lifts,
the throttle closes, the butterfly covers the hole and the
timing retards back to the static+centrifugal advance value.
From this description, one can see that this is NOT the
vacuum level that one needs to read to measure combustion
efficiency.
On a car set up like this, one has to look for (or add)
a port on the manifold as THAT is the vacuum level that
one needs to read.
If your vacuum advance is attached to the manifold,
then you can insert a "T" in the line and measure vacuum
there. Some of the gauges come with T-fittings just
for this purpose. As emphasized above, just don't use
them on a line going to the carb.
Now, with all that said, some carbs have TWO vacuum
ports -- one from the hole in the choke and one from
simply within the choke, downstreem of the butterfly.
In that case, use the downstream tap.
And for more variance, some carbs have only the
downstream tap and don't have the little hole that the
butterfly closes over!
The real answer is to know the carb and how the
vacuum is ported. You may even have to remove
one of them to get the answer BUT, you can sometimes
get the answer from empiricle data. That is, by observation
of the operation of the advance. With your engine on
slow idle, disconnect the vacuum line and plug the bit
going to the carb. Did the engine speed change (read:
timing change)? If so, the advance was working in response
to vacuum from the port. In this case, the port is probably
reading manifold vacuum. If no change occured either the
port is covered by the butterfly -- or your vacuum advance
is broken! The later condition is easily tested by other
means.
More than you ever want to know, eh?
> Also can someone recommend a brand? I like to have good
> quality tools.
While I can NEVER fault buying quality tools, I must
add that the quality of the vacuum gauge that you buy is
probably not a big issue if you use it only ocassionally.
Mine is a generic auto parts store variety and cost under
$20. It also doubles as a fuel pressure gauge. Remember,
you are tuning for maximum vacuum, not some absolute
number. The precision of the gauge is much less important
than repeatability of measurement.
> Skye
Best,
rick
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