-=> Quoting Alexander Bilan to Bryson Hughes <=-
 AB> "Most carburetors provide ports for timed spark and straight manifold
 AB> vacuum.  In general, a retarded spark reduces oxides of nitrogen to a
 AB> minimum by keeping peak pressures and temperatures at lower values
 AB> than would be obtained with an advanced spark setting.
This is true if all else remains the same on the engine. Also more of the
thermal energy is transfered into the exhaust and makes it hotter. This
heat aids in burning off emissions in the exhaust system. And it sharply
reduces the possibility of an exhaust backfire.
However compensations can regain peak pressures as noted in your post below.
 AB> ...this is good
 AB> for reducing emmisions-but bad for economy, driveability and heating of
 AB> coolant."
As for heating of coolant, or the engine in general, I have found retarded
timing to reduce thermal transfer into the combustion chamber and coolant,
if all other things are the same. One easy way to overheat an engine is to
have too much spark lead.
 AB> "The use of a retarded spark requires richer jetting in the idle and
 AB> main systems to get off-idle performance and driveability."
Only if the spark is retarded across the entire engine RPM range will
the main circuit need to be richer.
However the use of port vacuum only retards the timing at closed throttle
settings. Open the throttle and the timing will be the same as it would be
on manifold. The main can be the same. The idle will be set more open to
get the proper idle rpm, and the mixture MAY need to be slightly richer,
depending on other engine systems that come into play. On a stripped 
down hot rod engine the idle could need to be a bit richer on port
vacuum, than the hobbyest found he could get by with when he had manifold
hooked up.
 AB> "It must also be considered because high temperatures at idle due to
 AB> the retarded spark and high idle-speed settings definitely promote
 AB> dieseling or after-run."
I think the "high idle temperture" stuff is bogus. However the idle rpm
setting need be no different. It is just that to get it, the throttle will
have to be adjusted more open relative to an engine getting manifold vacuum
at idle. The wider throttle settings can cause after run. I have experienced
it, especially with higher compression engines. I prefer to use a throttle
stop solenoid that allows the throttle to fully shut when the ignition is
turned off.
 
 BH> For pure hot rodding, no vaccum advance is used very often. Also some
 BH> heavy duty and truck applications use no vacuum advance.
 AB> Point taken,  =) *Street* Rodding then.
 
If your car is nearly stock as are your systems, then making sure
everything is right and doing tuning tweeks here and there, starting
with factory specs and setups as a referance point. The biggest problem
I see with the engines compartments of El Leadfootos is that stuff is
in a mess and not hooked up or working. Distributors are a big offender.
 AB> Could one not argue that under idle to just 'a little beyond closed
 AB> throttle' manifold vacuum gives you an advantage for a standing start?
 AB> (becuase you have manifold vacuum, but no ported vacuum.)
 AB> Granted, the time period for this is short, and mechanical advance
 AB> will take over. But it is a possibility. =)
If throttle sharpness is a concern, the best answer is a fine tuning.
After that *maybe* a notch on the accelarator pump. Lowering friction
and improving operating efficiency are also good areas to work on in
a nearly stock engine.
Myself, I would put on a set of headers, windage tray, and an intake such
as a performer. Cams that are set up for 4WD truck engines tend to deliver
sharp throttle response and fairly normal drivability. Not really hot rod
stuff though. 
If your just wanting to stand on the throttle and move out quick, a higher
stall torque converter can give you an advantage if all else is the same.
Lower gear ratios might be the next step. Some of those things may get you
a fun street rod.
--- Blue Wave/Max v2.12 [NR]
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