This might help to explain. Yes it does affect engine temperature both at idle and cruise. BTW the Tony who wrote this is not me. My engine is modified to some extent, but I am running about 50 degrees at cruise and 30 degrees at idle on vacuum advance. Temps about 175 degrees normal driving. Get yourself a dial timing light.

TIMING AND VACUUM ADVANCE 101
By Tony (formally from GM)
- 1 -
The most important concept to understand is that lean mixtures, such as at idle
and steady highway cruise, take longer to burn than rich mixtures; idle in
particular, as idle mixture is affected by exhaust gas dilution. This requires that
lean mixtures have "the fire lit" earlier in the compression cycle (spark timing
advanced), allowing more burn time so that peak cylinder pressure is reached
just after TDC for peak efficiency and reduced exhaust gas temperature (wasted
combustion energy). Rich mixtures, on the other hand, burn faster than lean
mixtures, so they need to have "the fire lit" later in the compression cycle (spark
timing retarded slightly) so maximum cylinder pressure is still achieved at the
same point after TDC as with the lean mixture, for maximum efficiency.
The centrifugal advance system in a distributor advances spark timing purely as
a function of engine rpm (irrespective of engine load or operating conditions),
with the amount of advance and the rate at which it comes in determined by the
weights and springs on top of the autocam mechanism. The amount of advance
added by the distributor, combined with initial static timing, is "total timing" (i.e.,
the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has
absolutely nothing to do with total timing or performance, as when the throttle is
opened, manifold vacuum drops essentially to zero, and the vacuum advance
drops out entirely; it has no part in the "total timing" equation.
At idle, the engine needs additional spark advance in order to fire that lean,
diluted mixture earlier in order to develop maximum cylinder pressure at the
proper point, so the vacuum advance can (connected to manifold vacuum, not
"ported" vacuum - more on that aberration later) is activated by the high manifold
vacuum, and adds about 15 degrees of spark advance, on top of the initial static
timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around
25 degrees with the vacuum advance connected). The same thing occurs at
steady-state highway cruise; the mixture is lean, takes longer to burn, the load on
the engine is low, the manifold vacuum is high, so the vacuum advance is again
deployed, and if you had a timing light set up so you could see the balancer as
you were going down the highway, you'd see about 50 degrees advance (10
degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from
the vacuum advance) at steady-state cruise (it only takes about 40 horsepower
to cruise at 50mph).
When you accelerate, the mixture is instantly enriched (by the accelerator pump,
power valve, etc.), burns faster, doesn't need the additional spark advance, and
when the throttle plates open, manifold vacuum drops, and the vacuum advance
can returns to zero, retarding the spark timing back to what is provided by the
initial static timing plus the centrifugal advance provided by the distributor at that
engine rpm; the vacuum advance doesn't come back into play until you back off
the gas and manifold vacuum increases again as you return to steady-state
cruise, when the mixture again becomes lean.
The key difference is that centrifugal advance (in the distributor autocam via
TIMING AND VACUUM ADVANCE 101
By Tony (formally from GM)
- 2 -
weights and springs) is purely rpm-sensitive; nothing changes it except changes
in rpm. Vacuum advance, on the other hand, responds to engine load and
rapidly-changing operating conditions, providing the correct degree of spark
advance at any point in time based on engine load, to deal with both lean and
rich mixture conditions. By today's terms, this was a relatively crude mechanical
system, but it did a good job of optimizing engine efficiency, throttle response,
fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle
performance, as vacuum advance is inoperative under wide-open throttle
conditions. In modern cars with computerized engine controllers, all those
sensors and the controller change both mixture and spark timing 50 to 100 times
per second, and we don't even HAVE a distributor any more - it's all electronic.
Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After
30-40 years of controlling vacuum advance with full manifold vacuum, along
came emissions requirements, years before catalytic converter technology had
been developed, and all manner of crude band-aid systems were developed to
try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One
of these band-aids was "ported spark", which moved the vacuum pickup orifice in
the carburetor venturi from below the throttle plate (where it was exposed to full
manifold vacuum at idle) to above the throttle plate, where it saw no manifold
vacuum at all at idle. This meant the vacuum advance was inoperative at idle
(retarding spark timing from its optimum value), and these applications also had
VERY low initial static timing (usually 4 degrees or less, and some actually were
set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas
temperature (due to "lighting the fire late") to improve the effectiveness of the
"afterburning" of hydrocarbons by the air injected into the exhaust manifolds by
the A.I.R. system; as a result, these engines ran like crap, and an enormous
amount of wasted heat energy was transferred through the exhaust port walls
into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine
temperatures went up, combustion efficiency went down the drain, and fuel
economy went down with it.
If you look at the centrifugal advance calibrations for these "ported spark, latetimed"
engines, you'll see that instead of having 20 degrees of advance, they had
up to 34 degrees of advance in the distributor, in order to get back to the 34-36
degrees "total timing" at high rpm wide-open throttle to get some of the
performance back. The vacuum advance still worked at steady-state highway
cruise (lean mixture = low emissions), but it was inoperative at idle, which caused
all manner of problems - "ported vacuum" was strictly an early, pre-converter
crude emissions strategy, and nothing more.
What about the Harry high-school non-vacuum advance polished billet
"whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK
on a street-driven car, but some people keep buying them because they're "race
car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at
wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't
TIMING AND VACUUM ADVANCE 101
By Tony (formally from GM)
- 3 -
need a system (vacuum advance) to deal with the full range of driving conditions
encountered in street operation. Anyone driving a street-driven car without
manifold-connected vacuum advance is sacrificing idle cooling, throttle response,
engine efficiency, and fuel economy, probably because they don't understand
what vacuum advance is, how it works, and what it's for - there are lots of longtime
experienced "mechanics" who don't understand the principles and operation
of vacuum advance either, so they're not alone.
Vacuum advance calibrations are different between stock engines and modified
engines, especially if you have a lot of cam and have relatively low manifold
vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they
see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a
modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither”
in and out in response to the rapidly-changing manifold vacuum, constantly
varying the amount of vacuum advance, which creates an unstable idle. Modified
engines with more cam that generate less than 15” Hg. of vacuum at idle need a
vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum
less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-
1810 advance can (about $10 at NAPA) provides the same amount of advance
as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so
there is no variation in idle timing even with a stout cam.
For peak engine performance, driveability, idle cooling and efficiency in a streetdriven
car, you need vacuum advance, connected to full manifold vacuum.
Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand
it, they're on commission, and they want to sell "race car" parts.
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hope this helps