Originally Posted by JohnZ
Not having full vacuum advance at idle (timing retarded from where it should be at idle) increases exhaust gas temperature, which heats the coolant more, driving up temperature at idle; this was the factory design "ported spark" condition in the 60's with air-injection systems to ensure a good "afterburn" in the exhaust manifolds when the injected air hit the exhaust stream at idle, which was where emissions testing was done in those days.
Most of those "ported-spark" cars had factory base timing at 0 degrees (some were actually 2-4 degrees AFTER TDC), and had 30-34 degrees of centrifugal advance in the distributor to try and make up for it. Cars with full manifold vacuum on the distributor at idle had factory base timing set at 8-12 degrees, and their distributors had 20-26 degrees of centrifugal advance. Same WOT total timing for both, but the "ported spark" engines with air injection systems and retarded idle timing ran like crap at idle and part-throttle, and ran hot at idle.
Properly-matched vacuum advance can calibration (so it's fully-deployed at idle) with manifold vacuum makes for stable idle and a cooler engine at idle, better part-throttle driveability and throttle response, and improved fuel economy.
If you had a timing light rigged so you could see it cruising down the highway at 50-60mph with vacuum advance, you'd see 50-52 degrees advance at highway cruise; the lean cruise mixture takes longer to burn than a rich mixture, so the vacuum advance "lights the fire" sooner so maximum cylinder pressure is still reached just past TDC for peak fuel efficiency. When you accelerate, the mixture is instantly richer and takes less time to burn, so the vacuum advance drops off, retarding the timing slightly to accommodate the faster burn time, again maintaining peak fuel efficiency.
Without vacuum advance (just centrifugal), timing is only affected by rpm, not engine load (manifold vacuum), so optimum torque isn't reached and timing isn't optimized for engine load and mixture burn rate. Vacuum advance is useless on a race engine that is run at WOT all the time, but it's essential for a good-running street engine that has to run at idle and part-throttle too.
People that run fancy chrome-and-polished whizbang centrifugal-only distributors on a street engine have been suckered by the Summit/Jeg's race-car marketing hype, and are missing out on the joys of a really good-running street combination. Absolutely. Positively.
'69 Z28 Fathom Green
would the amount of vacuum available make a difference on these vacuum advance distributors you praise? I am just wondering, if a car has a RACE cam in it, shouldnt the parts follow suit?.........because my 396(.030 over) was a stock block, but the cam is a FAR, FAR ,FAR ,FAR cry from OE. 292magnum, 244 on the I/E with .550 lift and laying on a 110LSA.........would stock parts be a good selection????? just wondering PROF.
i consider my car streetable, but nothing like the 95 monte carlo, or 01 GMC pickup i drive....they are light years ahead....
i mean, a stock fuel pump runs out of gas, the stock carb just wont work well with that cam, the stock points setup is far from desirable when you spin to 6500, i had to have headers you know, i mean the race parts get that domino effect............i believe that last portion of your comment gave too little info to lump us all into being gullable, ignorant consumers....just my .02
should i throw my pro billet distributor in the trash, and go OE?