Dan,Dan Timberlake wrote: ↑Sun Nov 01, 2020 6:32 pm I think the likely perpetrator of the "emissions only" explanation was John Z (John Hinckley ) who used to post on various Corvette forums.
http://www.camaros.org/pdf/timing101.pdf
He was a manufacturing/process Engineer at GM and later at Chrysler.
His mostly good "TIMING AND VACUUM ADVANCE 101" unfortunately stated flat out -
" 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. "
I also think John may have conflated lean mixtures and low density intake charge, but perhaps to simplify the discussion.
Regards,
Dan T
Thank you for posting that. Those are great examples of pre-emissions use of ported vacuum. From my own experience in automotive manufacturingI long suspected the author was probably not involved in engine design and performance even if he was an engineer.
A classic example of a well meaning person beleiving his knowledge and therefore understanding being greater than it actually is.
Shrinker and a couple of others set me straight on this emissions control myth, and I don't understand why some have so much trouble recognizing the myth is based on a false premise.
Here's is the archived thread with a few more.
As I looked through factory lit, I found my own examples, such as this from Chrysler's 1959 Master Technician's Conference.
And then, recognizing the original 3310 carbs have timed vacuum ports, found examples of 49 state pre-1968 Chevy high performance engines. Its really not a matter of good or bad. Generally its a better design to use base for initial, mechanical advance to account for timing needs that change with rpm, and vacuum for addressing timing changes that occur with load. Idle was not traditionaly a 'lean' condition until emissions concerns tried to reduce both CO and HC. Whether to use vacuum advance at idle or not comes down to what can be done to shape the mechanical advance, how stable the vacuum is at idle, and how slow the burn is at idle. A lot guys don't want to get into shaping the advance on an emssions era distributor (esp 1968 to roughly late 70s), and sometimes an easy work around is to switch to manifold vac.
I'll load up an example of non-smog vs. smogged timing specs so folks can see how switching to manifold vac can work. CAP = Clean Air Package (in 1967 only used for California cars)
The other change that will likely have to be made is to richen the idle mixture from the CAP settings.