Ported vs manifold vacuum for ignition advance?

[novadude]

Not sure how that proves that PVA was used for emissions reasons? PVA was used in the 40s and 50s when performance and efficiency were a concern and emissions weren't given a second thought. This is well documented in automotive engineering texts and service manuals. I guess we'll have to agree to disagree on this.

What examples have you got ?

Here is just one example from a 1958 Ford manual (pre-dating emissions era). This is for a Y-block V8.

http://www.oldcarmanualproject.com/manu ... 2-018.html

Note that it says: "When the throttle plates open the distributor vacuum passage is exposed to manifold vacuum...". Clearly, they are saying that the vacuum canister was connected to a ported source. Lots of other examples in old 1940s /1950s manuals as well.

[Tom68]

Yer, I had a '59 VW when I was a kid, from memory VW ran a couple of different adv systems and power enrichment to. Back then before I knew about the emidsions angle I thought ported vac was to help tip in, so you didn't go from full vac adv to none when you cracked the throttle quick enough to drop manifold vac.

Another advantage of ported vac would be a poor mans start retard. No idle vac adv seems a high price to pay though.

Remember the old doomsday movies where anandoned idling cars would be boiling, no vac adv at idle on them.

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[Tuner]

The sidevalve 2 barrels had vacuum brake distributors, high air speed through the venturi ported distributor port braked the advance in the dizzy.

If you are referring to the vacuum brake distributors on Flathead Ford, 32-48, that isn't how they work. When no vacuum is on on the brake piston it applies and drags the advance mechanism to full retard. Load and large throttle opening drops the vacuum and allows the spring to push the brake against the slip ring to retard the timing.

When the brake is applied it has the effect of slowing the mechanical advance so it may not reach total until 3000 or higher RPM.

When the vacuum is high the brake is off and the mechanical advance is allowed to move to full advance, which was only 17 degrees on early and "Denver Head" engines and 25 degrees on later engines.

Early engines with light load high vacuum (brake off) were full advance at 12-1400 RPM. Later engines full at about 2000. Standard initial advance was 4 degrees.

The mechanically inclined user was encouraged to listen for knock and adjust the initial and vacuum brake spring tension accordingly.

3-bolt distributors up to '41 got manifold vacuum through an external steel line from the manifold below the carb and '42-'48 2-bold distributors got manifold vacuum through passages in the manifold and block and timing cover.

In 1949 Ford introduced the "Load-O-Matic" distributor advance system which used a vacuum diaphragm instead of the vacuum brake piston of the '32-'48. The Load-O-Matic used venturi vacuum to advance the WOT timing at full load and manifold vacuum for additional timing at part-load.

The carburetors for this system had internal passages arranged to change from manifold vacuum to venturi vacuum using steel and nylon ball check valves and a vacuum operated valve that looks like a Holley power valve, but its open/closed functions are opposite of a PV. That is the deal that looks like an external power valve on Ford carbs of the 49-56 model years when they used that Lode-O-Matic system on all Ford cars.

1957 and up the V8 got mechanical advance with ported vacuum advance, however the Load-O-Matic was was still used on 6 cylinders until the late '60s.

[Tom68]

Cheers tuner.
Grew up with a shed filled with SV gear, explored it all but never knew what went with what carb and dizzy wise.

Worked on one recently that had the venturi port to electronic vac adv dizzy.

[Tuner]

Take care to realize flatheads use a lot less timing than overheads. Also, depending on the "Ricardo Combustion Chamber" squish area percentage some only use 17 total, others 25. Hudson Hornet, 17 total at WOT.

[Tom68]

Old VWs some didn't have vac adv, looks like the ones that did were ported.

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[Geoff2]

Chrys engines used ported vac adv, one of the few things they got wrong. Had they used man vac adv, they would have had improved economy, cooler running etc.

[mag2555]

A whole bunch of GM V8 cars from 68 to early 70s used the TVS temperature referenced vacuum switch.
In some years this system was used on both auto and man trans vehicles.
If engine temp got up to 230 degrees at idle the switch brought in some 20 more degrees of advance until the engine got down to what was considered normal operating temp.

A lot of guys who have hopped up there street motors and are posting on many different boards about a overheating condition they have now don’t seem to look at the fact that they are running a distributor with no vacuum advance capability at all, or have a fully digital ignition system and still only run less then 40 degrees of total advance when highway driving.

There’s nothing like having a strong street motor trying idle cool with 32 degrees and having unused fuel still burning in the exh ports and or headers nicely added to the engine temps being seen!