Ported vs manifold vacuum

[howzzzit]

My 72 C10 1/2 ton has always used ported vacuum per the factory manuals. I'm thinking about trying full manifold vacuum. I read this was done to keep the emissions down when using the smog pump. Don't have the pump installed any longer. Maybe a little smoother idle? Ideas please. SBC 350 and quadrajet.

[MikeB]

Google "why ported vacuum?" and you'll see many discussions. Bottom line is it came about during the emissions control era to reduce emissions at idle. But it has no other redeeming values. Typical side effects are stumbling off idle. If you look around long enough you may may find a rant by a GM engineer who explains why we have it and why it doesn't work that great.

[j_cst_10]

There is a very good article that MikeB references. You want to use manifold vacuum so have advance at idle where you need it.

At idle, there is no ported vacuum signal and thus no vacuum advance. Engines need plenty of ignition advance to light off lean mixtures at idle or part throttle cruising.

[Rich72C10]

Yep, I moved to manifold on mine - mucho better.

[72SB]

go manifold vacuum so the VA works at idle. You want 20-24 degrees of timing at idle (base + VA)

[Dead Parrot]

Another vote for manifold. The smog pump system was an efficiency killer. They tuned the system rich so there was unburned gas in the exhaust. Then when the pump pumped air into the hot exhaust stream, a 2nd burning happened which resulted in cleaner exhaust vs the older no pump system. It did help with tailpipe emissions but made gas mileage and power less.

With no pump, largely ignore the timing recommendations on the sticker or factory manual. Assuming a mostly stock 350, most like about 10~14 BTC indicated with vacuum pot disconnected. (initial timing) You might go through a couple of cycles of adjust timing, adjust idle speed, adjust idle mix before its happy. Then connect the vacuum advance and readjust idle speed and maybe idle mix.

If it becomes hard to start hot, back off a couple of degrees on initial timing.

[Rich72C10]

This is why I love my progressionignition.com distributor but it does cost a arm and a leg!

It actively retards the timing during starting and retards / advances timings in different running conditions, based on your RPM setting for idle. It retards/advances the timings during idle (say at a stop light) by trying to keep your expected idle RMPs in the settings. I have two main charts for two different seasonal idle RPM engine conditions: Summer (AC expected to be used) and Winter (not expected to use AC). Basically, I adjust my carburetor's idle twice a year, depending on expected AC usage. Then load the required Progression Ignition distributor chart after adjusting the carburetor's idle.

Also, I really get hypnotized watching the active chart / bouncing timing bubble on my iPhone! Some photos attached, getting the starting up "10" was hard as it's only there for a moment before the timing bumps up.

[Ironangel]

Originally Posted by Rich72C10

This is why I love my progressionignition.com distributor but it does cost a arm and a leg!

It actively retards the timing during starting and retards / advances timings in different running conditions, based on your RPM setting for idle. It retards/advances the timings during idle (say at a stop light) by trying to keep your expected idle RMPs in the settings. I have two main charts for two different seasonal idle RPM engine conditions: Summer (AC expected to be used) and Winter (not expected to use AC). Basically, I adjust my carburetor's idle twice a year, depending on expected AC usage. Then load the required Progression Ignition distributor chart after adjusting the carburetor's idle.

Also, I really get hypnotized watching the active chart / bouncing timing bubble on my iPhone! Some photos attached, getting the starting up "10" was hard as it's only there for a moment before the timing bumps up.

All good info except in the first sentence. The Progression Ignition Distributors are only $30 dollars more than the MSD pro billet dizzy's and the Progression dizzy's include the coil where MSD's 8360 pro billet dizzy does not. Progression Ignition dizzy's are made in the USA where MSD sold out to Holley and China...If you live in a state where they stick sniffers in your tail pipe to pass emission standards then keep the ported vacuum set up. Otherwise, go manifold vacuum and learn how to tune your timing and the Quadrajet... https://progressionignition.com/prod...ei-distributor https://www.jegs.com/i/MSD-Ignition/121/8360/10002/-1

[Rich72C10]

I was comparing if you put in pertronix... I don't think I ever looked at a MSD Pro Billet... my error / ignorance.

[68 P.O.S.]

Originally Posted by howzzzit

My 72 C10 1/2 ton has always used ported vacuum per the factory manuals. I'm thinking about trying full manifold vacuum. I read this was done to keep the emissions down when using the smog pump. Don't have the pump installed any longer. Maybe a little smoother idle? Ideas please. SBC 350 and quadrajet.

Everything you need to know is right here: http://www.camaros.org/pdf/timing101.pdf

[MikeB]

Several years ago I splurged and bought an MSD E-Curve distributor for a small block. It had no mechanical advance weights & springs, and just a hose nipple for a manifold vacuum hose. It was all configured with dip switches, but did not have any electronic user interface.

I have seen at least one TV show where the shop guys were stumped by the setup, because you have to think backwards. In other words, start off by physically adjusting the distributor to the max advance you want, and then set switches to retard advance to, let's say, 12 degrees at 800 RPM and 25 degrees at 3000 RPM. That's because electronics can retard a spark, but they can't advance it. Got it?

I see where that distributor now costs literally 65% more than I paid for it 10-11 years ago. So if Holley is actually having them built in China, you'd think the price would be less or flat due to significantly lower labor costs, even with U.S. inflation.

[leftybass209]

I ran ported a long time, as a way to cover for low compression and a vacuum can that added too many total degrees of advance.

Engine ran slightly hot at all times and temp would creep up at lights and drive-thru's, but it didn't smell too rich at idle.

Eventually limited the total degrees the vacuum advance added, and switched to manifold vacuum.

Truck now runs cool and doesn't push temps (104 here lately) even in traffic, with a minor bump in MPG. Slightly more rich at idle, but could be dialed in better with an 02 sensor.

Ported vacuum was for emissions, and emissions is not to be confused with fuel economy.

[MikeB]

Originally Posted by leftybass209

I ran ported a long time, as a way to cover for low compression and a vacuum can that added too many total degrees of advance.

So many vacuum canister have 20-30 degrees advance to make up for the small amount of initial and mechanical advance. Years ago I bought one from Crane, and was surprised to learn it has >20 degrees. After doing some research, I ended up getting this one:

I don't remember the current P/N, but over the years it was known as a VC1838 or AR12. Max advance was 7 degrees at 10-12" vacuum.

Tons of information in this document. Be sure to read the section by Duke Williams starting on page 9. Note that there are canister listings for non-HEI distributors and HEI.