Ported or not for vacuum advance

[bubbasimms]

Whats the general consensus: 250 3 speed

Do I want my vacuum Advance on the HEI hooked to ported or non ported vacuum?

[Sheepdip]

A cut and paste from another site you can make up your own mind:

This was written by a former GM engineer as a response to a similar question on a Corvette board:


As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

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 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, late-timed" 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 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 long-time 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 street-driven 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.

[68 P.O.S.]

Manifold vacuum. The GM engineers figured this out long ago. Here's a link to the magazine article John Hinkley wrote as well. Same info, different format explaining the same thing. http://www.camaros.org/pdf/timing101.pdf

Also, here's a link to mapping your advance curve written by him as well. http://www.stl-vettes.com/65Vette/co...ng_Advance.pdf

[HotWheelsFan]

Loads of info on this out there.

While I am not familiar with the consensus here, I do visit other sites and non ported is the big winner. I did a quick search to see what comes up "Ported Vacuum vs. Manifold Vacuum" and familiar posts I have read come up. The Chevelle sites are what I am familiar with, but there's plenty to read.

Ported vacuum is best when running an EGR valve.

Bob

[tdangle]

Just throwing this out there for discussion. Here is GM performance HEI specifications. This is the distributor GM puts on the deluxe and complete crate engines.

[BRL]

I was running manifold vacuum on my engine forever and there was a bog when I mashed the pedal from a dead stop.

Probably due to manifold vacuum plunging drastically when you first crack open the throttle, thus killing advance for a brief second.

Watched an Edelbrock video and afterwards tried pluging into ported vacuum.

The idle was reduced when taking away the advance, (auto trans in gear) so I had to turn in the idle screw a little to compensate.

Dang thing jumps to life right off the line now!

[geezer#99]

Think they got a typo there Terry.
Last line should be 3000 rpm - 22 degrees.

As for which vac port to use.
If you got a rowdy cam that needs lots of throttle opening To idle near 750 rpm which results in idling on the power circuit, then use manifold vac port. Increased timing allows throttle closing, transfer slot covered, mix screws working, doesn’t run rich.
If you have A mild
cam that lets you close the throttle, covers transfer slot, mix screws work, then use ported
vac port.
If manifold vac gives too much extra timing at idle then limit the amount. Use an adjustable can or a stop.

[tdangle]

Originally Posted by geezer#99

Think they got a typo there Terry.
Last line should be 3000 rpm - 22 degrees.

As for which vac port to use.
If you got a rowdy cam that needs lots of throttle opening To idle near 750 rpm which results in idling on the power circuit, then use manifold vac port. Increased timing allows throttle closing, transfer slot covered, mix screws working, doesn’t run rich.
If you have A mild
cam that lets you close the throttle, covers transfer slot, mix screws work, then use ported
vac port.
If manifold vac gives too much extra timing at idle then limit the amount. Use an adjustable can or a stop.

I agree, I think it's suppose to be 3000. Once I get a better timing light with a tach built in I will verify as this is the dist I have. I have a mild cam but with a low 108 LSA. the chassis dyne shop set it up with ported vacuum, I have since changed it to manifold vacuum and limited the amount of vac advance to 10 degrees. Thinking I may go back to the original settings as it seems to have a bog hesitation it never had before. The reason I went to manifold vac is to try eliminate the run on I somethings get. Think I'll just shut it off in gear as a habit.

It's just to dam hot to mess with it this weekend. 117 right now and same forecast for tomorrow.

[garyd1961]

If you are running high compression you want manifold vacuum . When running high compression you don't want the timing to advance when you step on it, may cause detonation.

[RichardJ]

My '67 came with the Rochester B. That carb was used back as far as about 1950 and had only one vacuum port. A Ported Vacuum port or sometimes calle Timed Vacuum Port.

There is NO Manifold Vacuum Port on the carb.

I'm now using a Carter AVS from a 1970 383 Mopar. One Ported Vacuum Port for the dist, one large PCV port, one port for choke pull-off and there is NO Manifold Vacuum Port on the carb.

These must both be pre-pre-pre-emissions carbs.

I'm not a fan of your CS130. In my experience they don't have a long life span.

[geezer#99]

Originally Posted by RichardJ

My '67 came with the Rochester B. That carb was used back as far as about 1950 and had only one vacuum port. A Ported Vacuum port or sometimes calle Timed Vacuum Port.

There is NO Manifold Vacuum Port on the carb.

I'm now using a Carter AVS from a 1970 383 Mopar. One Ported Vacuum Port for the dist, one large PCV port, one port for choke pull-off and there is NO Manifold Vacuum Port on the carb.

Sure about that Richard.
Here’s the avs from that time frame.

Not 1, not 2 but 3 manifold vacuum ports.
Easy to see the passages from the underside.

https://www.allpar.com/fix/fuel/carter-AVS.html

I had an original 70 Satellite (bought from original owner) with a 383 and avs. Had those same 3 manifold vac ports.

[bubbasimms]

Originally Posted by RichardJ

My '67 came with the Rochester B. That carb was used back as far as about 1950 and had only one vacuum port. A Ported Vacuum port or sometimes calle Timed Vacuum Port.

There is NO Manifold Vacuum Port on the carb.

I'm now using a Carter AVS from a 1970 383 Mopar. One Ported Vacuum Port for the dist, one large PCV port, one port for choke pull-off and there is NO Manifold Vacuum Port on the carb.

These must both be pre-pre-pre-emissions carbs.

I'm not a fan of your CS130. In my experience they don't have a long life span.

I hope the alt lives a long life. If not, easy enough to go a different route.

[RichardJ]

Well, I'll give you the one. Very few have the rear port for a brake booster and they are usually larger.
Nearly all were set up with divorced choke and used that side port for the pull-off.

In addition to the one on my '67 GMC,

I have not 1, not 2, but 3 Carter AVS carbs out in the garage. All have the choke pull-off and none have the brake booster port.

I actually just bought the one in the second picture off ebay last week. It is an exact match for the C9 4615 SA that came off my '69 383 4spd roadrunner. Someday i hope to put the pair on a Cross-Ram manifold that now has holleys.