vacume advance question

[bryanw1968]

Originally Posted by 71chevylowrider

Originally Posted by Billla View Post
Ummm...again The vacuum advance SHOULD be connected to manifold vacuum as ported vacuum will be zero at idle.


Bryan, when I read that it sounds like he is saying it "should be connected to manifold vaccume", & that is what I did, & that is what edelbrock says in the owners manual if you can't get enough ported vacuume, as in a long duration, high lift, radical camshaft.

Everything I've heard, read, seen, experienced says the vacuum advance should be hooked up to ported vacuum not direct.

[71chevylowrider]

Hey Brian, I meant no dis-respect bud, I believe You've helped me on some issues with my Demon before, I'm sure you kow your stuff, & I know You are right that normally the way to hook it up is to ported vacuum, I just wanted to ad in that in some case's like mine, I had to hook it up to direct vacuum & i thought it was interesting that in the edelbrock owners manual they say it's okay to hook it to manifold vaccum when you have a long duration/radical type of Cam that might require more vacuum at idle.

This is a good discussion topic, believe me I was kinda freaking out that I had to hook it up wrong until I found that section in the owners manual.

[bryanw1968]

A day with out leanring something new is a day wasted
There are for sure a lot of different ways of doing things. There are so many differnet applications that one or the other can't be wrong. Billa, chime in
Or for that matter anyone else

[gator2511]

I just put a new GMPP HEI dizzy in my truck last week. I am getting too much vacuum advance with it hooked up to manifold vacuum. This Sat. I'll put the adjustable vac can on it and adjust it to about 20 Deg. advance hooked to full manifold vac. Initial will be at 8 deg btc. Then I'll have to play with the springs on the weights to get about 36 deg. of advance total. My truck likes to run with a lot of advance at idle so I give it all I can without pinging and it runs a LOT better! Do any of you guys use bushings on the weights to control or limit the mechanical advance? I've never seen them but someone told me they were available. That might be easier than the springs.

[crazy longhorn]

Originally Posted by bryanw1968

A day with out leanring something new is a day wasted
There are for sure a lot of different ways of doing things. There are so many differnet applications that one or the other can't be wrong. Billa, chime in
Or for that matter anyone else

The old 60's stuff was hooked to manifold vacumn from the factory, whereas into the early 70s the trend was ported from the factory(emissions). Now you will get about a lot of different veiws on the proper way to set up the manifold vacumn, but as a rule a true performance eng should be hooked to manifold vacumn. Your run of the mill streeter (close to stock cam ect), may work out better in some apps on ported spark. Full manifold vacumn will smooth the idle, give better throttle response, & keep you engine running cooler(cyl head temps). My 383 (specs in the sig), likes a 34 degree total (all in @ 2800-3000) + 12 degrees extra in the vac can (hooked to manifold vacumn). The trouble with the HEI distribs, & too much advance, is that they need to be recurved to run properly in these old trucks. They were setup to run ported spark, & to run full manifold vac @ the can, you need an adjustable can , such as a Crane unit to cut the vac advance back from 20 or 22 degrees(that the can has) , to the more useable 10-12 that the hipo engins like. The only reason to disable the vac can on a street motor, is with a cam that is so "rowdy" that you dont pull enough vacumn to be able to use the can. you also dont need that vac can for racing, but you will like the mileage & throttle response that it gives on your streeter. Good luck tuning..... L

[jimfulco]

Some race motors can benefit from vacuum advance. It won't do squat at full-throttle on the track, but it can help it run smoother and cooler in the pits.

I had a 292 with a 310* cam in a Nova for a short time, and hooking up the vacuum advance (manifold vacuum, please) made it run smoother at low rpm, such as in residential areas & school zones.

Ported vacuum was introduced for smog devices, and the HEI distributor was designed as a smog device. Recalibrate it like Crazy said and it should work much better. Just don't tell the smog police.

[bryanw1968]

Originally Posted by crazy longhorn

The old 60's stuff was hooked to manifold vacumn from the factory, whereas into the early 70s the trend was ported from the factory(emissions). Now you will get about a lot of different veiws on the proper way to set up the manifold vacumn, but as a rule a true performance eng should be hooked to manifold vacumn. Your run of the mill streeter (close to stock cam ect), may work out better in some apps on ported spark. Full manifold vacumn will smooth the idle, give better throttle response, & keep you engine running cooler(cyl head temps). My 383 (specs in the sig), likes a 34 degree total (all in @ 2800-3000) + 12 degrees extra in the vac can (hooked to manifold vacumn). The trouble with the HEI distribs, & too much advance, is that they need to be recurved to run properly in these old trucks. They were setup to run ported spark, & to run full manifold vac @ the can, you need an adjustable can , such as a Crane unit to cut the vac advance back from 20 or 22 degrees(that the can has) , to the more useable 10-12 that the hipo engins like. The only reason to disable the vac can on a street motor, is with a cam that is so "rowdy" that you dont pull enough vacumn to be able to use the can. you also dont need that vac can for racing, but you will like the mileage & throttle response that it gives on your streeter. Good luck tuning..... L

Interesting.....

If you had the VA hooked up to direct vacuum down here where it's hot most of the time wouldn't that make the motor drag when starting? Or would there not be enough vacuum present when starting to make a difference?

[jimfulco]

Never bothered anything of mine. I don't think there's enough vacuum at starting rpm to mess with it.

[PICKMUP]

All HEIs are not created equal. Most drop in, set up pretty close, and run good...but some of these have just way too much dist advance built in (vac & mech combined) and have to be "fixed" to make them compatible with our heavy trucks.
This is one of those. I don't have separate vac & mech figures on this one yet, but total is over 50.
My 68 GM serv manual (396) calls for mech: 28 @ 4200rpm (only 15 @ 2000) and vac: 15 @ 15"+ with 6 initial setting.
My 79 GM serv manual (454) call for even less with mech: 20 @ 4200 (only 14 @ 2800) and vac: 10 @ 13"+ with 4 initial setting.
Sorry, I don't have a '71 manual close, but it looks like GM set these heavy trucks with BB engines to run around 30 degrees total advance for running in mid-range cruising rpms at part throttle. That seems to agree with what has worked best in the past.
So, until I can get an adj vac adv pot or limit the mech adv to shorten up the curve...I will have to leave the vac adj off. Not best, but the pinging will burn a hole in the pistons over time...and it drives me crazy.

[Billla]

Originally Posted by bryanw1968

Everything I've heard, read, seen, experienced says the vacuum advance should be hooked up to ported vacuum not direct.

This gets into a discussion about overall timing - initial, mechanical, vacuum...and the build level of the engine.

Here's the best explanation I've ever seen from a VERY HIGHLY respected tuner.

[GREASEMONKEY72]

i have a stock 350 with the points dizzy and i have the VA hose connected to the carb (qjet) and its set at 8* initial, i have no idea what the total is

what would you all say to that? right/wrong, what?

all the talk thats gone on so far is giving me a headache, so i thought id just ask about mine instead

[76bonanza]

One other question, I am from the camp of full vacuum at idle also. But I have always wondered about tuning the carb. Do you tune the carb idle screws with advance plug in or not. Because if you tune it with advance disconnected then when you connect it to full manifold vac the idle tends to go up also?

[bryanw1968]

Originally Posted by Billla

This gets into a discussion about overall timing - initial, mechanical, vacuum...and the build level of the engine.

Here's the best explanation I've ever seen from a VERY HIGHLY respected tuner.

That was awesome! Thanks for sharing.
B

[67chevemall]

Man I put a edel 1406 on my truck a few months ago and I can't get it to stop pinging and poping.
This brings more light no the subject 4 sure

I think I am going to try turning the dizzy to full advance and see what happens with out the VA.......

Good thread keep er going

[Billla]

Originally Posted by 67chevemall

Man I put a edel 1406 on my truck a few months ago and I can't get it to stop pinging and poping.
This brings more light no the subject 4 sure

I think I am going to try turning the dizzy to full advance and see what happens with out the VA.......

Good thread keep er going

If it's pinging you have TOO MUCH advance. Do you have an adjustable timing light?

[76bonanza]

What does pinging sound like exactly? I have what sounds like an exhaust leak louder under truck and very metalic sounding, but maybe it is pinging it does it idle.

[piecesparts]

In most cases the so called "Pinging", otherwise known as spark knock happens under load or acceleration. If you have a metallic noise at idle, that would be another thing. I would try driving with the window down and listen as you accelerate your truck to get out onto a street or highway. If you here something that sounds like a light rod knock, then you are experiencing the "pinging" we have referenced to.

[rage'nrat638]

sounds like a bag of marbles in a tin can

[gator2511]

The pdf file posted by Billa is very good with one exception. The author states "and, since most vacuum advance control units pull in about 16 degrees of vaccum advance at cruise speed (where the full centrifugal advance will also be deployed), the 36 degree setting will produce 52 degrees of total combined advance at cruise with the vac advance fully deployed."
I don't know about you but I don't have full mechanical advance at cruising speed. I have always set my mechanical advance to be fully deployed at around 3,000 rpms. I would have to be cruising at 100 mph to get to 3,000 rpms and cut in FULL mechanical advance.Maybe I'm not getting something but if your throttle plates are open enough to get 3,000 rpms your manifold vac will be very low. If I'm reading this wrong please educate me.

[76bonanza]

One reason I like full vacuum at idle is my carb has no choke and I find it really smooths my idle out and makes it easier to keep running on cool mornings.

[Billla]

Originally Posted by gator2511

The pdf file posted by Billa is very good with one exception. The author states "and, since most vacuum advance control units pull in about 16 degrees of vaccum advance at cruise speed (where the full centrifugal advance will also be deployed), the 36 degree setting will produce 52 degrees of total combined advance at cruise with the vac advance fully deployed."
I don't know about you but I don't have full mechanical advance at cruising speed. I have always set my mechanical advance to be fully deployed at around 3,000 rpms. I would have to be cruising at 100 mph to get to 3,000 rpms and cut in FULL mechanical advance.Maybe I'm not getting something but if your throttle plates are open enough to get 3,000 rpms your manifold vac will be very low. If I'm reading this wrong please educate me.

Completely agree that mechanical should be all in before 3000 RPM; what do use cruise at? With 3.08s and a 3 speed, I cruise at 2750...and have set my mechanical advance accordingly.

Typically at cruise manifold vacuum is enough to get most of the vac advance in; 4-5 inches.

The biggest help here is to know what your cruise RPM and manifold vacuum is - this isn't hard to get

[Jim_PA]

There's a lot of questions flying around about timing. I've been looking around for this for days, and finally found it. It covers about everything you need to know about timing. I copied and pasted from www.camaros.net. Hopefully many of you will find it helpful!

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.
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.

[fleetsidefive8]

I am a little bit confused by all the views. so let me come in late! If I am running a big roller cam is it better for me to use the full manifold vacumm? not the ported vacumm from the carb. But I have another question?...what do i adjust the dizzy vacumm can advance to? if my initial time is @ 8* and everybody agrees you need to have between 32-34* total advance?

[WorkinLonghorn]

"Total advance" referrs to the mechanical advance ( the centrifical weights system) plus what the INITIAL setting is (set by positioning the distrib itself). VACUUM advance is set for CRUISING or part trottle operation only. On a stock engine the vacuum advance goes back to 0 degrees as the throttle opens past the little port in the venturi that opens vacuum to the advance unit.The reason for vacuum advance is that under part throttle the cyinders are just partly filled which results in a low compression ratio and much more advance can be tollerated and is needed for economical operation.Racers don't give a rats rear about economical cruising so they don't have any vac unit,and I've heard that the guys with rad cams like to hook the vac up to full vac for better idle etc.But if you want good mileage you will install an ADJUSTABLE vac advance and experiment until you are just a bit below the knock point at cruising/part throttle operation.-Brian

[fleetsidefive8]

my own words of advice anyone with timing issues...1. be sure to check uder the rotor and make sure the springs are opening and closing and operational. 2. always seperate the plug wires from each other. 3. plug vacumm advance and trans vacumm and carb vacumm to set initial timing. when dropping dizzy in harmonic @ 8*-12*. nothin new but..... just simplified.