Another timing question

[Kctom]

The Demon carb has two ports to hook the vac advance up to. One is ported vac. and the other is full time for testing. Which do I want to hook up for vac advance for the dist.
TOM

[Fat Cat]

In a SBC you want it hooked to full vacuum not ported.

[rooster]

In a SBC you want it hooked to full vacuum not ported.

FatCat im using a holly 600 cfm in a stock 350, hei (aftermarket) w/vac adv , are you saying that the hose from advance canister should be connected to the carb port that is allways sucking even at idel? If so I have that assed up!

The  vacume advance canister that came with my dizzy requires ENGINE vacume level lower than 12 in/hg at idle, how can I measure this? Is there a special guage?

[beatnik]

Yes, that's what he's saying and he's right, and it applies to Chevy, Ford, Mopar, and every other Hot Rod Engine I've ever dealt with.

Other wise at WOT you end up with the vacuum advance, and centrifical advance coming on and that's way too much timing.

[rooster]

Yes, that's what he's saying and he's right, and it applies to Chevy, Ford, Mopar, and every other Hot Rod Engine I've ever dealt with.

Other wise at WOT you end up with the vacuum advance, and centrifical advance coming on and that's way too much timing.

thanks!

I rescently got a small miss at idle and hesatation when throttle begins to open, Ive checked and cleaned and gaped the plugs (45), checked eack wire with meter,inspected cap, replaced the coil needlessly. This all started all started within 10 minutes of buying some fuel going to the power tour a few weeks ago HUMM!

Im going to retime the car today with vac hose in the right location a seen if it makes a difference.

[Mikej]

I hate to disagree but they will work on port vaccuum also.
A smoother idle on manifold vaccuum and better fuel milage maybe.
Rougher idle and better throttle responds on port vaccuum.( my opinion) I don't think manifold vaccuum was the way the motors of my youth came from the factory.
The initial timing won't change for either method. But you will have to adjust the idle speed a little.

[enjenjo]

I hate to disagree but they will work on port vaccuum also.
A smoother idle on manifold vaccuum and better fuel milage maybe.
Rougher idle and better throttle responds on port vaccuum.( my opinion) I don't think manifold vaccuum was the way the motors of my youth came from the factory.
The initial timing won't change for either method. But you will have to adjust the idle speed a little.

Yes it will work. It will also overheat idling in gear. Depending on the axle ratio, it may even overheat driving down the road at slow speeds. All GM engines used manifold vacuum from the factory well into the 70's, and only changed for emissions.

Fords did use ported vacuum, and the engine can spark knock at certain speeds if hooked up to manifold vacuum

The timing will change, the correct proceedure it to disconect and plug the vacuum line, set the timing, and reconnect the vacuum.

At the car shows, the cars with coolant dribbling out of the overflow when shut off, generally have the vacuum conected to ported vacuum.

On my 65 Buick, bone stock, with ported vacuum, at a stop light, the temp would rise 30 degrees in the length of a light. Two light in a row, and it was 240. This was with a new radiator, water pump, clutch fan, and hoses. With manifold vacuum, it doesn't heat up even with the AC on.

327 in a 69 Chevelle, distributor had the vacuum disconnected, and total timed. In the staging lanes, at the drags, it would overheat badly. Hooked up the vacuum to a manifold source,and you could idle it all day.

[Mikej]

Are you sure that they didn't change to manifold vaccuum in the 70's?

[rooster]

I re-timed the engine ,Dist vac dis=connected  and pluged the carb ports, At operating temp and idle set the timing to 12 D (before), tighted dizzy down, checked again, then connected hose to carb port that sucks all the time ( port vac). Done!

Then raseid the RPM's to around 2500 and put the light on it again, this placed the timing mark well off the scale, then! with the timing light dial moved the mark seen on damper to 12 degrees. The dial on the light was moved 9 degrees. this all being done at 2500 rpm.

took it for a test drive, hesatation still there and sounds rough at idle. I use regular gasoline, can it make that much difference?

[Mikej]

If this all started after you bought gas, you may need to clean out the passages in your carb.
The port that you are using now is the manifold vaccuum port( all the time). Port vaccuum is the one that increases as you open the throttle.
And yes you can buy a vaccuum gage. Not to expensive. If the canister you have now say under 12 at idle, you are on the wrong port. Unless you have a big cam.

[Kctom]

If this all started after you bought gas, you may need to clean out the passages in your carb.
The port that you are using now is the manifold vaccuum port( all the time). Port vaccuum is the one that increases as you open the throttle.
And yes you can buy a vaccuum gage. Not to expensive. If the canister you have now say under 12 at idle, you are on the wrong port. Unless you have a big cam.

The Demon carb instructions say ported vac. If you hook it up to full time vac. which is what I did. The cannister on the dist. Fills up with gas after a while. I used a clear hose for the vac advance and noticed when I revved up the motor gas was sucked from the dist. Called Barry Grant and was told that was one of the reasons you used ported vac. The new MSD dist I installed also said to use only ported vac. What do you think ? I'm running 18 deg. of idle advannc and 20 deg. of mech.

[Fat Cat]

The Demon carb instructions say ported vac. If you hook it up to full time vac. which is what I did. The cannister on the dist. Fills up with gas after a while. I used a clear hose for the vac advance and noticed when I revved up the motor gas was sucked from the dist. Called Barry Grant and was told that was one of the reasons you used ported vac. The new MSD dist I installed also said to use only ported vac. What do you think ? I'm running 18 deg. of idle advannc and 20 deg. of mech.

Something I found and saved after this came up the first time.

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.

[Bruce Dorsi]

Thanks for re-posting that, Frank!  
  I know you're quite busy now, so I don't expect it to be done soon, but this is an example of when the "printer friendly" page format is most valuable.

Whenever we change the board software, we lose a lot of this info.  .....That is why I prefer to save good tech info on paper.

Sorry for nagging 'ya, but since you're single again, I thought you could tolerate some nagging!

[Mikej]

Thanks Frank, good info.
You could also put a barb in the manifold so you don't get fuel, in the canister, from the carb.

[Phat]

Just one question?  Who is the author?    I dont agree and have no problems with overheating on anything we build ......sounds a little sour grapes with the summit Jegs comment as they sell stock and mod dizzys. (i dont buy from either ) Hey i guess you do what works for yah. It seems like more of and opinion than fact.