Vacuum for HEI Dist Advance and Power Brakes - JeepForum.com
 
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post #1 of 13 Old 04-28-2010, 01:49 PM Thread Starter
hybridcj
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Vacuum for HEI Dist Advance and Power Brakes

Quick question. Trying to fix up some of the millions of plugged vac lines.
If i have HEI running, should this be on ported or manifold vacuum? ie, passenger side of carb under bowl vent or should i use the manifold vac above the egr valve?

These are my thoughts but just wanted to confirm. If i use manifold on the dist, i will be advanced all of the time. However, the carb vac source doesn't really provide much vac, and i'm unsure if it sufficient to advance the timing.

Also adding power brakes from YJ. Pretty confident that the source should be the manifold but wanted to check to be sure.

Details: no computer, carter carb, 84 258.

Thanks!

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post #2 of 13 Old 04-28-2010, 02:48 PM
JeepHammer
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Always spark ported vacuum from the side of the carb.
That's why it's called 'Spark Ported Vacuum Source'.

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post #3 of 13 Old 04-28-2010, 02:55 PM
Jeepican
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Hey Jeephammer.... sorry to hijack.... but what happens when you use a regular vacuum source? Say off manifold or w/ pcv?

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post #4 of 13 Old 04-28-2010, 03:20 PM
TheGhost
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I'm a manifold vac guy - always have been. Here's a little write-up that I read a while back that I think does a good job of explaining the whys and why nots:

"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."
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post #5 of 13 Old 04-28-2010, 03:25 PM
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Laymans term please?

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post #6 of 13 Old 04-28-2010, 03:29 PM
Lordwrench
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So if one switched to manifold vacuum,are there other changes necessary?
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post #7 of 13 Old 04-28-2010, 03:43 PM
TheGhost
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Quote:
Originally Posted by Lordwrench View Post
So if one switched to manifold vacuum,are there other changes necessary?
I think that sort of depends on where you start. If this is a change you want to make, I'd start by setting your static timing in the 8* range - switch to manifold vac. - then set your mixture / idle. Drive a little to make sure you don't have any pre-ignition (spark rattle) - if you do, back you timing off a couple degrees at a time until it's gone.

Mine is at 8* and I don't have any issues.
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post #8 of 13 Old 04-28-2010, 04:00 PM
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I use a vacuum gauge to set it as I am unsure of po's internal mods and timing marker is unreadable.I was more concerned about parts changes I.E. springs weights etc. I would definitely reset idle/mix then re-time until smoothThanks for the info.
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post #9 of 13 Old 04-28-2010, 05:18 PM
Shawn Watson
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I agree with Ghost 100%. I've been running manifold vacuum for a lot of years and on many different engines and it's worked better than ported in every case.

Ideally, you want to use the adjustment screw inside the vacuum canister and set it so that it barely pulls full advance at idle vacuum, but not before, and provides no advance by roughly 6 or 7 inches. To get close to that, you'll probably have to go counter-clockwise on the adjustment a couple of turns.

If you're getting detonation after the change, I would disconnect the vacuum from the distributor and run the test again. If it goes away then you know you need to pull advance from the vacuum canister. If it doesn't go away then you'll need to lower the initial advance.

The benefits I've always seen have been better cold starts, better throttle response and better gas mileage.

Hope it helps,

Shawn

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Last edited by Shawn Watson; 04-28-2010 at 06:54 PM.
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post #10 of 13 Old 04-28-2010, 09:55 PM
Fjguercio
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Use Ported Vac for the Distributor.

Just like the AMC Engineers, Ford Engineers (Ford Duraspark), most of your carb directions will say to use Spark Ported Vacuum. You want the advance to work with the engine load.

No reason to have all that advance from manifold advance when you start your engine. That extra 10-12 deg of vac advance will give harder starts not easier starts. That is why the Ford Duarspark (Jeep) ignition has 7-8 degree taken out of time when you start your engine....... for easier starts..... it a nice feature of the stock ingition in the CJ.

Lean mixtures are easier to light off. That is why gas vapors, dust vapors, and such will light off very easily nad have a viloent explosion.

Rich mixtures are harder to light off...... that is why when some run over rich they more more advance to burn the overly rich mixture

CJ Jeeps are putz vehicles mostly. If you idle thru the trails and woods you do not want alll that advance (manifold) and in a moderate load situation. Most of us operate the 258s from 600 to 3000 rpms and usually below 2500. When ported vac is used the advance and load and engine needs run together and safer for your engine.

I tuned a guys jeep this weekend and could not get ported vac of the std 2100 Carb and it was not pluged. Worked ok but evertime you left from stop normally it had soooo much advance it would stumble. We replaced a HEI with hard gear that ran like crap. We found another port to use.... PORTED..... and it ran great.... the off stop stumble was corrected and ran much better with ported.

PORTED PORTED PORTED...
Carb says it, Ford, AMC, and every other car mfg and eng says to use ported vac. There will always be differ opinion. The safe is ported, the correct is ported, and unless you have some short comming you cannot correct properly there are few if any reasons to use ported in a CJ...
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post #11 of 13 Old 04-28-2010, 10:07 PM
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Not to start another fight with people but the above post is just plain backwards. The flame front moves slower through a lean mixture than it does a dense one.

Once a firm grasp on combustion dynamics and flame-front propagation are understood, a person will realize that manifold vacuum makes much more sence than ported. Manifold vacuum is the only accurate way to sense engine load. Believe me, I've tested the ported signal vs. manifold signal on several different carbs.

Manifold vacuum will never interfere with engine start-up either as there's not nearly enough manifold vacuum under cranking speed to activate the canister.

We can discuss theory and right vs wrong according to some FSM that uses ported for one model year and manifold vacuum for another model year but the truth is that the results speak for themselves. Many people on the forum who have made the switch say they'll never go back to ported vacuum.

Manifold vacuum simply works better and it does so every single time. The Ghosts quote above explains it in pretty good detail and my suggestion is for people go back and read it with an open mind with the intention of learning.

Shawn

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Last edited by Shawn Watson; 04-28-2010 at 10:25 PM.
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post #12 of 13 Old 04-29-2010, 07:32 AM
Andy5150
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Swatson454, I,m with you brutha! When I switched to manifold vacuum, the idle was smoother and cooler.(360, performer plus cam, 600 holley,hedman headers). I can be in bumper to bumper traffic on a 95 degree florida day, and the water temp. will sit on 180 with a 180 thermostsat. It cranks over easy with 12 degrees initial, and when it starts, theres another 15 degrees vacuum of vacuum advance. 27 degrees total at idle. Smooth as glass. Makes me wonder what the volumetric efficiency, and the actual running compression ratio of an idling motor might be compared to wide open throttle. The incoming mixture burns about as fast as duraflame log, and therefore need to be lit sooner. I understand about the AMC guys, the Ford guys, and other smart guys, but they had to keep the federal government happy- I don,t. You can use the factory stock ignition set up/ curve and you,re motor will run fine; mine did. It just runs better now. Results will vary.
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post #13 of 13 Old 04-29-2010, 02:17 PM
TropicalNusselt
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Without going into Thermodynamics Principles, Otto Cycle, Orsat Analysis, etc; I'm in 100%++ with manifold vacuum. I'll take miles of pages on a thread to convince all "Carburetor Spark Ported Vacuum Advance Source" guys. Just give it up and let them believe they have the ultimate 100% efficient machine.

Raph

TropicalNusselt- 1986 CJ7 w/ 258, T5, D300, D30/D44(3.31:1), Weber 38DGES, DUI, 2.5" Lift, 33" X 12.5" X 16", DC1
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