Distrvacuum or not?

[Belgium R/T -68]

My vacuum for the distributor is still plugged, should I connect it since running in is done? It's a MP electronic stock system.

Per

[firefighter3931]

Leave it blocked off...tune without vac advance ; your engine will be happier 


Ron

[Big Sugar]

Hey Ron:

Not to Challange your vast knowlage but there was a rightup in Feb 2010 Mopar Action ( Jay Leno {Turbine}) in which Rich Ehrenberg Stated....

"OK yours truly has never been known for subtlety.And this cant be candy-coated. So I'll just say it:Anyone who drives on the street and doesn't run
Vacuum advance is an idiot."

As opposed to me simply posting Quotes from Rich
I'll find a link to the story or at least Scan and Copy it, It was well written Story.


Ron

[Big Sugar]

Here's a a scan











http://i438.photobucket.com/albums/qq101/Big-Sugar/vacuum1.jpg
http://i438.photobucket.com/albums/qq101/Big-Sugar/vaccum2.jpg
http://i438.photobucket.com/albums/qq101/Big-Sugar/vacuum3.jpg
http://i438.photobucket.com/albums/qq101/Big-Sugar/vacuum4.jpg

[Belgium R/T -68]

Leave it blocked off...tune without vac advance ; your engine will be happier 


Ron

You build my engine and it's running great so I go with your advice.

Per

[FLG]

Per I know your not running a stock motor so, yes keep it plugged.

Vac advance helps on stock engines that pull a lot of vac, not for a built motor or a motor that is on a track car. Track cars don't run vac advance because they only see wot and at wot your intake pressure matches the atmosphere so you have no vac thus no vac advance, so its not required.

[firefighter3931]

Leave it blocked off...tune without vac advance ; your engine will be happier  


Ron

You build my engine and it's running great so I go with your advice.

Per

I'm not a fan of vacuum advance in a performance build. Per, just check the total timing and make sure it doesn't exceed 35-36* and it will run great....and you won't need to worry about detonation.  


Ron aka Big Sugar ; I've read that article....it's probably OK for a very mild build but i prefer not to pull extra timing into the engine if i can avoid it.  



Ron

[elacruze]

I'm a fan of vacuum advance

HOWEVER

It has it's place, and that place is VERY SMALL.
Advancing timing beyond that required for best power/detonation (your max mechanical) serves no purpose except to improve fuel mileage at light throttle/high vacuum. If you do not travel far enough often enough to put a lot of effort into saving fuel, then vacuum advance is either wasted effort or academic study for you (which I strongly support)

Vacuum advance, like every other system, must be finely tuned for max effectiveness. On a stock engine, one can expect to make use of much more VA than on a performance engine-primarily because the efficiency in the performance build is already much closer to optimum than stock. So, you may spend just as much time tuning your VA as you would tuning your carburetor to find the perfect settings. You must consider not only the total timing spread, but the speed of advance and return, and the measured vacuum level at which the VA comes in and out-all 3 must be dialed in perfectly together. That takes a lot of time and a keen eye and ear.

Even if you spend 50 man-hours tuning your VA, you may discover that the benefit is near or at zero, depending on your engine.
Given the potential for engine damage from too much advance, and knowing that the sole purpose of the VA is to input too much advance, one should weigh very carefully the benefits. For most of us there is no measurable return on investment, particularly considering usage, thus the wise recommendations to omit.
Personally, I plan to run 15-20,000 miles next summer, and I'll dial in my VA over time. If you don't care about mileage, or use and carefully measure many tanks of fuel and look for detonation during every trial, you've only wasted time.

Heh I guess after all, that puts me in the 'No Vacuum Advance' advice column about 99% of the time.

[Belgium R/T -68]

Thanks, I find it fantastic how much you can learn here. Since I will only do some sundaytrips a couple of times
a year plus the obligated number of burnouts I guess I don't care about fuelconsumption.
What I do learn here is that I should watch out for detonation, the non positive one that is since normally our engines works because of detonation so where is the limit between "bad" detonation due to wrong timing and "normal" detonation beeing in my eyes
what's happening when air/fuel meets spark.

Or am I just stupid here?

Per

[skip68]

I made a mechanical vac (3\8 rubber hose) that runs from my distributer up to my dash.   This way if I need more vac all I need to do is give the hose a quick suck or two.  Now I can apply total control sucking for just the right spark. LoL.........

[elacruze]

Thanks, I find it fantastic how much you can learn here. Since I will only do some sundaytrips a couple of times
a year plus the obligated number of burnouts I guess I don't care about fuelconsumption.
What I do learn here is that I should watch out for detonation, the non positive one that is since normally our engines works because of detonation so where is the limit between "bad" detonation due to wrong timing and "normal" detonation beeing in my eyes
what's happening when air/fuel meets spark.
Per

'Detonation' is a very specific term. Detonation is the act of exploding. Normal combustion chamber activity is 'Burning' which is a sequential oxidation of the fuel mixture from one point to completion. Detonation is precisely what the term implies-the explosion of the mixture; most or all of the unburned mixture oxidizes at once, at all points in the mix. Fuel/Air mixture can only tolerate a finite temperature/pressure before exploding; thus, if your timing is too far advanced, as the normal combustion moves and creates pressure eventually the threshold is overcome and the remaining mixture explodes.
During normal combustion, as the temperature and pressure rise, the piston moves away from TDC, increasing volume, lowering pressure and keeping the maximum cylinder pressure under control and below the detonation threshold. Detonation causes an instant rise in combustion pressure, which happens too suddenly for the increasing volume to control, and that pressure and associated heat can destroy parts. Pretty simple, really.

[six-tee-nine]

Per,

You want to ignite the fuel mixture in the chamber when the piston is all the way up. Just before it starts to go down again. Then you're fine. If you iginte the mixture too early then the expantion of the mixture will push your piston down (or at least it will hold it up) when it is still travelling upwards.

Might be a silly explanation but thats were it comes to basicly

[b5blue]

Now my mind is pinging.....

[John_Kunkel]

A couple of comments.

Instead of a blanket yes or no on whether to use vacuum, I'd ask "how do you drive the car?" If it's street driven the majority of the time I'd recommend trying it both ways and make the decision based on whether or not it's more driveable with the vacuum hooked up.

And instead of simply hooking up the vacuum I'd recommend trying to tune it as per the E-booger article. The vacuum advance is about driveability not maximum performance, if you value driveability give the vacuum advance a fair try before thumbs downing it.

[Just 6T9 CHGR]

When I was running my Holley with the Pro-Form main body, Don over at FBO recurved my dist & locked out my vacuum advance for that carb & performance.

Going back to the stock Carter AVS, I found out that I needed the vac advance to get the carb to run right (severe off idle stumble).  Don, however recurved the dist to use the manifold vacuum on the advance rather than the ported vacuum.  He did it this way so that at WOT I am running nothing but the prescribed 34º mech advance & no further vacuum advance to prevent the detonation.

I still have some tuning to get this carter dialed in right (choke adjustments & fine tuning of the off idle circuit)

[Musicman]

Don will tell you that any "street driven" car that generates 10" of vacuum of more will benefit from a Vacuum Advance distributor.  The key is to get as much initial timing in the motor as it'll take without kicking back. Then set up the distributor to take it to advance levels that will help it burn the fuel at low RPM.  By setting the initial timing up "correctly" you keep the vacuum timing low and prevent the lifting of the advance plate which will drastically improve light throttle/cruising fuel economy.

Don is also a fan of connecting the pot to a constant manifold vacuum source (as it was originally intended), as opposed the ever popular ported vacuum which came about in the 60's as an emissions measure. I for one never use a ported vacuum source.

I know Don likes to see Mechanical Distributors in performance applications... but of course, I believe Don's definition of a "performance application" is probably a little different than most of us view it.

Don is a real nice guy, and real easy to get a hold of if you have questions. He's always by the phone, and he also has a Live Chat feature attached to his web page when he is on-line.
http://4secondsflat.com/

[flyinlow]

I run vacuum advance on my mild build 446 Eddy heads 10.5 cr. ..........However.........


I limited it to 8* above about 10 inches of vacuum.


Initial timing  24* + centrifical 12* by  about 2400rpm + plus 8* vacuum at cruise  = 44 *


Does not Improve full throttle acceleration, but seams to help gas mileage and part throttle "feel"   

I only go to the track once a year, do a few burnout at cruise night a the Nationals and have driven it  12,000 miles in the last two years.   Works for me. 

[A383Wing]

Some more reading here --> http://www.setyourtiming.com/

[Challenger340]

IMO
Another word for Vacuum Advance is "Economy" Advance ?

I never understood the "why" behind, hooking up an "Economy advance", on Engines we've just "built up" with Larger Camshafts & Fuel Breathing curves ?

Thats NOT to say, a Guy may get a little better mileage with the Vacuum Advance hooked up, you might ?
Just,
That I personally don't see the Risk/Benefit as being worth it ?

At High speed Cruise, and High Vacuum/Light Throttle conditions where the Vacuum Advance typically "advances", is exactly "where" higher duration @ .050" Cammed Engines tend to be a little leaner ?
NOT the place,
again IMO,
to be initiating the Flame Front sooner with a Vacuum Advance  ?

The amount of Carbon Buildup Ehrenberg describes, by NOT having it hooked up, is not enough worry to me, considering the Trade Off on the Tune generally applied to Stroker/Performance builds. Especially if a Guy DRIVES it once in a while ?

Long story short, I just think the "application" needs more scrutiny ?

Just my opinion, no wars wanted
Bob out.

[John_Kunkel]

IMO
Another word for Vacuum Advance is "Economy" Advance ?

I never understood the "why" behind, hooking up an "Economy advance", on Engines we've just "built up" with Larger Camshafts & Fuel Breathing curves ?

If you built that engine mostly for street, what percentage of the time do you have your foot buried in it as opposed to just cruising at part throttle?

It's not all about economy it's also about driveability, if the vacuum advance will make the car more driveable for the vast amount of time you spend just cruising why not take benefit from it?

[1969chargerrtse]

Leave it blocked off...tune without vac advance ; your engine will be happier 


Ron

Wow, interesting.  Mine is blocked off by me and I was wondering if that was ok to do?  I time by ear also. 

[flyinlow]

Use to see lots of cars come in for tune ups with the vacuum advance unit not working or the hose got brittle and fell off.  It never seamed to bother the engine. The customers just complained about the milage and pick-up not being what they use to be.

[Mick70RR]

IMO
Another word for Vacuum Advance is "Economy" Advance ?

I never understood the "why" behind, hooking up an "Economy advance", on Engines we've just "built up" with Larger Camshafts & Fuel Breathing curves ?

If you built that engine mostly for street, what percentage of the time do you have your foot buried in it as opposed to just cruising at part throttle?

It's not all about economy it's also about driveability, if the vacuum advance will make the car more driveable for the vast amount of time you spend just cruising why not take benefit from it?

I agree, it's all about how the car drives for me. I've tried blocking off the vacuum advance but felt I needed more throttle for the same cruise speed. I've tried manifold vacuum but didn't like the way the car drove off idle. I use ported vacuum but limit the vacuum advance to 10 degrees and it feels right. My car is driven daily during the warmer months but gets plenty of track time too, the vacuum advance stays connected on the track because at full throttle it does nothing.

[b5blue]

  Since this was posted in May of last year I've overhauled my ignition system. Due to an intermittent crank but no spark condition (I found the issue inside the old harness after taking it all apart) and a nagging issue of trying to tune out the ping yet still have good start and idle that was making me nuts.
  I bought and installed a new engine harness made for ECU ignition and had Don at FBO curve, phase and calibrate a stock Mopar Dist. with vacuum advance for my exact build. I used his "A688 kit" with matched components to go with my Firecore wires from Ron and his recommended XR-5 plugs. What a freaking difference, my idle and bottom end love the vacuum advance and the transition out of vacuum advance into mechanical advance is seamless and gave me more usable power across the board right up to top end.
  My build is "mild" with 60 over 9.5 to one Hyper Speed-Pro's a "Resto" Mopar cam, stone stock six-pack and stock 346 heads. No amount of fiddling around with the dist. previously got me anywhere close I know now. The quality of my idle really was the biggest surprise, it's so noticeably stronger and rock steady. Mileage is way up and now I'll tweak out the six pack this summer.
  The ported vacuum advance is a compromise to emissions if you read up on it, FBO uses full manifold vacuum and it works very well. That will let me use an adjustable center carb metering block from Pro Max and still have vacuum advance.
  I've seen others struggle with ignition here just want to say for a mild street build (Having run for many years without it.) vacuum advance is worth trying, in fact with today's crappy fuels and high prices it's going to pay for itself in short order if you drive as much as I do.          

[tan top]

  i think distributor vacuum advance    for a street car , what you want for cruising part throttle !   but what ever works for your set up & your preference   

having said that since swapping to a thunder 800avs & electronic ignition  , still not got round to dialing my in