Vac advance or not?

I am looking to this as more of a poll. Let's say you are finishing your engine that is going into your traditional street driven rod. (keywords....street driven) If you are buying a new electronic distributor, which would you be more likely to purchase.....one with or without vacuum advance? Thanks.

 

Where's the poll?

 

WITH for the street, no question.

 

I don't have it and have no problems with driveability on the street. I put in an advance curve kit so and shimmed the distributor. It starts advancing on the mechanical side very quick off idle.

 

with. no question. i fought the elec BS for 2 distributors. completely satisfed with vac advan.

summit magnetic vacuum advance

 

I try to buy mechanical advance distributers whenever possible. No reason other than I like them.

 

The key word being "street" go with the adj vacuum advance can. Helps driveability with the correct curve etc, especially with the crappy gas, and runs cooler when idling.

 

It would depend on the motor and build specs. You should call Jim Linder "Bubba" and have him fix you up anyways

 

All that vacuum advance does, is save you gas. That's it. If you don't want to save gas, then don't use it.

I doubt I'd be buying a new electronic distributor to go in a traditional street driven rod.

 

I posted this before.......Lots of good info on why to use vacuum advance.

Hear is some info on ported vs manifold vacuum

Quotation thanks to JohnZ G.M engineer

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.
____________

 

We had to remove the vacuum advance mechanism on the Unilite distributor on my Son's 455 Olds because it was not clearing the tripower setup. We called Mallory and they said to just bump up the timing a couple of degrees more and it would be fine, and since the car runs great it looks like they were right.

I have heard that vacuum advance will make an engine run cooler in traffic, but I have never confirmed that by my own experiences.

Don

 

I vote for vacuum advance on a street car.

 

No matter how well your street car runs without vacuum advance, it WILL run better and get better mileage with vacuum advance. But, you have to know how to tune it properly to get the most out of it. The right vacuum can, weights, springs, and controlling your rate and amount of advance can really make a huge difference.
If you only do 1/4 mile blasts, no need for vacuum advance, since it doesn't do anything at WOT.

 

Great info, I'll print this and keep it in my " now what should I do " file.

 

Vacuum advance on my street car?..I'm all for it.

 

Mine seemed to do better without it, so it is plugged off.

 

My mileage is fine, sure I would really love to get 40 MPG and when I have enough money I am going to buy me one of them hybrids.

 

I went from 11 to 12 mpg with vacuum advance...that $10 part saved me a lot more than $10 of gas over 15 years

 

I would bet money it would be better with vacuum advance. I have yet to see a car not improve with it. To each his own, you don't NEED to run it, but it does work. I'm not telling anyone they're wrong for not running it.

On my 55, there is a noticeable loss in throttle response, mileage, and it smells pig rich while cruising with no vacuum advance. Like I said above, if you don't notice a difference, it's not set up properly.

 

I wont have one on my motor because I want the visual appeal of the old Mallory. I'll have to forgo the gas mileage help. If I did not have this distributor, I'd use a period dist. with the vac advance and install a pertronx for the period look. I took out a modern ugly billet electronic distributor to run this old Mallory which has been converted to a Chrysler electronic ignition. It's about the look for me.

 

It is not likely that you would win that bet. I do agree it is easy to say "well you just didn't set it up correctly." And you would be right with 99% of the people that you meet.

Mechanical advance needs to be set up properly just like a vac advance. It is comletey adjustable just like vac advance is and it is not dependent on vacumm or the lack there of.

I consistantly get 22 out of my small block without an OD. There is not a vac advance small block on this board that can boast that. You don't have to take my word for it drop raven a note, he has ridden in the truck and whitnessed the mileage.
Of course it is like you said you have to know how to tune it.

Mr Sinister on a side note your avitar makes me feel like you are being mean to me.

 

So what weights and springs work best?
What does your advance curve look like on a graph?
just asking to learn.

 

Both mechanical and vacuum advance do need to be set up to work properly for the intended use of the vehicle. But they serve two very different purposes there is no doubt that a vac canister on a distributor that is set up properly will return better mileage and usually smoother running at light throttle loads, like cruising down the highway. In that instance, the motor likes a bunch of lead (50° or more in many cases) and if the mechanical is set up to provide that, it is going to detonate like crazy when you bury your foot in it.
Using the example that you get 22 MPG as proof that vac isn't needed is ludicrous, how do you know you wouldn't get better than that if you added a properly set up vac canister? And saying "There is not a vac advance small block on this board that can boast that." is also crazy, how do you know what every other small block powered car on this board, which had to number in the tens of thousands are getting for mileage? It isn't like it either vac advance or mechanical advance - It's mechancial advance only or mech and vac working together.
Sorry, but I have yet to see a street driven naturally aspirated motor that couldn't or didn't benefit from vac advance. We get these inquires everyday in the ignition department here at work and the answer is always the same; "yes, you want vac advance on your street car"

 

I have a Factory Dual point Tack drive Chevy Dist that I
have been running for years
with no Problem on my 50 Merc/chevy engine
and gas miliage is 18 mpg

just my 3.5 cents

 

On the street, with, absolutely!!!!

 

Me? Never!!

 

What works on one car might not work as well on another. Advance curves should be fine tuned for your specific combination of parts and your powerband.
You're best off starting out with stock weights, and adding one silver and one blue spring. This provides the best general setting, and shows an improvement on just about every street engine. Most distributors are set up to work in a wide variety of applications, but are not optimal for any, typically.
Depending on your idle vacuum, you need an advance can that will work with your level of vacuum. If you have low vacuum, you need a can that is fully deployed at low vacuum.
You also need to limit the total amount of vacuum advance. Most do so by fabricating a stop plate that limits the vacuum can arm movement inside the distributor.
There's also the argument for full manifold vacuum or ported vacuum. I won't even get into that. I know what works best for me, but it might not work best for you.
I'm still learning about this stuff myself, so the best thing to do is google searches, and asking around on performance boards. I don't have a graph to show you, but you will feel it when it's improving, and you will know it when you've gone the wrong direction. I've gotten as far as selecting the optimum springs, weights, vacuum can, and fabbing and installing the limiter plate, but there's still more to learn. If you have a Chevy car or engine, Chevytalk.com is a great place to start. Learn all you can, then start the fine tuning. Believe me when I say it's not something you can learn in one post on a message board, I've been on and off with it for the past year and a half, but the difference from where I started is huge.

 

If you have a vacuum pot them which is the better source, manifold or ported vacuum?
Would you have the same 'tune-up' with either source?

 

I ran a dual point mallory with no vacuum advance, and now run single point GM with vacucm advance.


Both curves were similar, and they advacned at similar rates.

It does run somewhat cooler with the vac can, really no other difference btween both, and I have not checked my mileage.

I run my vac to manifold vaccum, per the reason HotrodDon posted.

Google crane adjustable vacuum advacne kits, the come with different advacne springs and they show the curvve of each one.

Keep in mind thatt the cam, type of tranny, gears etc will determine how fast of a curve, and how much mech advance you can use in the dizzy.

Also GM does have different vac cans that add different amounts of vaccum....

 

That's the argument. For me, I've had better luck with ported, but some guys swear by full manifold. The difference being you get no vacuum advance at idle on ported due to the port being above the throttle blades, and full vacuum advance at idle on manifold. As soon as you step on the gas, you get vacuum through the ported port, and your vacuum advance kicks in. When you're cruising, both sources provide the same vacuum signal. Low RPM cruising is where vacuum advance helps the most.
I've tried both, and the problem I had with manifold vacuum was a weak idle. It idles at the speed I wanted, but it was just really faint and weak sounding. Any variation in idle speed due to the decent sized cam I use was also magnified. It would also drop idle in gear occasionally when I'd stop. Instead of idling at the usual 750, it would drop to 600 and chug badly, causing my power brakes to go to hell.
My engine idles much stronger with no vacuum advance at idle than with, so I switched back to ported.