Article on Vac advance

Best technical articles on the forums. Send nominations to the admins. You can reply to topics but only admins can make new ones.

Moderator: Moderators

Do you use vac advance on your street driven car

Poll ended at Thu Aug 31, 2006 1:57 pm

Yes
19
95%
No
1
5%
 
Total votes : 20

Article on Vac advance

Postby Buscop » Tue Aug 01, 2006 1:55 pm

Here is an article I found after browsing for hours looking for the diffinitive answer on vacuum advance...pros and cons..

(Long post)

There appears to be a lot of conversation on timing, vac advance, distributor tuning and, in general, hacking up a perfectly good ignition system to compensate for a poorly tuned carburetor.

from
JohnZ
Tech Team


posted 03-23-2003 03:41 PM


The following two articles review the basics of distributor tuning quite well and have worked for years and years and are based on sound engineering principals. I thought it would be helpful for some to review these prior to hacking up their distributors. Hacking up your distributor to compensate for a poorly tuned, misapplied or defective carburetor is not very sound engineering, for a street application or otherwise.

Here's an interesting article on vacuum advance written by a GM 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 15â€
User avatar
Buscop
 
Posts: 402
Joined: Sun Jun 25, 2006 2:43 pm
Location: Cobourg, Ontario

1979 Chevrolet Monza Wagon


Postby gerbsinmd » Tue Aug 01, 2006 2:10 pm

Russ,

That is some awesome information!!! It actually explains a lot of things!!

Darrel
1997 Blazer - Being driven by my daughter :S
2008 Town & Country - Kid transport
2002 Saturn SL2 Commuter car 36mpg!!!!

1977 Monza Mirage being Restified!!
User avatar
gerbsinmd
 
Posts: 2013
Joined: Mon Jun 26, 2006 2:48 pm
Location: Dunkirk, MD

1977 Chevrolet Monza Mirage

Postby V8Spyder33 » Tue Aug 01, 2006 4:12 pm

Great articles. Thanks!
User avatar
V8Spyder33
 
Posts: 25
Joined: Thu Jul 06, 2006 9:08 pm
Location: Southeastern MA

1980 Chevrolet Monza Spyder

Postby SunbirdMan » Wed Aug 02, 2006 11:22 pm

Do a search on his name and you'll find his tech papers on ignition, carbs, alternators, etc. He's a Corvette guy and hangs out at some Corvette forums.


"by Lars Grimsrud
Colorado Corvette Crazies
The Ultimate Corvette Tuning and Beer Drinking Fraternity
Lafayette, CO "
Roger---------------------------------------------------------------
UNLESS OTHERWISE SPECIFIED
DIMENSIONS ARE IN INCHES
User avatar
SunbirdMan
 
Posts: 1270
Joined: Sun Jun 25, 2006 8:16 pm
Location: SantaMaria,CA

1978 Pontiac Sunbird Sport Coupe

VACUMN ADVANCE TIMING

Postby MIKESMONZA » Thu Aug 03, 2006 6:51 pm

TODAY I CHANGED THE SPRING'S AND ADVANCE WEIGH'TS, AND BUSHING'S ON MY 75 MONZA HATCHBACK 262 2BBL AUTO ORIGINAL MOTOR, THE OLD SPRINGS WERE RUST COLORED RUST, ALSO I STUCK MY TEST EQUIPMENT VACUMN HAND PUMP WITH GUAGE, TO MEASURE VACUMN AT HOSE TO ADVANCE UNIT ,,,,WOULD NOT MOVE ADVANCE AT ALL,,, WOULD NOT HOLD VACUMN,,,, DOUBLE CHECKED HOSE IT WAS GOOD,,,,REPLACED WITH NEW ADVANCE,,, HOOKED UP TIMING LIGHT AND TACH WENT TO SET TIMING THEN STOPPED TO COME INSIDE AND GET ON LINE FIND OUT WHAT THE REST OF THE H- BODY HERD IS DOING FOR TIMING,,,WHAT ARE YOU DOING????? SUBLIMINAL SUGESTION"S THOUGHT PROJECTION,,,,,PHYCHIC TIMING THREADS,,,,,I WANT MORE ,,,,,THOUGHTS,,, MIKE,,, :idea: :idea: :idea: :idea: 8) 8) ,
User avatar
MIKESMONZA
 
Posts: 1262
Joined: Thu Jun 29, 2006 2:02 am
Location: CUT N' SHOOT,TEXAS,, EAST TEXAS AREA

1975 Chevrolet Monza 2+2

Postby cjbiagi » Thu Aug 03, 2006 7:41 pm

A stock 75 monza V8 distributor will provide 22 degrees of advance. While I do have custom springs, I am running the stock weights along with about 14 degrees initial. This give me, in theory, 36 degrees total not counting the vacuum advance. Seems to work well on my 350.
Clyde.........75 Monza 2+2
User avatar
cjbiagi
 
Posts: 7991
Joined: Sun Jun 25, 2006 2:37 pm
Location: Glenwood, Illinois

1975 Chevrolet Monza

VACUMN ADVANCE

Postby MIKESMONZA » Fri Aug 04, 2006 1:59 pm

THANK'S CLYDE;,,, :idea: THAT'S WHAT I WAS LOOKING FOR AND RUSSELL THAT ARTICLE IS A POSITIVE PLUS FOR ALL OF US,,, :?: :?: , ANY BODY ELSE ,,,,HAVE A SET UP,,,WITH OR WITHOUT PROBLEMS ,,,, :?: THE SYMPTOM,S ON MINE WERE SERGING AT 65 -75 MPH AND RE APLYING THE THROTLE AT 50 TO PASS AFTER CRUISING AND SLOWING DOWN FOR A SLOW VEHICLE,,,, THE WEIGHT"S WERE ELIPITICAL IN THE HOLE"S WHERE THEY SWING,,,, USED THE SILVER SPRING"S AND THE UP GRADE CENTER AND MODIFIED WEIGHT'S ,,,,WITH BUSHING"S,,, :idea: , I LIKE THE IDEA OF SWITCHING BACK TO STOCK CENTER AND WEIGHT"S HAVE TO LOOK AT MY SPARE DIST,, AND SEE I CAN GET THE PARTS TO TRY WITHOUT THE EXCESSIVE WEAR IN HOLES AND SPRING TENSION"S THAT MY OLD ORIGINAL,,,, HEI DIST HAS ,, MIKE 8) 8)
User avatar
MIKESMONZA
 
Posts: 1262
Joined: Thu Jun 29, 2006 2:02 am
Location: CUT N' SHOOT,TEXAS,, EAST TEXAS AREA

1975 Chevrolet Monza 2+2

Postby cjbiagi » Fri Aug 04, 2006 6:43 pm

As a general rule, the weights will determine the amount of advance, the springs will determine the rate of advance. There are some things you can do to allow the weights to give a different amount, but if you keep the stock weights and just modify the springs I think it will put you pretty close to where you want to be. That was a very good article on vacuum advance, seems we should be running straight manifold instead of ported according to those thoughts. Anybody tried it?
Clyde.........75 Monza 2+2
User avatar
cjbiagi
 
Posts: 7991
Joined: Sun Jun 25, 2006 2:37 pm
Location: Glenwood, Illinois

1975 Chevrolet Monza

Postby gerbsinmd » Fri Aug 04, 2006 8:36 pm

That's the way I used to run my V6 Starfire, Manifold vacuum. I liked the idle it had better than on the ported vacuum. And I ran it with a little more initial advance and it seemed to like that as well. Either one, the more timing or manifold vacuum made a difference but the combination made for a little more zip.

Darrel
1997 Blazer - Being driven by my daughter :S
2008 Town & Country - Kid transport
2002 Saturn SL2 Commuter car 36mpg!!!!

1977 Monza Mirage being Restified!!
User avatar
gerbsinmd
 
Posts: 2013
Joined: Mon Jun 26, 2006 2:48 pm
Location: Dunkirk, MD

1977 Chevrolet Monza Mirage

Postby marco_1978_spyder » Sat Aug 05, 2006 7:16 pm

I switched to manifold this year, after doing some research and reading the same article.

It makes sence, once you read and understand where and why its there to function.

Maybe AstreGT will be interested in it....since it could help cool his ride down on city streets.
Last edited by marco_1978_spyder on Sat Aug 05, 2006 8:07 pm, edited 1 time in total.
1978 Monza Spyder V8 4speed, posi
1978 Sunbird Formula V6, 5 Speed, Hatchback
1978 Sunbird notchback 4cyl Auto
2006 Chevrolet Aveo 5 speed Hatchback
Yes, you can refer to me as Mark
Please visit my blog... http://chevymonza.blogspot.com/ Follow if you like!
Featuring special guests; Carl Beraytor and Ray D'atore

My Red spyder now has a youtube channel please Like and Subscribe!!!
https://www.youtube.com/channel/UCOzzSR ... ISuing7KLA
User avatar
marco_1978_spyder
Senior Moderator
 
Posts: 5242
Joined: Sun Jun 25, 2006 5:20 pm
Location: Western, New YorK / Rochester

Vac source

Postby Buscop » Sat Aug 05, 2006 7:49 pm

:arrow: Marco, where are you tapping in for a vac source? Is the timed spark vac pipe on the Holley Carb on the front metering block manifold or ported vacuum? :?: :?:
Russell G Hilder
1949 Model Wife
1979 Monza Wagon
2007 Harley FLHTCU
2009 Dodge Dakota Laramie
User avatar
Buscop
 
Posts: 402
Joined: Sun Jun 25, 2006 2:43 pm
Location: Cobourg, Ontario

1979 Chevrolet Monza Wagon

Postby marco_1978_spyder » Sat Aug 05, 2006 8:10 pm

I'm not sure exactly which nipple I used....(I run man.vac. on my 79 with the rochester 2bbl and my 78 with a holley)

To make sure its manifold I just check them all with a vac. gauge, and determine which are which.

This is where an adjustable vacuum can from crane is nice......i
1978 Monza Spyder V8 4speed, posi
1978 Sunbird Formula V6, 5 Speed, Hatchback
1978 Sunbird notchback 4cyl Auto
2006 Chevrolet Aveo 5 speed Hatchback
Yes, you can refer to me as Mark
Please visit my blog... http://chevymonza.blogspot.com/ Follow if you like!
Featuring special guests; Carl Beraytor and Ray D'atore

My Red spyder now has a youtube channel please Like and Subscribe!!!
https://www.youtube.com/channel/UCOzzSR ... ISuing7KLA
User avatar
marco_1978_spyder
Senior Moderator
 
Posts: 5242
Joined: Sun Jun 25, 2006 5:20 pm
Location: Western, New YorK / Rochester


Return to Best Of

Who is online

Users browsing this forum: No registered users and 1 guest