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Vacuum Advance Explained

one finger john

Active Member
A while back I read a very interesting article about the function of vacuum advances in the modern automobile. It was written by a retired GM engineer and was eye opening and a good read. I promptly lost it until last night when I Googled the phrase "distributer vacuum advance explained G.M. engineer" and a lot came up. Found the original paper by the engineer and I also found on 460Ford.com the paper plus a listing of ALL the GM vacuum cans for HEI distributers plus how many degrees they pull @ how many inches vacuum. Leave it to those Ford guys.

Here is the site http://www.460ford.com/forum/showthread.php?t=117504 I think maybe Lee or Exjunk might have brought this up before, but if not bon appetite !

John
 
Mike, this article isn't getting a lot of action but I think it could be a great addition to the tech archives. How do I go about getting it in there?
 
I think that article is by Lars Grimsrud. Do a search for articles by him and you'll find a world of good tech info includeing:

Timing
Quadrajet
vacuum advace
and more.

He was a GM engineer at the Corvette plant i think.
 
Metal (or anyone else), care to expound on the nuances of retarding timing for more top end ?

Well, I could go on and on about Spark timing, and cam timing....which is just as important. Motors that I work with only usually idle maybe 15 seconds max after they're brought up to temperature, or warmed up. I deal with 80% alcohol motors of various race types....and as long as it stays running long enough to get itself to the starting line....everything else is tuned and built to make maximum horses at maximum revs, everythings geared towards winning races. The other 20% are taking hits on Nitro....and that stuff is really hard on parts. Alcohol is really easy on parts.
Timing for maximum power is a double-edged sword. You have to run as much advance as the motor can stand, and because you have to turn rpms to make horses, with the piston speeds involved, you've gotta lead that piston and have it hitting at the right time to make horsepower. Usually we run 2 magneto's and they're locked down, so-to-speak. But with the ignition systems we're running now, we can use computer software that can adjust things so fine, we make alot more power than we did 20 years ago.
That has trickled down to the street stuff....but the good old street motors....I really enjoy their durable, simple, reliability. By that I mean running for years, set a certain way, and you never have to worry about them.

Take oino's situation. Simple, but once the problem was addressed....it straightened itself out. I have always thought of the timing thing as someone skeet shooting. Here comes a piston flying down the bore over 1500-2000 feet per second....the thing has to slow down as it changes directions....and you have to shoot the mixture with a spark at the right time for optimum performance. A little early, makes it run better at high speeds. A little later makes it run better at low speeds....

Its as simple as you make it.....
 
Hmmmm .... So what you are saying is that being able to have advanced timing at lower rpms and then retarding the timing at higher rpms has come about only AFTER the advent of electronic timing devices. Could mechanical retarding be brought about in the "old" distributers and magnetos by reverse centrifugal weights ? I.E. By locking the mags in at 40* total and then have the timing retard as the rpms increased ?

As for Oino, at this point, we don't know what caused the of loss of vacuum signal. He replaced two items and the problem was mysteriously repaired. I would have liked to have first replaced rotor and see if that repaired the low vacuum situation. If it did not, then pull the rotor and replace the cap and see if that helped. Then you could see which part was bad and why they were bad. I'm probably being picky at this point. Maybe it was just carbon tracking and a corroded electrode. End result is that the car runs way better after the cap & rotor were replaced.

John
 
Well, last night it wouldn't let me finish my post....so here it is....


Its as simple as you make it.....in Nov. at Pomona, we'd really tore a car up be messing with the timing. At Famoso in March....hopefully....some others won't be making the same mistakes. Melting a mill down after a race is sometimes expected for a all-out run....but folks don't like it when it actually happens....
Its great if your in the points race and you melt one coming to be the champ....but burning one down coming in less than 2nd place, eyebrows start being raised....
 
Hmmmm .... So what you are saying is that being able to have advanced timing at lower rpms and then retarding the timing at higher rpms has come about only AFTER the advent of electronic timing devices. Could mechanical retarding be brought about in the "old" distributers and magnetos by reverse centrifugal weights ? I.E. By locking the mags in at 40* total and then have the timing retard as the rpms increased ?

As for Oino, at this point, we don't know what caused the of loss of vacuum signal. He replaced two items and the problem was mysteriously repaired. I would have liked to have first replaced rotor and see if that repaired the low vacuum situation. If it did not, then pull the rotor and replace the cap and see if that helped. Then you could see which part was bad and why they were bad. I'm probably being picky at this point. Maybe it was just carbon tracking and a corroded electrode. End result is that the car runs way better after the cap & rotor were replaced.

John

Yes, to the first paragraph, it can be done like that.

On the 2nd paragraph, I think oino has a pixie or a fairy hiding in that T somewhere and it just likes 'jacking' with the motor....
 
Screamin metal i think that vacuum advance just sucks.
 
Screamin metal i think that vacuum advance just sucks.

Thats why I like running a Mechanical advance....Like Ted said on many occassions, and he is totally right....vacuum advance is for gas milage and somewhat for emissions....
The early performance motors only has mechanicals on them, and they were street driven high horse high performance mills.
Alot of them got decent milage as long as you didn't hang your foot off into it.
 
A while back I read a very interesting article about the function of vacuum advances in the modern automobile. It was written by a retired GM engineer and was eye opening and a good read. I promptly lost it until last night when I Googled the phrase "distributer vacuum advance explained G.M. engineer" and a lot came up. Found the original paper by the engineer and I also found on 460Ford.com the paper plus a listing of ALL the GM vacuum cans for HEI distributers plus how many degrees they pull @ how many inches vacuum. Leave it to those Ford guys.

Here is the site http://www.460ford.c...ad.php?t=117504 I think maybe Lee or Exjunk might have brought this up before, but if not bon appetite !

John

Thanks for posting that link John, it answered all the questions I had and more. :nod:
 
Thats why I like running a Mechanical advance....Like Ted said on many occassions, and he is totally right....vacuum advance is for gas milage and somewhat for emissions....
The early performance motors only has mechanicals on them, and they were street driven high horse high performance mills.
Alot of them got decent milage as long as you didn't hang your foot off into it.
One more advantage in running plenty of (vacuum'ed) advance in your street engine is that the chambers don't load up with carbon near as bad. This is because the hard to burn "heavy ends" of the fuel get combusted completely and disposed of out the tailpipe instead of becoming a tarry residue that stays as carbon.
 
One more advantage in running plenty of (vacuum'ed) advance in your street engine is that the chambers don't load up with carbon near as bad. This is because the hard to burn "heavy ends" of the fuel get combusted completely and disposed of out the tailpipe instead of becoming a tarry residue that stays as carbon.



What he said!
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