Vacuum advance on triples

[Reverend]

I read about somewhere (i think it was in this forum) that vacuum advance in properly tuned engine/carburator can improve the fuel efficiency with triples as well. It was somekind of research, cant remember where it was.

Im very close to finishing my project with triple carb setup and i would like to hear about your experience in using vacuum advance with 3 carbs. I have SK carbs and they have vacuum port fitted, though only in one out of 3. Rest are sealed for some reason. Does one barrel provide enough vacuum for distributor?

My engine is stock, with MSA header but i have no-points dizzy. I think timing was set to 17 degrees.

Any info/help would be appreciated.

edit. haha, i had saved the pdf to my dropbox. take a look:

https://www.dropbox.com/s/p0jo85kt4mbbcpe/Weber%20DCOE%20Fuel%20Economy%20Study.pdf

Edited February 11, 2014 by Reverend

[madkaw]

Certain dizzys pull in too much vacuum advance and so we don't run them. I run a 81 dizzy on a 9.55 CRAtio so I elect to have 20 initial with 37 total with no vacuum advance.

You only need one port for the advance. Additional advance at cruise would be helpful, but it would be tough to make work with the ignition curve

[Reverend]

So does standard 240Z dizzy pull too much vacuum? Could it work as is or does it need to be modified?

[Reverend]

http://www.classiczcars.com/forums/carburetors-s30/28489-tripple-webers-vacuum-advance.html

"The issue with triple webers is that they just don't generate enough vacuum from any one throat to pull/move the diaphragm in the dizzy."

Im confused. Does it pull too much or too little?

[LeonV]

Try it out and see how it goes. I'm sure ganging up 3 runners together would do the trick if the signal from one is too messy. You can also try to dampen out noise if you'd like to stay with using one runner. When properly set up, vacuum advance is always a good thing.

[240260280z]

https://www.classiczcars.com/forums/attachments/carburetors-s30/56286d1347626058-dcoe-documents-weber-dcoe-fuel-economy-study.pdf

[Reverend]

Blue: the same pdf is on my dropbox link. All i need to know is it dangerous to experiment with standard dizzy and vacuum advance. I dont want to break anything so i rather ask stupid questions.

[madkaw]

http://www.classiczcars.com/forums/electrical-s30/34192-280zx-distributor-advance-curve-reference.html

Look at this thread. i wish the moderator would make this a sticky.

When i make the statement 'pulling in too much vacuum advance', I am referencing the amount of actual degrees of advance. It doesn't take much vacuum to pull the lever in your dizzy to actuate full advance. Some dizzys this is 17 degrees or more.

Look up your dizzy by the number on the side and determine exactly how much advance you have. Some early dizzys have 24 degrees mechanical, now you advance your initial timing by 5 degrees and throw in the vacuum advance and you are running 50+ degrees total on todays gas.

[240260280z]

They used manifold vacuum in the experiment. Ported will work better for efficiency. Do't worry, keep reading and you'll understand advancing. It takes a bit of head scratching at first.

[Captain_Zeros]

TANGENT ALERT: Directed at folks with crappy high vac advance malaise era 280zx distributors!

Now this is something I wished I would've documented, by I am running a 1981 non-turbo electronic distributor... you know... the one with about a billion degrees of vacuum advance (30 to be specific, more than twice the 14 degrees my car came with stock, but I don't want to deal with points). It wasn't particularly drivable, so I figured what the hell, and cored out the little epoxy plug in the end of the vacuum advance servo. (pointy hand tools and patience are suitable, I think I used a pocket knife)

Inside there were two screws within each other, a large coarse insert that changed the inner volume of the diaphragm chamber by a bit, and a long skinny central narrow one that serves as a bump-stop for total throw distance. I monkied with it using the high tech approach of throwing a protractor on it and eyeballing it, corroborated by measuring the travel of the vacuum servo when I sucked on it with a piece of nice clean vacuum hose and doing some bar napkin math and was able to achieve somewhere in the vicinity of 7.5 degrees of vac advance in the distributor plus or minus a degree or so (15 at the crank, gearing and all that). Then capped it off with a new plug in the end made of RTV (the screws are NOT airtight). A timing light corroborated that yes, in fact, I had achieved fairly close to 15 degrees of advance at the crank.

All this being unscientific and from the angle of "this part is junk to me if I can't modify it, so if I break it modifying it... oh well". All things considered I don't have much of any control over the characteristics of that advance because I haven't change the diaphragm itself, only it's overall travel limit, but I've been daily driving my Z on it for over a year now and it feels great, some day I'll rip it apart and take pictures, I didn't at the time because it was all experimental and I wasn't expecting much. Considering I don't have as much mechanical advance as I might like, I DO have to run quite a bit more static timing, I forget the numbers offhand, but it doesn't cause any issues for me, I just set timing appropriately at full mechanical advance. Anyhow, I haven't seen this particular modification documented anywhere, just people saying "Some distributors are not suitable", so I figured I'd throw in my two bits.

End Tangent Alert

Edited February 12, 2014 by Captain_Zeros

[Reverend]

I found that nice spreadsheet from link provided by madkaw. Thanks. I have D606-52 dizzy, sooo.... it seems to be safe to use with vacuum advance....?

[madkaw]

I'd say you will be okay. That dizzy won't be the best for performance with only 29 degrees total advance WOT, but it's always a compromise.