babbah

Look up "ported vacuum" on the net - You will get this explanation:

"Full vac port is where the port/nipple is below the throttle blade. You have full vac on these ports at idle and as you floor the throttle the vac goes away."

"Timed or ported vac ports are above the throttle blades and has no vac on it at idle but as you throttle up, it draws a vac."

Which is exactly what I wrote above........

lars

Todd -
Since you advanced the timing a little, and since you probably have a 17-degree vacuum advance control unit on your distributor right now, the California gasoline may be causing you a little bit of erratic behavior with all that advance at idle. I'll custom build you a 12-degree vacuum advance control unit that will match your engine's requirements. E-Mail me your address (along with a reminder of how much vacuum you're pulling at idle) and I'll send you a nice unit that will fix the problem.

Lars
V8fastCars@msn.com

Frankie the Fink

I'm sorry but what are the disadvantages of just going with an adjustable vacuum advance? I run one on both my '61 283 ad '67 396.

Not being a wise guy just seeking some knowledge!

toddalin

Thanks Lars. PM sent.

babbah

Hi Frankie - What adjustable vacuum advance Can are you running on the 396?? Brand? model#? Thanks Bro!

Frankie the Fink

By using different weight springs, mechanical limit tabs and this vacuum can you can dial in nearly any advance curve desired:
http://www.jegs.com/i/Accel/110/31034/10002/-1

My '61 283ci/270HP engine is 12* initial timing; goes to 36* at about 2800 RPM (all in mechanically) and the vacuum can is adjusted to add another 12* on top of that for a total of 48*. A near perfect curve for an SBC per Lars's writings...

I'm still tweaking the 396 in the Chevelle but hope for similar success...

lars

I seriously hope you didn't really "PM" me after me asking for an e-Mail:

http://forums.corvetteforum.com/c3-t...results-2.html

See response #34.

JohnZ

You're not really sure how ported vac works, are you? Cheers, GA[/QUOTE]

That's EXACTLY how ported vacuum operates.

http://www.camaros.org/pdf/timing101.pdf

toddalin

Was e-mailed. I guess I use PM as generic for contact off-line.

snydes

I was running one or Cranes adjustable VACs, the only problem I found was that when adjusted to the minimum setting it would take 11" of vacuum to pull it in, which is fine if that meets the requirements of your particular engine. If your engine has less than that at idle then it may not be a good choice. Again this was a Crane VAC, another manufacturers unit may be different.

QIK59

Usually doesn't change the AMOUNT of vacuum advance just the rate.

What total advance do you have on the Corvette at idle ??

w1ctc

When setting timing we must consider that we don't have the same gas as was available in the 60's. Less octane and energy. So the factory set up will only be a starting point

Plasticman

Not true.

The total amount of advance is adjusted with a Vacuum Timing Limiter (at least the Crane unit does).

Per the Crane instructions:

The adjustment made through the vacuum port of the Adjustable Vacuum Advance Kit cannister adjusts the rate of vacuum timing change as engine vacuum changes.

Adjustments made with the adjustable Timing Limiter will change the amount of that vacuum timing.

You can see it included with this listing on Summit:
http://www.summitracing.com/parts/CRN-99601-1/

Also available separately:
http://www.summitracing.com/parts/CRN-99619-1/

Plasticman

QIK59

Yep the forum is still the same as when I last left it in disgust.
Every time you post ; some one has to show how much more they THINK they know then you.

Key word plasticman in my response to FF was "usually".

I addressed FF's question re adjustable vacuum CANS - most vacuum cans DO NOT LIMIT AMOUNT OF VACUUM ADVANCE.

The only adjustable vacuum advance (that I know of) that has a vacuum advance limiter is the Crane KIT ; which includes a Mickey Mouse little piece of metal that you ***** around with to limit the travel of the pull rod.

I don't really need the links to Summit et al as I have one (Crane 99601) in my stock of vacuum advance units for direct reference.
Actually earlier this evening I tried it on my Sun Distributor Test machine just to see what it's range of rate adjustment was.

Frankie said how the Accel unit worked so well for him - reason for my question as to how much total advance he see's at idle with it.

Plasticman

You make an incorrect statement and then get corrected, then have a hissy fit.
Sorry for upsetting you with a fact.

Plasticman

QIK59

Man I can't believe it ! I was wrong twice in one day !
Here I thought illiteracy had been eliminated in the deep south ?!

Frankie the Fink

I don't remember the total advance at idle QIK....I'd have to hook the dial back light up again to recheck. I DO know the car runs phenomenally well for a lil 'ole 283ci. The car is on full manifold vacuum so I'm sure that affects idle advance but can't say right off the bat how much.

I researched the Accel adjustable vacuum advance before purchase and it does indeed change the amount of advance you can watch it move each time you tweak it. My '61 pulls a solid 17" of vacuum at idle which is plenty to activate the can.

The instructions also show 3 different advance curve charts showing how many turns of the adjustment screw provide how much advance depending on the engine's base vacuum...and those are actually quite close to what I experienced in tweaking things.

The point of my original question was why have Lars build a vacuum can with a custom, static curve when adjustable cans are around for $25 ?? Maybe the adjustables have a higher failure rate, aren't original looking enough, don't fit on some distributors, etc... Pure curiosity.

65tripleblack

I have an Accel adjustable on mine and it works very well. The lowest "cut in" vacuum that you can get with it, however, is about 5.5 in-hg, which is not quite as low as the 4 in-hg that the VC1810/B28 begins to deploy at. I idle mine at 1100 to maintain the "2 inch rule".

There is no need for a "custom" vac advance can, unless you're looking for originality.

I have the Accel rate graphs for posting if anybody's interested.

INMYBLOOD

That's EXACTLY how ported vacuum operates.

http://www.camaros.org/pdf/timing101.pdf[/QUOTE]


John I believe I've always understood how each type works, but know I see why manifold vacuum is the better choice. Thanks.

Now for a question. When I used to run manifold vacuum. My exhaust gases would burn your eyes and people would complain. I've seen no cooling issues either way. So I'm guessing my issue was air/fuel ratio? With Ported vacuum it just seemed to idle and smell normal? Now I do have the mechanical set @ about 12-14 BTDC as a low.

Currently I run no vacuum advance with the blower. I'm afraid I'll rattle it and break a piston.

Thanks for the great article.

QIK59

Thank you guys !
I am going to respond and continue this thread on the previous "Vacuum Advance Can" thread which I believe is more appropriate to the current discussion.
Jerry