toddalin

Most people into street performance will connect the distributor vacuum advance can to full manifold vacuum, which obviously has a considerable impact on idle. Minor variations in moment-to-moment operation manifest themselves in vacuum and this is amplified through the action of the vacuum can as it pulls on the distributor. In this mode, my vacuum continually varies, but within about 1/4" hg. This make the idle "hunt" just a bit.

On the other hand, some people connect the can above the throttle blades leaving the idle unaffected. When I use this port, I get a rock steady idle as the minor moment-to-moment variations are not magnified through the vacuum. But you have to rev the engine considerably to get to the ported vacuum to finally kick in at the vacuum can, and this creates a lag and sudden surge.

So what if we "T" the two ports together and send that signal to the vacuum can?

My first impression was that the manifold vacuum should simply be "filled" from air coming in through the vacuum port located above the throttle blades, essentually creating a vacuum "leak," and very little vacuum should be available at the can during idle. Off-idle should be no different than before.

But that is not what I found at all!

When connected in this manner, I found that I only lost about 1" to 1-1/2" of vacuum at idle at the distributor. But the resultant vacuum (and idle) was rock steady with no moment-to-moment variation. And, the vacuum response is instantaneous with none of the prior lag when just using the ported vacuum fitting.

I'm impressed and need to buy a proper size "T" to make this permanent.

This is on a Holley 600 dp. Can other people give this a try and let us know your results?

Thanks guys.

anyChevy

Interesting idea Todd.
If more initial advance is required for a better idle without excessive total advance, the standard practice is to recurve the distributor, limit the total and use a different vacuum can, connected above the blades for a liitle more advance on cruise.
But that's alot of trial and error if you don't have a machine to do it on.
Is that what you're trying to achieve ?
Sounds like you're on to something I've never tried.

Mike Ward

Sounds like you have the wrong vacuum can if it's 'hunting' at idle.

MasterDave

The manifold vacuum would simply overpower the ported IMO, unless manifold vacuum is lessened some when using the 'T' by pulling some air from the ported...OR the combination of butterflys not adjusted properly and manifold vacuum is screwing with your vac can and idle.

toddalin

Vacuum can was installed by Lars when he went through the distributor and carb, so as to being the wrong one or incorrectly set-up, no, I don't think so.

Rather than condem the idea, how about giving it a try and posting your results/impressions, assuming you are using a Holley dp.

DansYellow66

Interesting idea. Someone who can hook up an interior vacuum gage and note the results in the two connection methods might come up with some interesting data. However, it seems that once you go off-idle the vacuum from both ports would even up. I guess there might be some benfit in the transtition phase though.

For me - I barely pull 8 inches vacuum at idle - I can't afford to give up that 1 to 1-1/2 inch vacuum signal.

65tripleblack

1. That's the reason the vac can should be tailored so that it begins to deploy at least 2 in-hg below idle vacuum.

2. That's because you created MANIFOLD vacuum leak, but the result on engine operation is not as you describe. Vacuum is backing up through the tee and above the throttle plates to the "ported" side. The flow will reverse when you open the throttles enough to overpower manifold vac when manifold vac drops and ported vac increases. Meanwhile, you created a vacuum leak at idle.

YOU ARE NOT THE INVENTOR OF YOUR SCHEME. IT WAS USED IN DIFFERENT ITERATIONS DURING THE SEVENTIES WHEN PORTED/MANIFOLD COMBOS WERE USED AS BAND AIDS. It was combined with thermal switches/electronic solenoids and/or check valves.

Either as previously stated, your vac can is not the right one, it's gone bad, or something else has changed. It's deploying too late; too close to idle vacuum. If your vac can begins to deploy at anything higher than 2 in-hg LOWER than your idle vacuum, then you are looking for trouble.

The easy "fix" for you, is to put your vacuum back to full manifold, and put the tee back into your tee-storage locker. Increase the idle speed such that your idle vacuum increases to minimum 2 in-hg higher than the vac can deploy point.

If I were you, I'd install the right vac can, or, better yet, install an adjustable unit by Moroso or Mr. Gasket Co. This is the better fix, and the right way to do it.

Mike Ward

I see your point but don't need to jump off a cliff a second time to see if it hurts when I hit bottom, nor do I think that turning up the radio to drown out an unusual engine clatter is a good idea.

Obviously your car didn't operate this way when new, nor did the umpty million other Chevs that GM built, and I doubt that Lars would have made a fundamental mistake in setting up your distributor, so it's more than likely that 'something has changed' in the interim and your car is now not operating correctly. Address the root cause, not mask it.

lars

Todd -
A correctly matched and operating vacuum advance control unit should not allow the timing to change with minor throttle input, and should not cause the erratic idle you describe - there should be adequate "margin" between your manifold vacuum and the "full pull-in" spec on the control unit.

It's been a lot of years since we last put that unit in, and they do go bad and stiffen up with exposure to the modern pump gas fumes - things may have changed a little. What unit do you have installed (i.e., "B1," "B26," "B28," etc) and how much manifold vacuum are you pulling at idle? I can send you a replacement unit and fix the problem with a good-working unit matched to the requirements of the engine.

Lars

toddalin

Wow, defensive! I never said I was the "inventor" of the scheme.


Lars, I pull about 13.5-14" vacuum at ~850 RPM. While its been a few years since we did the job, it's only been <1,000 miles. I would have to pull the shielding to determine which can we put in.

The "hunting" and vacuum shifts are very minor, less than 25 RPM and less than 1/4". Most probably wouldn't even notice it. But with the "T" both are rock steady.

toddalin

Of course not. GM never put a high rise manifold with a 1/2" open spacer and a 600 Holley dp with a milled choke horn and full length headers on anything. Car came with a low rise, AFB, and exhaust manifolds. Sure it operates a bit different now.

MikeM

Maybe your self induced vacuum leak just smoothed out a rich idle mixture.

babbah

Mine is steady as a rock connected to full manifold vacuum. I only have 8 inches of vacuum at idle (large duration cam) but it's rock solid and the B28 can on mine works perfectly.

My BB idles perfectly and rock steady as well.

IMO, connecting the can to both full and ported manifold sources makes no sense to me; as in that scenario the advance can would be advancing the ignition timing all the time - under light and heavy loads and would cause poor performance and harmful detonation.

toddalin

That is no different than connecting it right to the intake manifold vacuum, which is what you've done.

babbah

?? Your initial post suggested to connect via a T-fitting to BOTH the ported and un-ported vacuum sources to the advance can.

I do not have any "ported" vacuum source connected to my advance can. At idle and under light load its all in, When standing on it no vacuum advance - exactly the way it should work.

Full manifold vacuum = under the carb plates or off manifold - full vacuum at idle and light loads

Ported Vacuum = above the carb throttle plates - zero vacuum at idle - vacuum only when accelerating

toddalin

I believe that you are confused.

Above you said "IMO, connecting the can to both full and ported manifold sources makes no sense to me; as in that scenario the advance can would be advancing the ignition timing all the time - under light and heavy loads and would cause poor performance and harmful detonation."

But then you say that you've connected your can to the manifold vacuum so that "at idle and light throttle it is all in."

So, that being the case, in accordance with your set-up it would be "advancing the ignition timing all the time - under light and heavy loads and would cause poor performance and harmful detonation."

babbah

No actually you are the one that is confused, read my replies carefully....

When the correct vacuum advance can is installed on a performance engine to full manifold vacuum (NONPORTED), under HEAVY load the vacuum drops almost to zero and the advance can does NOT advance the timing. It advances it when the vacuum returns to the required level to re-engage the advance which is under no load or cruising conditions.

Your original post suggested to have both non ported and ported sources tied together to the can which "would cause poor performance and harmful detonation".
Cappeche?

stingrayl76

At idle the manifold vacuum will be sucking air thru the ported vacuum path and under load the ported vacuum would be sucking air thru the manifold vacuum path which theoretically render the vacuum advance inoperative. Using you "dual" set up' tee in a vacuum gauge at the vacuum canister and see what you read at idle and under heavy load.

After re-reading the original post I see you have done this at idle and lost 1 to 1 1/2 in/hg at idle which confirms you have a vacuum leak or the engine RPM has dropped. What does it read under load.
If you have the correct vacuum and 10 in/hg at idle, the idle is not varying because of the vacuum advance. Verify which canister you are using. I have a 365 with the 30-30 cam and using a B28 canister it idles at a steady 850 rpm pulling 10 in/hg.

toddalin

When Lars set up the car, he set the mechanical advance at 34 degrees. But subsequent dyno testing showed that the car liked ~36.5 degrees, so the distributor was pushed over a bit.

Obviously, this also advances the timing at idle, raising the idle and making it a bit more erratic. Loosing a bit of vacuum at idle seems to take a bit of this timing out to where the engine likes it better, so runs a bit smoother.

For those who say that the factory had it right and "why vary from the factory way/settings?", my manual shows the factory only specifies 30 degrees of mechanical advance @ 4,700 RPM for a '66 327/350.

Certainly if I kick the advance back 6.5 degrees, I'll get a nice smooth idle, but I'm not about to give up that horsepower.

I'll pick up a proper T fitting and use the dial back timing light to see what is actually happening at idle for the various cases.

troutster71

Ported Vacuum = above the carb throttle plates - zero vacuum at idle - vacuum only when accelerating[/QUOTE]

You're not really sure how ported vac works, are you?

Todd, good for you for thinking out of the box, but like Lars, I think something

has changed. Not many miles, but alot of gas fumes...


Cheers, GA