Riley's35089rs+

I was reading this tech artical about timing....Under.." How do I change the Mechanical and Vaccuum Advance settings?" It says to connect to a full vaccuum port. Could someone read this artical and tell me if it is correct as to there explanation...I have mine to a ported vaccuum , "above the blades connection" It says with todays fuels the vaccuum advance connected to a ported scorce can cause "detonation, over heating and other grief"

http://www.centuryperformance.com/timing.asp

Riley's35089rs+

ttt

ME Leigh

It should be the other way around. When the vacuum is connected to the full vacuum port it will start advancing the timing sooner and even at idle. This can cause timing to be to advanced causing detontation problems. The ported vacuum will only advance the timing when the rpm is higher where more timing advance is required by the engine.

But some people have found that having it connected to the full vacuum port makes more low-end power and makes the engine "peppier."

BTW i have mine connected to full vacuum and don't have any problems.

AJ_92RS

WRONG!!!

The only difference between "ported" vacuum, and full vacuum is timing at idle.

The guy is a moron. When the throttle blades are open enough to expose the port above the throttle blades, there is no difference in vacuum readings at part throttle, nor WOT. Don't believe me? Use a vacuum gauge and try it yourself.

If there is any engine vacuum at high RPMs at WOT, it's because the induction system is somewhat restrictive. It has nothing to do with where the vacuum port is located (above or below the throttle blades).

If an engine wants 750 CFM @ 6500 RPM and the carb is rated at 600 CFM, then the pressure drop BELOW the carb will be higher. A pressure drop is what creates vacuum.

If the engine wants 750 CFM @ 6500 RPM and the carb is rated at 850 CFM, the pressure drop will be lower below the carb. This is what inhibits vacuum.

At idle the throttle blades are below the vacuum port when hooked to "ported" vacuum. This means the pressure ABOVE the throttle blades is higher (close, if not AT atmospheric pressure) than below the blades.

When the vacuum canister is hooked to a “full” vacuum source, it sees vacuum at nearly every throttle position other than WOT.

That’s the only difference.

As far as which to use... it's all a matter of personal preference. I prefer “ported” because IMO, you don’t need additional timing at idle. You aren’t putting a load on the engine, so what’s the point? To me, it’s like putting 100+ octane fuel in a car that’s perfectly happy with 89.

Some people have claimed they get a better, more stable idle when hooked to full vacuum. IMHO, that just means it’s being used as a band aid. If they have to advance the timing at idle, that means the cylinder efficiency is low and they’re using advanced timing to make up for it.

Some also claim the engine runs cooler when using a full manifold vacuum source. That could be true (I honestly don’t know), but FWIW, if the engine is getting noticeably hotter with “ported” vacuum being used, again full manifold vacuum is being used as a band aid. I would think a less efficient engine would run hotter if it didn’t have some help.

That’s just my view on it. I’ve never used full manifold vacuum on any car or truck I’ve ever owned and I never had idle stability problems, nor any problems with overheating at idle.

Riley's35089rs+

I thought the artical was a bit misleading. I just wanted to make sure I wasnt missing out on something. Thanks for the input guys!

five7kid

iTrader: (14)

He doesn't quite have his history right. The '57 had manifold vacuum advance, as did the '66. Ported came in during the emissions era to reduce nitrogen oxides.

On the '57 today, the Proform doesn't have a ported vacuum port. So, I have to use manifold if I want vacuum advance (and I do). I had been using ported with the Holley. Idle quality is much better now.

With computer controls, you'll notice that the advance mimics manifold vacuum advance. So, since they could do anything they wanted, why did they do it this way?

Really confusing question: Computer controlled timing is a result of emissions requirements. Why did they start using ported vacuum for emissions in the 70's, and now mimic manifold for emissions?

(There is an answer...)

si_camaro

The thing I don't understand is that I don't seem to have a choice. Plugging my vacuum advance distributor into a "full" vacuum port raises my idle rate something awful (about 2k IIRC).

Am I doing something wrong here? The base timing is about 4* without the vacuum attatched, and I can't lower my idle any more, the screws are all the way out.

Streetiron85

I've tried it both ways, ported and full vaccum.
Guys sometimes say that it should always be ported, so I give it a try, but always end up going back to full manifold vaccum.
Every engine (carbed) I've had seemed to idle nicely @12-15*, with the highest vaccum ga reading being at that advance, and was also more responsive. But for starting that much advance can be too much, so the vaccum cannister will pull some timing til the engine fires.
Also I like to set up my vac cannister with a lot of advance, cause then it will be at about 40-45* at cruise rpm... good for economy.*
At least that's the way I do it.
Til someone shows me a way that I like better I'll keep doing it that way.

*I'm going from memory on this, I haven't looked at my distributor in a while, It's possible the figures gould be slightly off.

SSC

I experimented with this recently to gain a little better fuel economy. I saw no gain going from ported to full. I actually had to use more throttle to take off or I would stall. This of course is a milder setup. If you have a rumpity, rumpity cam and are running a vacuum advance distributor full vacuum will help with idle quality like on my 66 Malibu. But that is soon to have a full mechanical distributor to match the 427's two speeds, stop and 7K.