The last time I went to the track, I found that I went 3-4/10 faster with my vac. advance disconnected. I posted that here, and we couldn;t really figure out why. Well last night I did..my Crane adjustable vacuum advance canister was blown. All I got out of connecting it was a vacuum leak.

Just FYI.

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"American made baby. 100% American iron. The muscle among the masses. My hero. Yep, you can take your ergonomically designed, space age, computer controlled, 4 door, cup holding map lighted split double wishbone split fold down retractable cargo covered moon roof piece of transportation and keep it. For I have felt the thunder. And I know the difference!"
JSP Motorsports
ICON Motorsports

 

There wouldn't be any vacuum advance anyway in a drag race.

You went faster with the vacuum leak plugged? It's not running as lean as it was before. The engine likes more fuel. Go up another couple of jet sizes until the ET drops off.

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Follow my racing progress on Stephen's racing page
and check out the race car

87 IROC-Z Pro ET Bracket Race Car
383 stroker (carbed) with double hump cast iron heads and pump gas
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Best ET on a time slip: 11.857 altitude corrected to 11.163
Best MPH on a time slip: 117.87 altitude corrected to 126.10
Altitude corrected rear wheel HP based on power to weight ratio: 476.5
Best 60 foot: 1.662

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!
Member of the Calgary Drag Racing Association

87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley

 

Well thats why I was so surprised at the .4 sec et drop when I left the vac advance discinnected and plugged one time, couldn;t figure it out.

The jets are perfect for the engine..I tuned it before it developed that leak and go leaned out. Moot point anyway, That engine is on an old tire in the corner now


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"American made baby. 100% American iron. The muscle among the masses. My hero. Yep, you can take your ergonomically designed, space age, computer controlled, 4 door, cup holding map lighted split double wishbone split fold down retractable cargo covered moon roof piece of transportation and keep it. For I have felt the thunder. And I know the difference!"
JSP Motorsports
ICON Motorsports

 

sounds like you connect your vac-advance hose to ported vacuum instead of manifold vacuum. That method is nothing but tuning problems waiting to happen.
You DO get vacuum from a ported source in a drag race.. that's exactly why you shouldn't use ported vacuum.

ODB

 

If you have a vacuum gauge in the car you will find almost no vacuum at all under full throttle conditions, from either source. The two sources are the same after the throttle is open. If you have more than an inch or two of vacuum under full throttle your carb is too small.

Bear Tracks

 

ported vacuum comes from the carburetor venturi area. If there is no vacuum (pressure drop) in this area at full throttle, then how do you suppose fuel gets metered in via the booster? gravity maybe?

 

Ported vac is almost nothing at idle, and gets stronger as the throttle opens. Manifold vac is the other way around, right? So what I'm hearing is that manifold vac is for some reason better than ported vac for spark advance? Is this only in certain situations/uses/whatever, or should I switch mine over? Just wondering the benefits and drawbacks to both sources.

 

MikeS is exactly right.

manifold vacuum works much better in performance applications.

With manifold vacuum you get your vacuum advance under light loads, and less under heavy loads.... this is exactly what you want and the whole reason vacuum advance exists in the first place.

At full throttle the advance should be RPM based, so the mechanical advance is perfect for this and never varies.

ODB

 

I must still disagree and you can prove it with the afore mentioned vacuum gauge. When the throttle is opened slightly it uncovers the ported vacuum port. After that it is connected to the manifold directly. This has nothing to do with the vacuum produced in the venturies of the carb. Just hook a gauge to the proted vacumm port and another to a full time vacuum tap. The only diference should be at idle. Ported vacuum is used to retard the ignition timing at idle for emmissions reasons. There are vacuum signals internal to the carb that vary directly with air flow.

 

Goshdangit is there no documented correct place to hook up the vac advance? I had always heard ported, and it seems to work fine for me.

You want less advance at idle, with advance quickly being added in at light throttle right, more as you give it more and more throttle right up to max? Isn't that what ported vacuum would yield? I have no idea dangit, don't think my MSD distrib specified where to hook it up at...

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Ray87Z
-Vortec headed 350.
86 IROC w/ a cammed 305 TPI.
Formerly Ray86IROC.
www.inter-scape.com/Ray

 

Ported is the documented "correct" place.

Personally I also think it's the best place, but obviously opinions vary. There is no good reason to run 20-30 degrees of advance at idle..which is what you get with manifold vacuum..the most advance when you need it least.

Mechanical advance only adds 20 degrees, give or take depending on that set of wieghts. A typical base timing is 6-8 degrees. At WOT, 30-38 degrees of advance is typically desired. The extra 4-10 degrees is designed to come from vacuum connected to a ported source, which you don't get with manifold vacuum. What you do get with manifold is pinging at part throttle, not enough advance at WOT, and extra wear on your starter.

------------------
"American made baby. 100% American iron. The muscle among the masses. My hero. Yep, you can take your ergonomically designed, space age, computer controlled, 4 door, cup holding map lighted split double wishbone split fold down retractable cargo covered moon roof piece of transportation and keep it. For I have felt the thunder. And I know the difference!"
JSP Motorsports
ICON Motorsports

 

.



[This message has been edited by The ODB (edited May 03, 2001).]

 

It's possible. would you care to elaborate?

 

Is it better to run the vac. advance on the distributor off of the carb. or the manifold vacuum?

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---Grady---
1984 Berlinetta.
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Daily Driver.

Personal Quote: "The Rice-Boys all tremble with fear when the old American V8 draws near."

 

quote "There is no good reason to run 20-30 degrees of advance at idle..which is what you get with manifold vacuum..the most advance when you need it least."

**
*The air density inside the intake manifold is only 1/10 the density of intake air at full throttle..
*Airflow in CFM is only a tiny fraction at idle because the throttle blades are almost closed and present an extreme restriction.
*There is almost no ramming or hemholtz effect at idle speeds which greatly reduces combustion charge density even further.
*Slow piston speed at idle does little for charge mixing or swirl which makes the burn slower and less efficient.

Considering only these factors, it should be obvious that the air-fuel charge will be burning very slowly and cool at idle. The cure for this is to begin ignition very early in the cycle to ensure that more of the fuel is burned efficiently. This means running more timing advance... ala vacuum advance. TPI cars run with 6 or 10 degrees of initail timing, but do you think that's all there is when at idle? No, there is usually between 30 and 45 degrees of advance at idle thanks to the ECM adding the necessary advance for idle and low-load conditions like cruise and decelleration.
Manifold vacuum is a great indicator of engine loading. When vacuum is high, the intake charge burns much slower and needs more advance. When vacuum is low, the extra advance from the vacuum advance goes away and advance is then based only no engine RPM thanks to centrifugal advance. This is how things should be and is how they work best.

Higher RPMs require more advance because fast piston speeds leave less time for the air-fuel mixture to burn. In case some of you do not realize this, mechanical or centrifugal advance is also adjustable. You could run as little or as much (within reason) mechanical advance as you want by simply modifying the spring & weight mechanism. There is usually bushings, stops, or slots that control the total amount of centrifugal advance a given distributor will give you.

Adding advance via vacuum advance based simply on throttle openning is what makes no sense. I nor anyone I know and race with can think of any good argument for running ported vacuum advance over manifold vacuum advance. The only case I could think of that I would ever run from ported vacuum is if I bought the car that way and performance was not a concern.

Manifold vacuum advance is a very good thing for smooth idle and better fuel mileage, and has no effect at all on WOT performance. It allows you to run much lower initial advance for very easy starting.
It also allows you to run a slower centrifugal advance curve to keep you out of spark-knock & detonation. It does this because your extra spark advance goes away under loads ( when spark knock & detonation happen) and returns when the load is gone. You can then adjust your centrifugal advance to come in when you want it (RPM wise) and give you whatever total timing you desire.

ODB




[This message has been edited by The ODB (edited May 03, 2001).]

 

I emailed the MSD techs about it (since I have a MSD Pro billet). They stated: "Generally you want to go to ported vacuum. This way the timing is only affected off idle." Not too talkative but I guess that's the answer...

------------------
Ray87Z
-Vortec headed 350.
86 IROC w/ a cammed 305 TPI.
Formerly Ray86IROC.
www.inter-scape.com/Ray

[This message has been edited by Ray87Z (edited May 03, 2001).]