speedlvr

I was reading a long string of posts going back and forth - some said ported, some said manifold, some said matter of preference. I wrote Edelbrock because I have one. Here it is (His response first).

Yes.
I would connect it from one side to the other and see which one makes the engine perform the best.
And then leave it in the one that works best.

-----Original Message-----
From: speedlvr
Sent: Wednesday, October 17, 2001 8:42 PM
To: edelbrock@edelbrock.com
Subject: vacuum advance timing


I just bought a 750 cfm model 1407 Edelbrock and I need to know if I should use ported or manifold vacuum for the timing advance. Can any advantages be had by using one or the other?

Thanks,
Speedlvr

According to him, either one can work.

CamaroMike

Ported Vacuum is generally for emissions only.

Full Vacuum Advance will increase throttle response, improve mileage, and improve idle quality! Other than that I don't know why anyone would use it, HEHE!

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84 Camaro ZZ4 with HOT cam. Ported Heads, Installed Larger Race-Flo Valves, RPM Air Gap Intake, and 1 3/4" SLP Headers (Best 60',Quarter Mile, MPH) (1.88, 12.91, 106.77)
Other Mods: You name it and I have probably changed it.

2001 Dodge Ram Quad Cab 5.9L 4*2

The Bowtie
ASE Certified Auto Tech
LIVE AND DIE BY THE ALMIGHTY BOWTIE!

speedlvr

!

Damon

If you've got a lumpy cam with marginal manifold vacuum use of a ported vacuum source will help improve idle stability. If the vacuum advance can not be pulled FULLY on by maifold vacuum you will never get a stable idle. The idle speed will wander as vacuum fluctuations alter the timing and therefore, idle speed.

82camaro

Using manifold vacuum will give you full advance when the engine is not under load. Under full throttle(near zero vacuum) the advance will be gone. Ported vacuum will give you extra advance at part throttle driving--emissions, fuel economy, etc... Run ported vacuum advance, or don't use vacuum advance at all is what I would do.


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350 with stealth intake, holley carb, 470 lift cam. 700r4 with .5 boost valve, vette servo, tci lock-up kit, B&M megashifter. Richmond 3.73 gears, powertrax locker, timkin bearings, synthetic lube. Custom 3 inch single into 2 2.5 pipes. 1 1/2 drop springs, 1 5/16 solid front sway bar, 1 inch rear bar, custom subframe connectors, custom LCA relocation brackets. Kobel ground FX, currant red metallic paint. Lots of other stuff...
82camaro

fast_broker

iTrader: (1)

Manifold vacuum will always be higher than ported vacuum. End of story. You will get better mileage w/manifold vacuum, especially at lower throttle openings, where you will be on the highway.

82z

82 camaro is right. Ported vacuum comes from a hole above the throttle plates when they are in the idle position. At speeds just above idle the throttle plates move up and expose the ported vacuum port to manifold vacuum. Ported vacuum exists only off idle. Opening the throttle plates further will decrease the "ported vacuum". I see no reason to connect a vacuum advance canister to manifold vacuum under any circumstances. If you want more advance at idle just increase the base timing. Remember manifold vacuum decreases with throttle opening which would decresase your advance as engine speed decreases. This is the opposite of what you want to happen. Ported vacuum will increase advance under part throttle conditions to increase driveability and, mileage and decrease emissions. fast_broker, I'm sorry but you have no idea what you are talking about.

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82 z28 350cid, vortec heads, comp 262h cam, Holley 600cfm carb, 2in twice pipes, MSD ignition, turbo 350 trans, 3.73 posi, manly b&m megashifter

fast_broker

iTrader: (1)

ported vacuum and manifold vacuum are the same once the plates are open past the ported vac hole... There is no ported vacuum at low throttle setting as the hole is above the plates and tus, the hole ses no vacuum. Not brain surgery. I think you are the one you knows not what you speak of...

Run manifold vacuum on engines with large cams that have relatively low vacuum signals at idle. The EGR effect of a heavy overlap cam demands the extra idle advance, or the engine will not run peroperly! Proper vac advance should only be 10-12 deg anyway, not the 20deg of an OEM can, if you have enough initial advance, so adding full vac advance at idle is no big deal. Unless you run an OEM girly-cam. If you are talking OEM stuff, doesn't matter much what you use. Whatever feels good. Edelbrock 2102 Performer cam is mellow, If that is your cam, use whatever makes you feel good, as suggested... If you have the RPM cam, I guarantee you will be happier with manifold vacuum in a properly setup distributor. An OEM HEI is not a properly setup distributor, BTW...


EDIT: to address what you said, yes, the ported vacuum does increase (to MANIFOLD VACUUM LEVELS!) at part throttle setting BECAUSE IT WAS AT ZERO at idle! You said you want it to increase but manifold vacuum is already there... Ported vacuum does NOT increase to a value that is HIGHER THAN MANIFOLD vacuum, ever. ie, you essentially get manifold vacuum from the ported vacuum port once the throttle is cracked beyond the ported hole, or ported = manifold vacuum above part throttle. So, you get the same drivability, mileage and emmissions from either port from part throttle up to WOT. It is the idle/very small throttle opening advance that is under scrutiny here. This effect emmission, drivability, overall idle quality and throttle response.

Mellow cams sometimes desire ported (ie, NO) vacuum advance because at idle, emmissions are reduced vs manifold advance...

Don't beleive me? Simply run two vacuum gages in the cab, one to each fitting, portd and manifold. Then you'll be a believer...



[This message has been edited by fast_broker (edited October 19, 2001).]

CamaroMike

Fast Broker........opens a can of whoop *** ! Could not have said it better myself.

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84 Camaro ZZ4 with HOT cam. Ported Heads, Installed Larger Race-Flo Valves, RPM Air Gap Intake, and 1 3/4" SLP Headers (Best 60',Quarter Mile, MPH) (1.88, 12.91, 106.77)
Other Mods: You name it and I have probably changed it.

2001 Dodge Ram Quad Cab 5.9L 4*2

The Bowtie
ASE Certified Auto Tech
LIVE AND DIE BY THE ALMIGHTY BOWTIE!

fast_broker

iTrader: (1)

Just the facts... Only the facts, Maam...

Ray87Z

The MSD Tech told me to use ported vacuum on my Pro-billet distrib's vac advance. My idle speed shoots up to the damn moon if I plug it into mani vacuum. Why would you want all the vacuum advance just sitting at idle anyway? Makes more sense to get it at light/part throttle ala ported...

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

fast_broker

iTrader: (1)

DEpends on the cam, heads, intake and exhaust and stall speed combo... If you like it better, use ported.

How big is your cam?

EDIT: FYI, If you use manifold and your idle shoots up, just re-adjust your carb idle lower and try it for a while. Just see if you like ported or manifold. As Belushi said, "Don't cost nuthin'".

You only need it if you and or your combo like/need it.

Also, how much advance do you have in your canister??? It is adjustable for rate and overall advance...


[This message has been edited by fast_broker (edited October 22, 2001).]

Jester

Your combo doesn't matter. Ported vacuum is put into every carb for a reason. If advance was meant to be that high at low speeds..then all distributers would have locked out advance, and everyone would run 35-45 degrees at all RPM.

Unless you have a trailored drag car where part throttle doesn't matter...it's not supposed to be that high at idle. The advance curve is supposed to look like a slope, not a U. There would be no reason for advance wieghts/springs...no reason for computer timing in later cars.

In other words....run ported

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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

[This message has been edited by Jester (edited October 22, 2001).]

fast_broker

iTrader: (1)

I use manifold advance and only have 20 degrees TOTAL advance at idle, which is not much. My TBI even has an OEM of 17deg at idle...

Not sure what you guys have for distributors but if you use OEM curves/plates/weights/springs/canisters, there is better stuff out there. If you have a 20 degree canister, that is TOOOOOOOOOOOOOOO much vac advance for any engine, if you have worked on your spring/weight/plate combo... Plain and simple.

In other words, when you figure out what you are doing, run MANIFOLD! Try it, you'll like it!! I believe that there is even an HEI curve tech article in the main pages of this sight... Go check there for more info on peroper advance. It's not the best article, but not bad...

https://www.thirdgen.org/newdesign/tech/hei.shtml

One thing to note, auto tranny vehicles won't mind MANIFOLD vacuum advance while a standard operated at low rpm in gear can cause problems. then, mellow cam may demand Ported.

IMO, initial advance of 10-14, centrigugal advance of 20-24 and vacuum advance of 10-14 works best. My distributor almost always have 20-22 degrees of TOTAL advance at idle, which I consider quite low actually. An OEM HEI hooked to vac advance will have a MINIMUM of 32 deg advance and up to 38!, which is obviously too high.

32-38 degrees is all you need without vac advance connected. ie, WOT total advance.

[This message has been edited by fast_broker (edited October 22, 2001).]

Ed Maher

I use manifold vacuum in my 92 to determine total timing, and it likes having extra advance at idle.
TPI or not, in the PROM or in a can on the distributor it's the same principle. With practically no cylinder pressure present at idle you run more timing to make up for the lower cylinder pressure. As vacuum in the intake goes down, cylinder pressure rises, and less timing is desired.

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Ed Maher - Moderator @ The TPI & Carb Boards
92 Z28 Convertible - Quasar blue / Tan top
305 TPI A4 2.73 - 14.7 @ 93.6
Stock except ported plenum and dual cats
-=ICON Motorsports=-

- Definitely prototypes, high powered mutants of some kind. Too weird to live, too cool to die