Thread: best vacuum ports for dist. advance and trany modulator

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I am proceding with the installation of my 1985 Rochester quadrajet carbutator, and I am wondering what vacuum ports are the best to use for the distributor vacuum advance and for the turbo 350 modulator valve. I pulled the engine out of the donor car about 8 months ago and didn't check at the time. I have blocked all vacuum ports except for the large port at the rear of the carb near the base, which originally supplied vacuum to the power brake booster.---Is that a good choice for the HEI distributor vacuum advance line? There is a smaller vacuum port right beside that large one, which I could use for my transmission modulator valve vacuum line. Any help would be appreciated.

Brian,

Most HEI distributors require ported vacuum - that is, a port above the throttle blades. With the engine at idle, find a port that has zero (or practically zero) and hook your vacuum advance line to that one.

Find another port that will suck the chrome off of a trailer hitch and attach your modulator line to that one.

Hi Denny---Thanks a million for the marked up quadrajet pictures----just one thing---I am running a stock 1985 HEI distributor, and I think I know what Henry Rifle is talking about in terms of "ported vacuum" and wanting a port above the throttle plates for the vacuum advance on the distributor. Only thing is, the port which you point out for "vac advance" on the mark up is below the throttle plates. Can you please clarify that point for me?

........ Then get the trailer hitch re-chromed or painted soz' it doesn't rust on ya'.


I know ........................ that joke sux.

Brian,

Are you sure the INLET to that port is below the throttle plates? Those passages wander around inside the casting.

Originally posted by Henry Rifle
Brian,

Most HEI distributors require ported vacuum - that is, a port above the throttle blades. With the engine at idle, find a port that has zero (or practically zero) and hook your vacuum advance line to that one.

Find another port that will suck the chrome off of a trailer hitch and attach your modulator line to that one.

henry, I'm not sure you're right when you say most hei dist. run ported vac. i haven't read anything on this in about 25 yrs, but I've set the timing on a lot of cars in that time. you have to pull the vac. hose off of the vac. adv. before you set the timing, so the timing can retard back to base timing. if it was ported vac. it wouldn't make any dif. when you pulled the hose off. 9 out of 10 gm cars idle down when you pull the hose off of the vac. adv. and all you are doing is changing the timing im talking stock setting. that my story and im sticking to it. help denny where are you at?

Originally posted by DennyW
Here I am. I agree on this one. Unless your really built up, and not close to stock, If your camed over the top, the rpms' are so high, you need to set total advance settings, but, close to stock , you wanted a vac, suck your face off vac port, hahaha, then pull the hose off, set your timing, and re-install. One other nice tip, and it's so simple, the basic rule for timing, especially if you don't knoe!!, Warm the engine up, make sure the idle is not to high, hook up a vac gauge to the full vac port. Adjust the distributor to the highest vac setting,(you'll know when you went to far, cause it will start to drop, and the engine juggle a little,) Then back off 2". So, if you get a high of 20" reading, back down to 18". Most of the time you will be almost right on the time mark. The important thing is the breathing of the engine will be set. Hope this helps. I've seen the HEI both ways on some vehicles. I think Olds was like that. As far as I know.

thanks danny i agree

O.K. Denny, that settles it---I will use the port which you pointed out in the marked up picture for the port to run my HEI vacuum advance. I will use the small port at the rear of the carb beside the power brake vacuum port to supply vacuum to the transmission modulator valve, and I think that I will use the vacuum brake port to supply vacuum to a pcv valve from the passenger side valve cover. Thanks a million guys, I,m going out to the garage now to install the carb.

This vacuum thing goes round and round, doesn't it?

I just pulled the spec sheet for my Chevy crate motor, a Fast-burn 385.

Set spark timing at 32 degrees BTDC at 4000 RPM with the vacuum advance line to the distributor disconnected and plugged. This setting will produce 32 degrees of total advance at WOT. The HEI vacuum advance canister should be connected to a ported vacuum source (no vacuum at idle).

Also, check this article:

http://chevyhiperformance.com/howto/97438/

Which, in part, says this:

Vacuum canisters advance according to engine vacuum, which is why they must be connected to a ported vacuum source. Ported vacuum is drawn from just above the throttle blades to make sure the vacuum canister does not advance at idle.

The vacuum advance on HEI distributors is meant to control part-throttle timing, not initial or WOT timing. As you come off idle, you get max ported vacuum, and your engine timing jumps way up. As you go to WOT, ported vacuum (and vacuum advance) drops off as mechanical advance comes in. At WOT, you have full mechanical advance and no vacuum advance. That's the way the advance curves are designed.

Can you hook the distibutor to manifold vacuum? Sure, but the timing curves go wacky.

Is this true for ALL HEI distributors? I honestly don't know - but for most of them out there today, it is.

Originally posted by Henry Rifle
This vacuum thing goes round and round, doesn't it?

I just pulled the spec sheet for my Chevy crate motor, a Fast-burn 385. Also, check this article:

http://chevyhiperformance.com/howto/97438/

Which, in part, says this:The vacuum advance on HEI distributors is meant to control part-throttle timing, not initial or WOT timing. As you come off idle, you get max ported vacuum, and your engine timing jumps way up. As you go to WOT, ported vacuum (and vacuum advance) drops off as mechanical advance comes in. At WOT, you have full mechanical advance and no vacuum advance. That's the way the advance curves are designed.

Can you hook the distibutor to manifold vacuum? Sure, but the timing curves go wacky.

Is this true for ALL HEI distributors? I honestly don't know - but for most of them out there today, it is.

henry, this vac. thing shouldn't keep going around and around. this thread spilled over from a thread from yesterday in which the guy wanted to know what vac. line to use. it was based on a old q-jet, carb, 1985 motor and dist. and what he was told is the setting he needs. I've all ways been talking stock. not a fast-burn 385. if i had a fast-burn 385 i probably would do something dif. but that was not the ?

henry, this vac. thing shouldn't keep going around and around. this thread spilled over from a thread from yesterday in which the guy wanted to know what vac. line to use. it was based on a old q-jet, carb, 1985 motor and dist. and what he was told is the setting he needs. [i]I've all ways been talking stock. not a fast-burn 385. [/] if i had a fast-burn 385 i probably would do something dif. but that was not the ?

The fast-burn 385 is just another Chevy small block engine. There's no reason why a HEI distributor in this engine would perform any differently than a in stock one. Also, the article I posted referred to Chevy engines in general, stock engines included.

Your approach is not wrong, and neither is mine. If you use manifold vacuum, you get full advance at idle. Not necessarily a bad thing. It increases idle speed a little, but you can adjust it back.

Ported vacuum also works, it's Chevy's general recommendation, and often gives better response in the mid-range rpms.

Neither method makes a whit of difference at WOT.

I gave Brian the info that has worked best for me. You gave him what works best for you. My advice is that he try both and see what works better.

Not arguing - just providing information . . .

From Car Craft Magazine:

Choose Your Vacuum

Where’s the right vacuum port to connect to the vacuum advance, you ask? Most street carbs are equipped with both ported vacuum ports and manifold vacuum ports. Vacuum is generated by the engine’s pistons traveling to the bottom of the cylinders—air is sucked in to fill the voids, which in turn creates a signal to the carburetor to meter fuel. Manifold vacuum is present anytime the throttle blades aren’t fully open, including idle when they’re nearly closed. Ported vacuum isn’t “seen” at all when the engine is idling, because it’s taken from a port above the throttle blades.

Some gearheads have near-religious views on ported versus manifold vacuum. (Ain't that the truth) Patterson connects the vacuum advance to either source, and chooses the “right” port after test-driving both hookups. He does say that full manifold vacuum should help tip-in response, and this setting made Smith’s Mustang feel crisper off-idle. But if your car idles shaky and “nervously” with manifold vacuum advance, it’s a sign that there’s too much initial advance. So give the ported vacuum a try.

From MSD's Chevy Distributor instructions:

Total Timing: This is the total of the initial timing plus the centrifugal advance added together. Example: 10° Initial + 25° centrifugal = 35° Total Timing. (When checking Total timing, disconnect the vacuum canister and plug the vacuum source.) Vacuum Advance: The vacuum advance will advance the timing up to 10° during partial throttle driving (with 15 lbs of vacuum). The vacuum line should be routed to a ported vacuum outlet above the throttle plates.

From Mallory's instructions:

ADJUSTING THE VACUUM ADVANCE
The vacuum chamber is adjusted at the factory to produce 12° of additional advance at steady speeds. This may be too much for some engines. Connect a vacuum line from the vacuum chamber to a ported vacuum source on the carburetor.