[y2k600f4]

I have been reading several articles and they mention using either ported or full vacuum to obtain distributer vacuum advance depending on application. I know that at cruise and WOT they are the same but differ at idle/off idle.

I am starting a final tune of my recent 383 10:5-1 compression rebuild. The engine builder did a rough tune, set the initial timing at 10-11deg and total mech is 31deg, all in I think around 2500 RPM. Right now I am idleing around 800 RPM (4 spd). Currently the vacuum advance is disconnected but previously was connected to the ported vacuum. I am targeting economy and driveability.

Seems like running pretty good except some exhaust popping at idle/off idle and engine temps seem to be running 10-20 deg hotter than previously. I am thinking maybe I am running too lean at idle/off idle, timing retarded causing the backfiring and slight increase in temps ??? Should I try full maniflold vacuum for my vacuum advance ?

[willys36@aol.com]

Manifold vacuum in most cases is best for street engine drive-ability. Ported vacuum is for emission controlled engines. See this discussion.

[y2k600f4]

Manifold vacuum in most cases is best for street engine drive-ability. Ported vacuum is for emission controlled engines. See this discussion.

Thanks for that link !! Very informative, I read all the posts and it "appears" that for a modified engine that is not concerned with emmissions that manifold vacuum is the consensus and the way to go for my application. Should solve my problems.... Now I need to be concerned if my vacuum advance can on my dizzy will work for my application.

Anybody use a procomp distributer PC-6001-D ? I am 99% sure that is my dizzy and that the vacuum advance can be adjusted, not sure what the range is. If not will any of these vacuum advance canisters fit on this dizzy ?

[F-BIRD'88]

What camshaft is in this motor?

[y2k600f4]

What camshaft is in this motor?

Comp XE262H

[curtis73]

I agree with most of what is in that link, but I don't think that ported vacuum is as "emissions based" as most think it is.

You're right that at part throttle and WOT both vacuum sources are the same, but depending on the carburetor, where those two signals become the same can be very different. Some carbs are the same almost immediately off-idle, others you have to get up to 15% or more throttle before they're actually the same. The only way to really know is to hook up two vacuum gauges. The other thing to note is that manifold-operated cans have much heavier springs, so regardless of the fact that you're comparing identical vacuum signals at part throttle, the manifold can will provide less advance per inch of vacuum.

I ran ported on one of my street 454 builds, but it was a bit specific. It was a moderately cammed, low compression motor. I locked the mechanical at 34* then used ported vacuum to add more at cruise. It was a good recipe for that motor, but most people don't build engines that way. Most of the builds I do are strictly street, high torque engines, so I like the ported because I don't need insane amounts of idle advance. Using manifold vacuum is nice for lumpy-cam engines with lower cylinder pressures at idle since it adds full advance at idle, then backs off during loaded operation. Ported can be used for stock-type engines, or anywhere that the idle cylinder pressures are higher.

I might suggest ported for you. If you have a good satisfactory idle without the vac advance, and you're looking for fuel economy, the ported source would add more advance at cruise than a manifold source - because of the lighter springs in the can. I suggest you get an adjustable can so you can fine tune it, and that also means that you can adjust it to work with either source.

[F-BIRD'88]

I would bump the timing up to 34deg total (14inital base) and install slightly stiffer (medium) advance springs so that max advance does not occur until 3000-3200rpm.
You mechanical advance is a little too agressive.
The get a replacement adjustable (Crane, Mr Gasket) vacuum advance and dial it in for around 12-15deg (stop limit) and play with the vac adv rate in and out by test driving. (diaphram spring tension) I prefer ported vacuum source. Try both, and use what works for you. There are no hard and fast rules for vacuum advance.

Use good fuel. Your PC distributor is a GM HEI knock off. use common HEI type distributor replacement parts.
OEM GM HEI modules are fine. MSD makes a good HEI coil, cap and rotor. Crane and MR. Gasket both have replacement HEI adjustable vac advance w/limit plate included.

You get what you pay for.

[y2k600f4]

Thanks for the tips. The engine builder actually set up the intitial timing and said that was all I needed for the street. During the cam break in he disconnected the vacuum advance (was connected to the ported vacuum) said the timing was way too high and needed to be adjusted, but for now it was ok to run w/o vacuum advance. I will inquire more on why he set the timing so low and look into bumping it up.

As far as the PC dizzy...it was in the car when I got it and basically is new but taking a closer look at it it either it is not a PC or the vacuum advance can was swapped out and not sure if it is adjustable with an allen wrench ? Prior to the rebuild the engine with a bigger cam was set at 28deg total mech advance + vacuum advance was connected to ported vacuum of my Holley 4160 750 cfm 0-3310-11 Wondering why timing so far retarded ? Lots to consider and experiment with....thanks again.

[Frisco]

Originally Posted by y2k600f4

I am starting a final tune of my recent 383 10:5-1 compression rebuild. The engine builder did a rough tune, set the initial timing at 10-11deg and total mech is 31deg, all in I think around 2500 RPM. Right now I am idleing around 800 RPM (4 spd). Currently the vacuum advance is disconnected but previously was connected to the ported vacuum. I am targeting economy and driveability.

Seems like running pretty good except some exhaust popping at idle/off idle and engine temps seem to be running 10-20 deg hotter than previously. I am thinking maybe I am running too lean at idle/off idle, timing retarded causing the backfiring and slight increase in temps ??? Should I try full maniflold vacuum for my vacuum advance ?

Your temp increase and the popping is more than likely due to the timing being somewhat retarded based on what you have posted. Look to see if your exhaust pipes close to the headers aren't 'glowing' cherry red. This is a sure sign of retarded timing (or an extremely lean mixture).

With the vacuum advance dis-connected from the vacuum cannister and plugged off, set your Total mechanical timing so that it is "all in" to 34-38 degrees. You are looking for this to occur around 2500 RPM. Swapping the advance springs will enable you to get it in the 2500 RPM range. Usually a medium and a light spring will do the job.

Now re-connect the vacuum advance hose and have it coming from a FULL manifold vacuum source. Your idle RPM will increase. Re-adjust it at the throttle stop screw back down to the RPM you previously were running. Test drive. When going up a slight grade in high gear at a moderate steady speed you should not have any pinging. If you do, lower the timing 2 degrees and re-test. You are looking for the maximum amount of timing with no pinging.

Your temps should drop and your performance should increase.

While some folks like using ported vacuum, it was originally incorporated by the manufacturers to be able to run the initial timing lower to increase idle temps so that they could meet Federal emission guidelines. This worked for them at idle RPM's and severely decreased performance.

[y2k600f4]

I removed my vacuum advance canister, which was adjustable but seems like it was broken (if not defineately is now )

Anyway I need a replacement adjustable canister so I can get my vacuum advance working (gas mileage is terrible) and would like to get closer to the final tune.

I have read that the one to get for GM HEI is the crane 99600-1, which has the stop limit cam (also hard to find). Is this the only one that has something to limit the vacuum advance ? I was considering the acell (mr.gasket) 31035 that claims to be adjustable for amount of vacuum advance and rate of advance but looks like there is no stop limit mechanism. What about the generics (proform, procomp etc) that claim to be adjustable and are 1/3 the price (probably close to what I had on my distributer).

[ralc]

There are actually 3 different vacuum advance modes. Ported venturi vacuum. Responds to engine rpm Used on some emission engines.
Ported manifold vacuum Vacuum blocked at idle to keep rpm low Straight manifold vacuum-gives full vacuum advance at idle (depending on the canister.)
Some cars use a thermal switch to switch from ported to straight manifold at idle to increase rpm when engine gets too warm

[327NUT]

Ralc...good info but the last post was 2 years ago, you probably won't get a response from the OP. We have all made the error of replying to old threads...no biggy.

[lg1969]

Originally Posted by ralc

There are actually 3 different vacuum advance modes. Ported venturi vacuum. Responds to engine rpm Used on some emission engines.
Ported manifold vacuum Vacuum blocked at idle to keep rpm low Straight manifold vacuum-gives full vacuum advance at idle (depending on the canister.)
Some cars use a thermal switch to switch from ported to straight manifold at idle to increase rpm when engine gets too warm

There is really 4 different types of advance and that is full mechnical advance. No vacuum advance at all. There are times an engine has a radical cam where the vacuum is so low it will not give you enough advance when you apply gas off idle.

[oldbogie]

Originally Posted by y2k600f4

I have been reading several articles and they mention using either ported or full vacuum to obtain distributer vacuum advance depending on application. I know that at cruise and WOT they are the same but differ at idle/off idle.

I am starting a final tune of my recent 383 10:5-1 compression rebuild. The engine builder did a rough tune, set the initial timing at 10-11deg and total mech is 31deg, all in I think around 2500 RPM. Right now I am idleing around 800 RPM (4 spd). Currently the vacuum advance is disconnected but previously was connected to the ported vacuum. I am targeting economy and driveability.

Seems like running pretty good except some exhaust popping at idle/off idle and engine temps seem to be running 10-20 deg hotter than previously. I am thinking maybe I am running too lean at idle/off idle, timing retarded causing the backfiring and slight increase in temps ??? Should I try full maniflold vacuum for my vacuum advance ?

The cam and compression are the principle dictators of the ignition timing. The head and piston shapes also are big players. The SBC with a bit of cam and high compression likes more on the order of 34-36 degrees total.

Popping is usually associated with a lean mixture which is hard to light and burns slowly which is why the popping in the exhaust as combustion is late and still burning when the valve opens. A lack of enough advance will also play this way for the same reasons as a late mixture, that is the burn either didn't occur or is still happening when the valve opens. The XE262 cam is just on the edge where it likes to blow a lot of mixture back into the intake at low speeds which makes the same sounds for the same reasons. Adding fuel and or raising the advance should help as does a multi-spark module.


Tuning is finding out what the engine likes, when it comes to ignition timing with vacuum advance you can play some games. Generally as the cam gets bigger the engine will like more initial advance. This can make starting difficult so this gets to be a place where you can play with using the ported or non-ported advance. Using a bit less base can make the engine easier to crank then use unported vacuum to pull the advance up once it's running to get a smooth idle. Or you can tip in more base than use the ported vacuum to delay bringing the advance too far up too soon. Gears and tranny play into this as well. Stiff gears and or a manual gear box make this an easier transition. High gears out back will always be a problem at lower speeds as they encourage the engine to bog.

You have to careful when running high compression not to get too much advance too early. The base and vacuum are additive to each other, if the centrifugal comes up faster than the vacuum goes down this will also be additive, perhaps too much so. If you get pinging ,the advances can be adjusted especially the centrifugal which can be adjusted for both rate and amount. Different gears will like different amounts, lower gears will tolerate more advance put in quicker than the higher gears. For a non-computer system you have to find the worst case and dial in under that limit, the limit will be less than adequate in the stiffer gears, that's just the way it is without a black box that an be tuned to engine load.

Bogie

[cobalt327]

If this nearly 2-1/2 year old thread keeps coming up in searches, I'd suggest THIS thread.