full manifold vacuum advance help

[m.lander]

I have a accell 59107 hei distributor and want to switch to full manifold advance. i understand changes have to be made to do successfully make the change such as to screw the adjustable can in or out. the manual that came with the distributor says if you rotate the screw counter clockwise till it bottoms out then turn the screw 4 full rotations clockwise it will give you 7" dist. +- at 12" vacuum +-. with this being said how many full turns clockwise would i roughly want to turn the adjustment screw to run full manifold vacuum. also, what other changes would need to be made. i know this has been discussed a lot and trust me i have been reading on this subject for months and i cannot grasp the concept. thanks for any help in a less confusing way. all of my attempts have resulted in poor idle and just general failure to make the change.

[68gmsee]

I'm not familiar with the Accel HEI distributor, but I did have some questions?

Is the reason you're wanting to do this because of idling problems?

Did you check to see how it idles with the vacuum lines plugged up?

What have you done in the area of tuning (new or old plugs or cables, carb adjustments, etc.)

Sometimes idling problems are caused by other components, such as bad or worn valves, vacuum leaks, carb adjusments, flat cam lobes, bad gas or just defective or dirty ignition components.

[m.lander]

idling is not a problem. all electrical components(and most everything else on truck) is new. I was mainly looking for the performance upgrade and better mileage folks claim.

[Wrenchbender Ret]

You don,t need to change the setting for full manifold vacuum. The adjustment you mentioned sounds about right to begin with. You may want the advance all in at 8-10 in. of vacuum. It,s all pretty much experamental. Every engine is different. If it pings at light throttle or runs rough running free at about 2000 rpm it is too much. Sometimes with an auto trans. itys better to run it on ported vacuum. If it slams when you put it drive or reverse or slows the engine down too much then that is the case. It,s a lot of trial error.

[LONGHAIR]

Originally Posted by Wrenchbender Ret

Sometimes with an auto trans. itys better to run it on ported vacuum. If it slams when you put it drive or reverse or slows the engine down too much then that is the case. It,s a lot of trial error.

I have to disagree on this one...It is never better to run "ported vacuum" to your distributor.....ever. IMHO, the only time to run one that way is if it was originally set-up for ported...and you are not inclined to mess with it. It is not better, but it is the easiest.

If it "slams" when you put it in drive...your idle is too high. If it has to idle that fast in neutral to stay running in gear, your converter is too tight.

In general you should be looking to get about 10degrees initial (w/ vac plugged) and about 24 more from the centrifugal. You should also get around 10-12 more from the vacuum advance unit.
I am not familiar with that particular distributor though as far as the adjustments/limitations. I assume that the vacuum canister is adjustable for the vacuum amount and the actual degree of advance?

[airdale94]

Originally Posted by LONGHAIR

I have to disagree on this one...It is never better to run "ported vacuum" to your distributor.....ever. IMHO, the only time to run one that way is if it was originally set-up for ported...and you are not inclined to mess with it. It is not better, but it is the easiest.

If it "slams" when you put it in drive...your idle is too high. If it has to idle that fast in neutral to stay running in gear, your converter is too tight.

In general you should be looking to get about 10degrees initial (w/ vac plugged) and about 24 more from the centrifugal. You should also get around 10-12 more from the vacuum advance unit.
I am not familiar with that particular distributor though as far as the adjustments/limitations. I assume that the vacuum canister is adjustable for the vacuum amount and the actual degree of advance?

Not trying to start anything here, I see you are staff, and I'm not a subscbribing member but you are not accurate with your information. GM HEI's are always hooked up to ported vaccum.

There's always been a big debate in the performance community about which is better ported or full vaccum for advance, but all vaccum advances have been hooked to ported vaccum since the mid 70's emmissions restrictions came out.

As for "m.lander"s question, There's an excellent write up on timing/advance in the FAQ section. The adjustable vaccum adv. pod is a way to add more advance. The board seems to agree for small blocks 12 at idle (vaccum advance hose plugged) and 30 total, all in at the 3000 RPM range.

If want to experiment I would set the adjustable vac. at the intiall recommended spec. from Accel. Then set your 12 at idle. Then set your weights and springs. Then adjust your vaccum advance.

It's not really for the novice but not that hard either. I would stick with recommended settings, and if your having a drivabilty issue look elsewhere for the problem.

[LONGHAIR]

Originally Posted by airdale94

Not trying to start anything here, I see you are staff, and I'm not a subscbribing member but you are not accurate with your information. GM HEI's are always hooked up to ported vaccum.

There's always been a big debate in the performance community about which is better ported or full vaccum for advance, but all vaccum advances have been hooked to ported vaccum since the mid 70's emmissions restrictions came out.

"Staff" does not make me automatically right, nor does the lack of being a subscribing member take away your right to express your opinion.

That said, "Just because that's what the factory did" in the Smog Era, does NOT make it right. Ported Vacuum serves no good purpose....unless you are talking about a totally stock truck, that you have no intention of improving. You may run into a pinging issue if you just swap from ported to full vacuum w/o any other modification.
I figure that a guy with a "fully adjustable" (who asked about tuning it) is not concerned with that screwed-up emissions timing mess.....
Your engine will idle better, have better throttle response and get better fuel mileage from running full vacuum to the distributor.

[byrd]

My 350 with 65K HEI is running off ported vacuum. I know it has 15deg initial and 32 total timing. Now what all was done I am not sure. I just couldn't get it right and took it to my local Drag shop(Street and Strip performance) When I took the truck in, it was on full vac,8 deg and running rough(mostly carb was wrong) Anyway when I get it back they had changed the jets on the carb(edel 600) move the vac advance to ported, and bumped up the timing and installed NGK plugs.
Now I really do not understand carbs and timing real well, but I DO KNOW I was able to feel at least 20-30 HP gains when I picked it up, and picked up about 1/2 second off my 0-60 times. I will have some 1/8 mile truck times to back this up in a few weeks.All the while getting 14 MPG @ 75 MPH with no OD, just a 350th and 3:73's, I verified the MPG on my trip to the national board meet.

I am not trying to stir the pot, just letting you guys know how my truck is set up.

[ChevLoRay]

I installed an ACCEL HEI distributor in my truck and it was a replacement for the GM HEI that died. I used the same vacuum as the GM distributor. There were no issues with it. Idle speed, acceleration, all stayed the same, but it started RIGHT NOW! I do have the high output coil that ACCEL sells and run AC Rapidfire plugs. I have not had any servicablility or driveability issues since it was installed. I have no electrical kickdown on the TH400, but modified the valve body and I'm running the smallest vacuum modulator (black) that I could get. It will do a part-throttle kickdown, but I still need the electrical kickdown to work to be completely satisfied.

[m.lander]

this thread was and is not intended to argue which is right. I simply cant seem to "get it" on how to set up for full manifold. lets pretend both are lousy ways to tune. i just would like to grasp the concept of how to tune for full manifold vacuum

[ChevLoRay]

In my opinion, full manifold vacuum would be the amount of vacuum present in the intake manifold. A tapped manifold, which would then be a port to the manifold, would yield whatever vacuum is present at that point.

So, again in my opinion, accessing that source of full manifold vacuum would yield as much vacuum as your engine would be capable of generating. As you decelerate, the engine generates its highest level of vacuum. That is why you may have seen some older vehicles that would coast down a hill, then as the driver applied gas to the vehicle a puff of smoke would emanate from the tailpipe. The vacuum that was created pulled oil past the rings into the cylinders and that application of gas and the subsequent combustion created the puff of smoke.

But you can get the highest degree of vacuum from a tap into the manifold. The power brake booster gets vacuum from the manifold in that manner. If I am guilty in my previous reply of missing the point, I apologize. I get caught up in the tone of the post and sometimes I miss the crux of the issue.

[airdale94]

[QUOTE=LONGHAIR;2866291"Just because that's what the factory did" in the Smog Era, does NOT make it right.[/QUOTE]

How true it is!

I "would think" if you want to "tune" for full ported vaccum, you would want to set your intial timing lower, since you will be running advance at idle. How much? I guess you'll have to experiment. Then I would set weights and springs, depending on what RPM range you want full mechanical advance in and how fast you want it to come in. Then set your adjustable vaccum advance as to how much extra you want. Keep in mind though that if you are running off full vaccum, adjusting the vac. adv. is just going to give you more initial advance.

[haysonj]

Originally Posted by m.lander

this thread was and is not intended to argue which is right. I simply cant seem to "get it" on how to set up for full manifold. lets pretend both are lousy ways to tune. i just would like to grasp the concept of how to tune for full manifold vacuum

Go to the FAQ and read the thread by Alaskan(Timing 101). This is written by a Chevrolet engineer and explains the vacuum advance very well. Ported vacuum was an early attempt at smog control and really should not be used. Its all controlled by the computer on new motors.

[haysonj]

Originally Posted by airdale94

How true it is!

I "would think" if you want to "tune" for full ported vaccum, you would want to set your intial timing lower, since you will be running advance at idle. How much? I guess you'll have to experiment. Then I would set weights and springs, depending on what RPM range you want full mechanical advance in and how fast you want it to come in. Then set your adjustable vaccum advance as to how much extra you want. Keep in mind though that if you are running off full vaccum, adjusting the vac. adv. is just going to give you more initial advance.

The adjustment on the vac can just determines when full vac comes in , It can be set anywhere from 0 to 20 or whatever the can is built for. If you have a big cam that doesn't produce a lot of vac then you set to a lower vacuum. Vacuum advance also comes back in at cruise speed and you should have around 53 degrees total advance. Read Timing 101 in the FAQ section I found it very informing and learned a lot that I never Knew

[bryanw1968]

Here is what I "think" I know about this issue.

If you have a big cam your vacuum at idle will be low and inconsistent. In this situation it would make sense to me to run ported vacuum instead of manifold. That way you don't have to have a bunch of initial timing in at idle that could cause heating issues in traffic. Now a stock cam with high, consistent vacuum, it makes sense to me to run manifold vacuum to the vacuum advance on the distributor. Set your initial with it unplugged, then plug it in and turn the screw to add the rest. You're weights and springs will do the rest as the rpms come up.

Thoughts?
B

[LONGHAIR]

Thoughts on that are simple.....
This is exactly the point of an adjustable vacuum canister. You can adjust it to the point that the vacuum advance comes in at a lower level of vacuum.

[PICKMUP]

...and I thought the adjustable vac adv pot limited how much travel or degrees of advance that was applied when vac came in.
Do you know of vac adv pots that adjust "when" the the vac adv comes in?

[pumpkin]

Looks to me like a whole lot of opinions. Vacuum advance is used for fuel economy mostly. On any performance engine I've had I don't even use it. The main difference between man. and ported vacuum is that there is no vacuum at idle with ported, so which one you use is more of a tuning device. Both work equally well for economy. I rarely set timing with base timing unless I know the advance the distributor has. I set timing at 32- 38 deg full advance, no vacuum. From here I fine tune to the engine, all depending on grade of fuel used, compression, camshaft. I have found that if the engine will behave, that running no vacuum advance and having timing all in by 2000-2500 rpm has given me the best economy and power. Some engines however don't like this setup.

[Old Chevy Mech]

Originally Posted by PICKMUP

...and I thought the adjustable vac adv pot limited how much travel or degrees of advance that was applied when vac came in.
Do you know of vac adv pots that adjust "when" the the vac adv comes in?

I've got one of the accell vacuum installed on my Blazer. I've always been under the understanding that the adjustment is only an adjustment of max advance travel, so what you are adjusting is the max amount of advance you will get from the vacuum advance. Where you draw your vacuum from, according to the instructions as I remember them showed it coming from the manifold vacuum which tends to run higher and more consistant.

You can tune the timing just about anyway you want, but I was taught that the "proper" setup for a 350 chevy (which can be taken quite liberally!) was whn you were cruising down the highway you were at max advance because the engine was at max vacuum/best economy. If you hit a hill and the vacuum drops because you are giving it more throttle the advance drops off so the engine doesnt ping (as much), and being slightly retarted now has a lil more power (debatable). It also makes a big difference how the cam is setup, so if the engine is not stock in its cam or stock degrees of advance, what the advance setting is should really come from the mechanic/tech who set the cam. Thats just my thoughts.

Frank

[LONGHAIR]

Having no vacuum advance at all is a purely drag racing thing. Since you would theoretically be at full throttle only, it wouldn't do anything for you anyway, there would be no vacuum to make it work.

Your engine will run much better and get better mileage by running full vacuum on the street. The actual amount of vacuum in a "ported vacuum" application is the same, it is just not available to the distributor at idle. The port is above the throttle blades. As soon as the throttle is opened it is the same above and below. In theory there is no reason to "disconnect and plug" the vacuum line to the distributor in a ported vacuum situation, there is no vacuum available.

The thing is, in the vehicles where this was originally installed and spec'd by GM, the whole timing curve is a mess. It is way retarded in the initial timing, and no vacuum advance at idle (ported) then there is usually more built into the centrifugal advance, so that the total at high RPM is close to where it should be.
So, if you just switch to full vacuum and bump the initial up to where it was in earlier years, you end-up with too much total, because that is the way the distributor was made. This is why I generally don't reccommend this, and why you can just use any ol' junkyard distributor....you never know what it really is.
This is a very involed subject, much of it is "voodoo" to most people and many of the opinions expressed have no basis in fact. It is really not that big of a deal, it's just that you can't "see" what is happening.

...and I thought the adjustable vac adv pot limited how much travel or degrees of advance that was applied when vac came in.
Do you know of vac adv pots that adjust "when" the the vac adv comes in?

I was under the impression that this was part of the adjustability of this particular distributor? If it is not, the way to "adjust" this is with a different cannister itself. There are definately cannisters that operate on lower levels of vacuum. The 60s-70s muscle cars w/soild cams had lower idle vacuum and cannisters to match.

[MyakkaJeff]

I may be sorry to resurrect this thread but I need help. I looked at numerous engine bay pics and it seems most of you running edelbrocks are using ported vacuum to the distributor. Mine was hooked up that way (previous owner). From what I've been reading on this site regarding setting timing is to disconnect vacuum line to advance cannister and plug. There is no vacuum on the ported line at idle (left side) so I assume it must be full vacuum port (right side). Do I have it hooked up to the wrong port (left side)?

[gator2511]

On an Edelbrock 1406 the full vac port is on the drivers side of the carb.

[Indyuke]

I would agree with longhair. Manifold vacuum is preferred.

M. Lander, with your distributor you would not be able to accurately set your timing curve up by an idling engine. You would need to set the timing at various engine rpms and possibly loads.

What you ideally want is a mechanical advance (the counterweights) at 34deg, and total advance (vacuum plus counterweights at about 52-54deg. Your total advance would be at freeway cruise rpm. The advance at idle doesn't matter since you don't care about performance nor fuel economy at idle.

Assuming you don't have an overdrive transmission and your cruise rpm is 3000, you would want to setup your total advance (52deg) at that rpm. There is a simple way of setting it up, and it is also fairly accurate. You would need a tachometer and a timing light.

First you need to set up the mechanical advance with the vacuum source NOT plugged into the distributor. On a warm engine and using the tach bring the rpms up to about 2800-3000, and turn the distributor so it's at a 34deg advance. Once you've got that, bump the engine up to 4000-4500rpms for a moment to make sure the timing does NOT increase past 34deg. That is your mechanical advance. That is the advance you would see with the engine at full throttle and high load. Now let the engine idle at 700 or 800rpm... your advance should be 4-8deg. All this on a warm engine, and you might need to play with different counterweight springs to get what you want. Again, you want to set the distributor at 3000rpm, not idle. A specific number at idle doesn't matter. Without mechanical advance setup properly, your vacuum advance will never be right since it builds off the mechanical advance.

Now plug the manifold vacuum into the distributor. This will bump the idle up a bit, so turn it down at the throttle till you see 700-800rpms. Bring the engine up to again 3000rpms and your total advance (mechanical + vacuum) should be about 52deg. If it's too much or too little, adjust the screw on the distributor until you get 52deg at 3000rpms. Don't turn the distributor to try to bring the advance to where you want it, because that will just throw your mechanical advance out of wack. Once you've got your 52deg, let the engine idle. You should now see about 20deg of advance AT IDLE. That's as it should be!

This is the optimum setting for chevy v8's... in a perfect world. Most likely you will experience some pinging with this because gasoline isn't as volatile today as it once was. If you do, retard the timing by turning the DISTRIBUTOR 2deg at a time until it goes away. Turning the distributor reduces mechanical and total advance, so rather than cruising down the freeway at 52deg total advance, you'll be at 48deg or something. Rather than a full throttle mechanical advance at 34deg, you'll be at 29 or 30.

I hope this helps. This is what I did (plus a carburetor rebuild), and my old 350 pulls smooth and strong with no pinging..


Whoops! My bad! I just realized this is an older thread and M. Lander is probably already figured this out.

[Huck]

Interesting debate--which will go on for years!! I recently got a CD from edelbrock because my carb was running rich at idle---PS the CD was a waste!! It did however recommend running with ported vacuum. I've done it both ways and kind or leaned to use a vacuum gauge verses timing changes to guide my performance tuning. Of course everything we do with carbs, distributors, vacuum advance is all "old school" and can't compare to fuel injection and computer controlled advance. But, it's still fun to discuss ~~~~and argue!

[m.lander]

old thread but i very much appreciate the reply indyuke.