I'm in the process of selecting a new distributor for my zz383 crate engine I installed last summer. I currently have an Accell billetech magnetic dist with mech advance only (3/98). Prior to getting that one, I had a Mallory Unilite (8/88) Unilites used a program module and had no vac adv either.

Question: How many here with "hot" engines are running vacuum advance? What are the pros and cons?

I've heard running it will give slightly better mileage, and better low speed cooling.

I've also heard the arguement, "Whenever you see a high HP performance engine, they DON'T run vacuum advance."

Thanks,
Ozzie

If it's a street car, run the VA. If it's a track car, don't. At the track and WOT the VA doesn't come into play. On the street, you need more advance at cruise than you can safely build into your mechanical advance. And yes, she'll run cooler with a VA than without.

And if you'll run your VA to manifold vacuum she'll idle better and give you better off-idle performance.

All true!

All true!
Indeed!

I have vacuum advance. For my aplication its all I need to peroform all around. I personally like to drive mine to and from the track. It's not an all out track vehicle. I've know guys who have both - without having a whole lot of problems with either....


The choice is ultimately up to you, your car and you driving....

I run vacaum advance (on full manifold vac) as well, and recommend it on all street driven cars, unless the cam is VERY radical. Vac advance can be a bit of a problem if you can't get 10" or more vacaum at idle, particularly if you run a manual edit: lets make that an automatic trans! Even then, you may get the benefits of vac advance at cruise AND a stable idle if you use ported vacaum.

I run a ZZ383 in my 1969 Camaro. It's fired with a MSD 6a,MSD Blaster II coil, and MSD Pro-billet distributor with vacuum advance.

Thanks, Guys.

Pretty much what I was hoping to hear. I figure also, you can always elect to NOT hook it up if you don't want to use it. In my current situation, I don't even have that option because the dist doesn't have it.

I'm 90% sure I'm going to go with the MSD Street Pro-Billet (8361) model. It says it must be used with an MSD 6,7,8, or 10 ignition box. Is my Crane Fireball Hi6 the same?

Also, ritchiefarms: Did you use the gear that came with the MSD or did you replace it with a melonized GM gear before putting it in your zz383?

Thanks

Vac advance can be a bit of a problem if you can't get 10" or more vacaum at idle, particularly if you run a manual trans. Even then, you may get the benefits of vac advance at cruise AND a stable idle if you use ported vacaum.
The problem is not so much running a vac adv. but getting one that can be configured correctly to work within the range of cruise and off cruise vaccuums.
I've also heard the arguement, "Whenever you see a high HP performance engine, they DON'T run vacuum advance."
That is not the arguement ..it is the satetment that results from the reasoning.
The reasoning is:
A full on race car runs it either full throttle or off throttle/hard braking...it doesnt cruise as one does on the highway. The vac advance is only for highway cruise, and just off cruise...it therefore follows that the full race engine at no time has any NEED for a vac advance as it doesnt use it.
A strip/street type car does use a vac advance.... on the street.

If it's a street car, run the VA. If it's a track car, don't. At the track and WOT the VA doesn't come into play. On the street, you need more advance at cruise than you can safely build into your mechanical advance. And yes, she'll run cooler with a VA than without.

And if you'll run your VA to manifold vacuum she'll idle better and give you better off-idle performance.


Agreed and Disagree.

I run VA on PORTED vacuum on my street engines. The engine idles better and has amazing off idle throttle response.

I am glad that this topic came up. I run a Mallory HEI distributor and have not hooked up my VA yet. I have a 383, 9.7-1, XE274 flat tappet with vic jr manifold. Where should i run the vacuum line from to the distributor? What should I be looking for in terms of timing after the VA is hooked up? RIght now I have 14 initial at idle and 34 total.

it really depends on the engine and car and what it wants. I run mine on full manifold vacaum. This is a stick shift car with a mild cam, and I'm able to get one heck of a lumpy but 100% stable idle at a very low rpm.

With an automatic trans and a long duration high overlap cam, it can get to be a problem. You tune it for a good idle, then put it in gear, the rpm drops from the load, vacaum drops, the vacaum advance pulls advance out, and the motor stalls.

If you have or can get a vacaum can that will be fully deployed at the vacaum your engine produces at idle IN GEAR, that will probably work best.

Most stick shift cars should work well on full manifold vacaum.

Many radical engines will idle more reliably with no vac advance at idle, use ported vac, and lots of initial timing, 20 or more if it will crank hot.

My car see's 36 degrees at idle, and nearly 50 at a 2000 rpm 5th gear cruise. Total, of course, is 36.

My car is an 80 Z28 with an automatic 2600 stall and 3.73 rear gear. TH350. Holley 3310 carb. No lightening to the car so around 3600-3700 pounds I guess. What do you think now? Gotta go, wife is going into labour. NO ****!!!

iu think "take care of business!!"
Good luck with the baby!

Jim M is correct above. Most stock VA cans aren't fully "in" until about 15 inches of vacuum. If your idle vacuum is lower than this, and you're running manifold vacuum with a stock can, your idle will never be consistent and often people fix things that aren't broken. Everything is doing exactly what it's designed to do.

If you're running manifold vacuum to the can (no disrespect to donbe, but I'll always contend it's better for most any street rig) you need a can that's all "in" a couple of inches below your idle vacuum reading,..i.e, if you idle at 10 inches you need a can that all "in" at 8 inches (I'd use the Echlin VC-1810 in this case, which is all in at 8"). I have chart somewhere if you need a recommendation.

If you run ported vacuum to the can (stock or otherwise), and your coolant temps rise at the stoplights, manifold vacuum will could likely solve this problem.

Here's another advantage to manifold vacuum to the VA. If you have a larger cam and in order to idle, the throttle blades on your carb have to be more open than usual for good steady idle (causing all sorts of problems,..setting idle mixture, diesleing, etc), manifold vacuum with the correct VA can will raise your idle speed, thus allowing you to close your throttle blades more at idle and eliminating the problems described.

The reason late 60's and early 70's cars ran ported vacuum to the VA can, was that retarded timings are better at cooking off hydrocarbons which helped Chevy and Ford meet federal emission standards.

The understanding of all this will do more for you're ability to tune your engine than most anything.

Good luck!

Are you saying that the cans on most VA distributors can be swapped for others? Including a brand new MSD pro billet?

Most use a stock-type GM small vacuum advance can.

still don't think you can beat the crane adjustable can for fine tuning though, and usually well under $20 too!
Eric B

I used the gear that came with the MSD Pro-billet distributor. It dropped right in with no problems. I did have to install a larger amp diode than supplied with the MSD 6a to stop engine run-on.

I used the gear that came with the MSD Pro-billet distributor. It dropped right in with no problems. I did have to install a larger amp diode than supplied with the MSD 6a to stop engine run-on.

Why did you need the larger diode? The current in that circuit should be less than 0.5 amps and the PIV possibly 25v if you account for any transients. A 1N4001 should be sufficient.

ALL gm style vacaum cans are interchangeable.

Most aftermarket distributors that use GM style mechanical advance (wieghts and springs exposed uner the rotor, including msd) should be compatible with a stock style GM can.

Some aftermarket distributors use Ford style mechanical advance (wieghts and springs hidden under the breaker plate), older accel and mallory unilite for example, I don't know if these will take a gm can. The can on the old style accel i used to run "looked like" a gm can, but I never tried to swap it.

ritchiefarms:

Boy you had my mind racing there for a minute! I remember installing the diode years ago when I first got my Crane Hi-6 ignition box. I put it on the proper wire just ahead of my external regulator.

When I did the engine swap last year, I also upgraded a few other things. I went with a 100amp internally regulated alternator and the MAD Electrical rewiring kit. I almost forgot that there is a diode in the system (otherwise it wouldn't work correctly) and it's located in the wire running along the firewall before it gets to the alternator.

Whew.

Presently my trans vacuum line is hooked to the baseplate of the carb for vacuum. Can i put "T" this off to also run the vacuum advance? With the vacuum advance I will have 22 at idle and a full 39 all in. What do you guys think?

I need to do the same thing, but I'm running a T off my brake booster line. I don't want any changes with my advance Vac, I just finally got it right! My initial is 14* with vac its 29*at idle.

I set my vac adv up like this
1/ find my best performance of the centifical @ approx 3000 rpm eg 32 deg
2/ have a perminent manifold vac gauge in the car.
3/ note vac at cruise on the flat @ 60 mph EG 22"
4/ note vac at slight acceration up a a slight slope 13"
5/using the centifical (vac adv not hooked up yet) advance dizzy up and test under 3/ and 4/ until a faint ping or detonation (aprox 60 to 75mph)or 44 deg which ever comes 1st. If faint ping or detonation at say 43 deg, minus 3 deg = 40 total advamce
If ping is 40 and max perfoemance is 32 deg I put 8 deg into the canister starting at 13" (4/) and all in couple deg under cruise (3/)for this example 20"
I connect the vac advance to manifold vaccum.
U have to have a canister that works in the vac range AND amount of deg u require...there are literally 1000s of variations of stock canisters for every consedable engine power range vechile weight, diff ratio, wheel diameter, gear box, so it is not just a matter of picking one up from the local wreaking yard or parts store. The chances of ordering by spec u require are slim, the guy behind the counter will not know what the hell u are talking about.
Stock vac advance canister can be made adjustable.
Sometimes this means extending/shortening/moving the slot where the actuation arm moves.
Once u have a slot that some where in that slot your vac and deg range is u can make a couple of small tabs or 'stops' out of panel steel.
one attaches to each of the mounting vac advance scews at each end of the slot, limiting where (vac) and how much (deg) the vac actuation arm moves.
U can get a little fancy here...at the end of the tabs u can make several small rounded sections that are slightly diff lengths, like a cam.

After market adjustable canisters have an adjusting grub screw inside the canister where the vac hose fits...this adjusts the vaccuum range of operation. Even then the range may will be out the spec u require.

When going thru old vac adv units, to pick up something that is range of what u want, take along a hand vac tester ( a small hand vac pump that can be picked up at a automtive tools supplier..a handy tool for other uses too, like bleeding brakes etc).

I dont use vac adv on idle....Vac advance is used on street and street /strip cars.
Generally if the extra advance is needed at idle, the engine has mis matching components (cam, carb, compression, headers, heads etc)
And/Or
The engine is over built above the generally accepted parameters of street/strip and the builder is trying to run a race engine as street/strip.
Street strip IS the compomise between street and race, Race is not the compromise.

I connect the vac advance to manifold vaccum....

I dont use vac adv on idle....Vac advance is used on street and street /strip cars.
Generally if the extra advance is needed at idle, the engine has mis matching components (cam, carb, compression, headers, heads etc)


Would this be a contradiction?

Would this be a contradiction?

Yes.

I just checked the mileage on my basically stock rebult 302. Mostly surface street driving, not in traffic, with full vacuum advance set up per John Z. Yes I put my foot in it occasionally, but not too bad the same. I see 13-14 mpg on the hiway in the future.

A racer I know gave me a little eccentric plate that goes under the back screw that holds the vacuum cam on. It is made to stop the arm from pulling the breaker plate. He said it is made by Crane. It allowed me to adjust the vacuum advance to 15*. The darn thing came with almost 20* from NAPA.

Would this be a contradiction?

Yes ..if the idle has a vaccuum higher than
4/ note vac at slight acceration up a a slight slope 13"
The 13" is an arbortory figure, thu within the ball park, 12" to 14" is the rough range
As not many of use are running camaros with low end stock engines, and have tweaked cams of varing degees, our idle vaccums are closer to and even below 10"

I set up my vacuum advance just the opposite - all in at idle. It's not a race car, just an 11-second mild-mannered tow car with 16" of idle vacuum. I fail to see the connection between copious amounts of initial timing and mismatched components. Can you elaborate on this a little bit? Are you saying that if an engine runs good with high initial advance the components are mismatched? How much do you run?

I set up my vacuum advance just the opposite - all in at idle. It's not a race car, just an 11-second mild-mannered tow car with 16" of idle vacuum.
If u have the vac advance all in at idle this will put the idle vac up
What vac is the idle, vac adv disconected, and cetrifial all in set to your correct specs.
Are you saying that if an engine runs good with high initial advance the components are mismatched?
No. A engine with miss matched AND/OR a above low end stock cam will run a low vac, without vac advance included...It has a low vac because the cam is well out of its operating specs at idle and runs inefficently.

I have a 8 to 9 " idle vac, intial of 6 to 8 degs no cenrfical or vac advance at idle
Idle is 550 rpm, cent curve comes off at 625 rpm, all in at 2500 @32 deg
Vac advance starts at 13" all in at 20" with a total of about 38 deg.
The engine has been built for pure street economy stump puller between 1200 and 4500 rpm on 112 to 115 octane fuel.
Dyno shows in excess of 210HP @3800 rpm starts to lean out at around 4200 rpm (450 cfm carb)
The previous build idle 5 to 6" idle @ 550rpm, with 27 deg cent in @ 2500 with vac advance start at 14" all in @ 18" with 5 deg to a total of 32 deg advance at cruise. Same fuel
1/4 mile in 14,2 sec, in drive, changing @ just under 5000 rpm, still with tool box, picnic gear in trunk, and real crappy hard old tyres, still with a 450 cfm carb, cast exhaust, performer inlet, stock fuelies, 308 diff turbo 350..crappy inexperianced driver...and a screw up in the cam.
Due to a error in calculations on my part with the design of this billet cam I had a low vac idle and very loppy, and the cruise also low.My new designed cam /engine build is a huge improvement. I increased lift, decreased over lap changed a couple things like rate of lift. No 1/4 mile figures yet.

The engine has been built for pure street economy stump puller between 1200 and 4500 rpm on 112 to 115 octane fuel.
Dyno shows in excess of 210HP @3800 rpm starts to lean out at around 4200 rpm (450 cfm carb)
The previous build idle 5 to 6" idle @ 550rpm, with 27 deg cent in @ 2500 with vac advance start at 14" all in @ 18" with 5 deg to a total of 32 deg advance at cruise. Same fuel
1/4 mile in 14,2 sec, in drive, changing @ just under 5000 rpm

Here in the states, we shoot for a bit more performance than what you just posted.

They do here to m8....
I just choose to get the most out of the car for what I use it for...
But that certainly doesnt take away the principles to a car driven on the street....
What 1 guy gets at idle, cruise, off cruise and how much where, differs to the next guy, therefore they have different curves, just as Chevy do for each engine, model, diff, application, weight, engine.

If u have the vac advance all in at idle this will put the idle vac up
What vac is the idle, vac adv disconected, and cetrifial all in set to your correct specs.

No. A engine with miss matched AND/OR a above low end stock cam will run a low vac, without vac advance included...It has a low vac because the cam is well out of its operating specs at idle and runs inefficently.

I have a 8 to 9 " idle vac, intial of 6 to 8 degs no cenrfical or vac advance at idle
Idle is 550 rpm, cent curve comes off at 625 rpm, all in at 2500 @32 deg
Vac advance starts at 13" all in at 20" with a total of about 38 deg.
The engine has been built for pure street economy stump puller between 1200 and 4500 rpm on 112 to 115 octane fuel.
Dyno shows in excess of 210HP @3800 rpm starts to lean out at around 4200 rpm (450 cfm carb)
The previous build idle 5 to 6" idle @ 550rpm, with 27 deg cent in @ 2500 with vac advance start at 14" all in @ 18" with 5 deg to a total of 32 deg advance at cruise. Same fuel
1/4 mile in 14,2 sec, in drive, changing @ just under 5000 rpm, still with tool box, picnic gear in trunk, and real crappy hard old tyres, still with a 450 cfm carb, cast exhaust, performer inlet, stock fuelies, 308 diff turbo 350..crappy inexperianced driver...and a screw up in the cam.
Due to a error in calculations on my part with the design of this billet cam I had a low vac idle and very loppy, and the cruise also low.My new designed cam /engine build is a huge improvement. I increased lift, decreased over lap changed a couple things like rate of lift. No 1/4 mile figures yet.

It seems to me like your idle vacuum is about half of what it should be, given you have a cam that is all in at 4500 rpm. Any tuning procedure will have you adjusting the parameters for maximum vacuum, indicating that the engine is operating at its peak efficiency. If you can use the vacuum the engine produces to bootstrap the efficiency, you're ahead of the game.

If u note that idle is only 550...in drive and advance 6 to 8 deg, on a custom cam...yes will be low...And as the springs come off at 625 rpm, the idle has to be stable (and is) to stop hunting of the engine caused by springs moving advancing rpm at idle.
This is a little off subject to vac canisters..
Due to the nature and mixture ratios of the fuel, high lift is required ib the cam to flow the re'qd mixture volime...this results in a rich idle up to 850 to 900 rpm at which piont and up the engine then runs at max effoeny with good mixture ratios.
Simplfied, Basically the cam is modified a blower cam for 2500 to 7000 rpm advanced 2 deg and lobe centers/ramp rate changed.
What happens at idle realy is not important, other than not idling economically. What happens in power range is important.
At the top end what holds back is the 450 carb over 4200rpm...since this part is in access of over 100mph, and the engine is not req'd to go above this unless passing a cattle truck/trailer. It still pulls very well with critcal lean out around the 4800 rpm...this is only momentary and not enough time to do damage as passing cattle trucks/trailer on the open road is only occassional.
I could throw a larger or 2nd carb on...this would then give me my top end, but what is the piont of doing so..I dont need it, as I dont use it, and it will then put the carburator max effiency out out the range of what the car is used for
Daily city driving, open road cruising on holiday, and towing the trailers. Camping gear etc and the car transporter with the Bradford with max economy. (22mpg with car trailer loaded)yet it still should be able to pull mid to high 13 sec 1/4s...that respectable for such a daily car.

To close the loop:

I installed an MSD 8361 yesterday. Amazingly, I installed it correctly, hooked up the mag p/u wires to my Crane Fireball correctly, swapped in HEI plugs, and got the firing order correct on the first try. My engine specs are supposed to have 32* total advance. So I put in all blue springs and left the blue bushing for 21* advance in by 3000rpm or so and timed it at 11* initial.

The first thing I notice after hooking up the vacuum adv and readjusting idle is the throaty, aggressive personality of the engine seems somewhat tamer now. Is this normal for vacuum adv?

I did notice the idle temp now stays under 190* whereas it would creep towards 200* before in this heat. Only drove it to work and back, so don't have any mileage figures yet.

I just had my timing and carb adjusted today and he disconnected the Vacum advance. He said that if you were running anything more than a stock cam that you didn't need to run a VA. I did notice on the way home that it ran a few degrees hotter than normal, but other than that it preformed well. Should I hook the vacum advance back up. It is a 383 stroker with a pretty good cam. I don't know the specs,

Yes,..run the can on the street regardless of cam. You need VA for cruise and it's better idle as well. You'll need a can that fits your engine's vacuum characteristics. Most stock cans are all "in" at 15 inches. If you run a bigger cam for instance, you'd need a can that's all "in" at a lower vacuum reading.

The amount of advance affects spark plug temperatures. Your mech advance has to be extremely aggressive to do well without a vaccum advance on the street. I once had a less agressive mech curve with no vac advance, the plugs would get deposit buildup much quicker and that reduced performance when I applied full throttle. This was very very pronounced after a long freeway trip, the car would almost die. I finally got it sorted out with mech only, but had to use an initial advance at idle of 20 deg and it came in to full advance very quickly.

My wife's 73 Camaro 383/425 worked best with ported vac advance. The correct amount of transfer port was uncovered with ported spark, almost none uncovered with mainfold vac source, even with the special carb air adjuster they have. We have the 750dp Demon carb with four corner idle and PCV valve.

Well, that a bit of a coincidence. I have the zz383/425 engine but with a 650 Speed Demon. I currently have the VA connected to manifold vacuum. I didn't try ported vacuum. I read several threads about full manifold vA being the way to go.

If I went with the ported vacuum, I'm assuming the amount of advance would be less?

Anyone using the MSD all digital e-curve? Any issues? Comments?

that "Mean lope" at idle is actually the engine misfiring and generally running poorly due to too much overlap and low manifold vacaum. Since more advance at idle makes the engine fire more consistently, it will tame down the idle some. Note that you can often get that lope back just by reducing the idle speed, if you can achieve a stable idle at a lower speed.

it is real important the the vac can matches up well with the cam's vacaum at idle, particularly on an automatic trans car. If the idle vac in neutral is high enough to fully deploy the vac advance, but idle vac in gear is not, you'll need to use a different can, or ported vacaum to get a stable idsle in gear.

I copied this from the Sallee Chevrolet description of the engine I bought:

"Ignition Timing: 32° total @ 3000 RPM with vacuum advance line disconnected and plugged. This setting will produce 32° of total advance at wide open throttle (WOT). "

Does that mean with using VA I'm over the 32* they are specifying? Or are they telling you the way you go about measuring and setting it?

What it means is, at cruise you'll be running the 32* mechanical (if you're up to 3000 rpm) PLUS the vacuum advance of 8-18* (depending on the can) for a total of 40* to 50*. At WOT, the vacuum advance "lets go", and the timing is reduced back to the mechanical advance of 32*.

At cruise, the engine isn't under a load and timing can and should be advanced as much as safely possible for better gas mileage, a cooler running engine, and a spunkier feeling engine.

On four of five older Chevy's, I run the B28 can which advances the timing 8*,..and it's all "in" between 5.75-8 Hg. Since my cams are pulling from 10-13Hg at idle, that means my VA is always "on" except during WOT. If you run straight mechanical advance (no VA), you won't enjoy the benefits of VA at cruise and idle (I run manifold vacuum to the VA).

Many who have porblems with their carbs could solve a lot of issues by running manifold vacuum and a can that's all "in" below the idle vacuum reading. And many who have issues with overheating (at red lights or in traffic for instance) could perhaps solve their issues with the correct VA can and manifold vacuum.

Good luck!

I installed an MSD billet distributor, I shortened the total mech advance available using the bushing over the stop pin they provided, I knew I wanted 36 deg total advance and 12 to 15 deg at idle. That means I needed to have 21 to 24 degrees mech advance in the distributor, I used the closest advance bushing to that.
Vaccum advance is in addition to the mech advance, but it goes away when you floor the throttle.

OK I just had someone to adjust my timing and he disconnected my vacuum advance. He said that it was not needed in a engine with anything other than a stock cam. It runs good, a little warmer than before. From what I'm reading on here it needs to be hooked back up. Can I just hook it back up or does the other timing have to be adjusted if I hook it back up?

You'll want to do some checking to see what it does and if it's right for the combo, but yes, hooking it back up will improv e performance and economy on the street without affecting WOT.