Have 81 Z28 350 with all computer and sensers ripped out. Thinking about getting a new distributor. Do I need a vac advance on it or can I not use it. Most all the distributors have mechenical and vac. advance. Don't know what to do or get, or how to hook one up.

north1

Yes you need the old style mechanical advance distributor, preferably with vacuum advance for better mileage.

If you have mechanical advance you may be able to tune it in just fine i dont run vac advance my total is 36 at 2800, 22 initial
i have tried evry combo possible on my motor and my motor is happy as is track proven.

There is no disadvantage to running vacuum advance on the street (as long as there isnt too much). It helps with milage and general drivability. Just get a new HEI, hook up 12V, connect the vac. advance and go.

I agree with Bondo79. Vac advance does 0, nota, nothing when the engine is at WOT, but at all other times it adds more timing.

That can and will give you better street manners, more throttle responce, better fuel mileage, cleaner plugs, cleaner idle emissions, etc

No reason not to run vac advance on a street car or even a street/strip ride. Just get one with an adjustable vac can, so you can tune the vac advance to what your engine likes

Or head into NAPA and ask for a VC1853. It fits HEI and only gives about 15* advance. Works really well.
Oh, and hook it up to full manifold vacuum, not ported - having the vac advance at idle will let you back the idle speed screw off a turn or two and make it easier on the brakes at stoplights.
Also makes the idle a bit quiter, and more mellow (good or bad, depending on how you look at that)

Here's a link to a thread with a complete list of vacuum advance cans for regular and HEI distributors:

http://www.nastyz28.com/ubb/Forum8/HTML/009276.html

I run vacuum advance all the time on my street motors. As an extreme example.... most recently I hooked up vacuum advance to a pretty wildly cammed small block (over 240* duration cam, huge single plane intake, etc.) This was a LUMPY street/strip kinda combo.

Doing some real-world kinda testing with a vacuum gague I found we could still pull 10"+ of vacuum under most modest operating conditions (including 900 RPM idle against the high stall converter). So I dropped my tried-and-true Crane adjustable vacuum advance can in the engine's HEI distributor and set it for 12* max. vacuum advance. Starting at ~5" vacuum and maxing out at 10" vacuum. I hooked it to a full vacuum source (normally I use a ported, off-idle, vacuum source but this was an exception plus I knew I would have enough vacuum to keep it from "wandering" up and down at idle).

The engine became considerably more "streetable" in around-town driving. Idled smoother with less throttle opening and with higher vacuum (almost 12" now!) and was considerably "snappier" at part throttle. Fuel economy probably picked up some, too, but we weren't concerned about fuel economy with this combination.

Oh, and the big difference- it was LOT quieter driving around town at modest throttle. Did I mention that vacuum advance can quiet down the exhaust note quite a bit? Well, it can. Before installing vacuum advance it was difficult to talk inside the car at a 50 MPH cruise over the exhaust noise (headers, 3" exhaust and 2-chamber flowmasters with turn-downs before the axle). After installing the vacuum advance it was almost embarassingly quiet, at least for a grumpy, grumbly street bruiser like this one). Not an "imaginary" difference. Definite, noticable, no-questions-asked difference in cabin volume. You can now talk without raising your voice.

Excellent post Damon. That is a great example of the benefits of vac advance.

<font face="Arial,Verdana" size="2">Originally posted by Damon:
Oh, and the big difference- it was LOT quieter driving around town at modest throttle. Did I mention that vacuum advance can quiet down the exhaust note quite a bit? Well, it can. </font>

THANK YOU for saying that! People used to look at me funny when I told them that. I can vouch for it.

Now, can you back me up on this?: A larger carb tends to have a louder exhaust note(at least at idle). I noticed going from a 600 to a 750 (Holleys) on one particular car that the exhaust was noticeably louder.

I dont know should my car work better with vac adv? is there any reason it seems to run better without it?
Man i tell ya im getting more and more confused every day about this stuff there must be an ideal setup?

<font face="Arial,Verdana" size="2">Originally posted by Mwilson:
I dont know should my car work better with vac adv?

yes

is there any reason it seems to run better without it?

You have too much vac advance? (ie: stock mid 70's cans tend to give 20+ degrees) Or it is on ported advance?
I know my car ran kinda wierd on ported. Stopped a few miles down the road, switched back to full and never looked back.
Also, do you have a big, lumpy cam? You need all the advance to be in at your idle vacuum reading (some cans don't give full advance untill pretty high vacuum), otherwise as your engine lopes at idle, the vacuum advance will be changing, which can really make things act strange
</font>

Thanks onovakind67 for that link. I didn't check it back then after you'd posted the lists of cans. Those are excellent!

My distributor is an aftermarket "Proform"

Ive tried ported,manifold
it seems to idle better and have slightly better throttle response on manifold but it spark knocks real bad at light acceleration
on the street and thats with total at 36-37 at 2800, without the vac my initial is 22

It's interesting to me that most guys understand the concept of a mechanical advance curve and its general theory of operation but are basically clueless about vacuum advance. They know that if you use a large duration cam, you usually need more initial advance to make it work well, and you have to modify the curve to get the best response, and limit the total to keep it from detonating, but will take a off-the-shelf vacuum advance can, plug it into the vacuum line then bitch when it doesn't work. If the stock mechanical advance curve didn't work for your engine why would a stock VA curve? In order for the vacuum advance to work properly, it must be tailored just like the mechanical advance curve. You need to define the starting point, ending point and rate of change based on your engine. As pointed out by Damon, you don't want to be in the middle of the curve at idle or the speed will hunt incessantly. If you've only got 6" of vacuum at idle, your choices are limited.

what Should i adjust my vac advance to ?
30,40,50? i have no idea?

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692 hp 13.23 @ 132 and dropping
believe it or not no nitrous or blower!

You certainly don't want 30, 40 or 50 degrees. How did you establish that you needed 22 initial, 36-38 total and all in by 2600? How much vacuum do you have at idle?

<font face="Arial,Verdana" size="2">If the stock mechanical advance curve didn't work for your engine why would a stock VA curve?</font>

Exactly.

For a performance SBC or any other common 2-valve wedge-head V8 you're typically only going to want 8-12* of vacuum advance MAXIMUM once you have the initial and centrifugal advance dialed in for best WOT performance.

The amount of vacuum it will want to bring in all that vacuum advance will depend largely on your cam and how much vacuum the engine is capable of making in the real world. Stock engine you might want to top out the vacuum canister around 15" of vacuum, in an engine with a lumpy cam you might want it all in by only 8". Depends on the rest of the combo and what works well in that particular engine.

I have enough vac at idle for good brakes!
idont know how much, but i have a 6.5 P-valve in the front of the carb if that helps
I set my total, the initial ended up where it is, It feels pretty good there and i have not touched the distributor since initialy installing the motor, i did hook up the vac adv at the track manifold and ported but it ran a little slower, i know WOT has no vac at the vac adv but i think as rpms go up on the other end you regain a little vac not sure?

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262 hp with only one stage of nitrous!15.23 @ 87 and dropping

you can get a couple of inches on the topend, but most vac cans don't start at that low a level. If you have enough vacuum on the topend to give vac advance you probably need to ditch the single barell carb http://www.nastyz28.com/ubb/wink.gif

22* initial sounds like way too much.
That's odd - I have a proform HEI as well. Initial is around 12-15*, total is in the mid-low 30's. At idle with the vac advance I'm just shy of 30*. Mind you, that is just at idle, as soon as you hit the gas, the vac advance starts to drop out.

I'd look into changing the bushings in there that limit the total advance. Then you can set the initial at a reasonable level.
With 22* initial, I'd say that you don't need vac advance because almost every can out there will leave you with too much advance. Especially because the can that comes with the proform is a stock-style one with 20-something degrees of advance.

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-'77 Camaro - 357, home-ported heads and intake, Q-jet, headers, hyper flattops, ~8.5:1, 3.42 gears, 214/224 cam, 112LSA, rear discs coming soon
-'92 LeBaron LX sedan - all family owned, great daily driver and highway cruiser. 52mm TB, rear discs swapped. 5-sp swap in the works.
-'70 Camaro - 350, 4-sp, needs work, lotsa fun
-'65 Nova - 230 I6, 3 on the tree, nothing power. HEI swap. 5sp and 4 wheel discs in the works.

22* initial sounds like way too much.
That's odd - I have a proform HEI as well. Initial is around 12-15*, total is in the mid-low 30's. At idle with the vac advance I'm just shy of 30*. Mind you, that is just at idle, as soon as you hit the gas, the vac advance starts to drop out.

What's your advance when cruising at 3000 rpm with 15" of manifold vacuum?

I put lighter springs in?
My car idles at 950-1000

At 3000 id be at 37 right?

<font face="Arial,Verdana" size="2">Originally posted by Mwilson:
I put lighter springs in?
My car idles at 950-1000
At 3000 id be at 37 right?</font>

Depends on the limits of mechanical advance curve, the rate of the curve, the amount of intake vacuum, the rate of vacuum advance and the limits of the vacuum advance.
i dont run vac advance my total is 36 at 2800, 22 initial
With this setup you would be at 36 degrees. If you had a vacuum advance can that gave you 10 degrees advance at 10" vacuum, you would probably be at 46 degrees cruising at 3000 rpm. A can that took 10" to operate probably wouldn't work on your engine because you most likely don't have 10" at idle.

I set the total 36-37 at 2800
the 22 initial happened to be what it is
the timing is still 36-37 at 3000,4000,5000

<font face="Arial,Verdana" size="2">Originally posted by onovakind67:
22* initial sounds like way too much.
That's odd - I have a proform HEI as well. Initial is around 12-15*, total is in the mid-low 30's. At idle with the vac advance I'm just shy of 30*. Mind you, that is just at idle, as soon as you hit the gas, the vac advance starts to drop out.

What's your advance when cruising at 3000 rpm with 15" of manifold vacuum?</font>

Yea, it is probably around 50*.
Of course, the engine is only having to put out 50hp or so to cruise along at 70 so a bunch of advance isn't going to be a problem.

My timing never goes over 36-37 thats what the total is set at with 22 initial and 14 mechanical no vac advance

<font face="Arial,Verdana" size="2">Originally posted by onovakind67:
If you had a vacuum advance can that gave you 10 degrees advance at 10" vacuum, you would probably be at 46 degrees cruising at 3000 rpm.
</font>

Thats why my car spark knocks with Vac Adv hooked up under light acceleration and i just dont use it.

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307ci 14.21 spinning!