Does anyone know where I can find information about stock distributors?

I have an 80,81, or82 vacuum/mechanical advance distributor.
I doesn't have those plastic bushings, the weights have 41 stamped on em and it used to have a 5 pin module wich has been replaced with a 4-pin and a new pickup coil.
The distributor is in great shape, the pins aren't grooved or anything.

My question is, what is most total mechanical advance that you know of in a stock distributor?
The reason I ask is that even if I set the base timing @ 0 I still get spark knock at part throttle with the vacuum advance limiter on the highest notch with about 9-10 turns on the crane vacuum can.

I get about 21 in vacuum @ idle and have the blue/blue sping combo.

Any Ideas???

sorry for long post........

 

usually 18-22 degrees mechanical advance in a stock distributor.

 

They are all over the place. Everything from 10-12° in a truck one, up to 24-26° in older performance cars. I know of no HEIs that have that kind of curve though.

 

all three distributors i have tried in this car have had between 18-20 centrifugal adv.
there are quite a few people who think all v8 hei's are
20deg +/-1deg

 

I've known quite a few people that are wrong about all sorts of things....

The number of them that I've seen twisted up on a distrbutor machine has convinced me otherwise.

 

One of the reasons I question this is because most typical non-cc distributors I see look alot different than the one I have.

Like it doesn't have bushings, the plate that the centerpeice and weights are on is smaller and doesn't have those 2 little "ears" sticking off to the side, and the advance has noticeably more movement.

I completely disassembled the distributor and cleaned & lubed it before I installed it and didn't see any real wear & tear.

But thats not to say I don't have another problem, I haven't had enough time lately to check that and run without the vacuum can to see if that might be it.

Also could be some other problem, maybe I'm running lean.
Lately I've noticed that cooling ticking noise when you shut her off, and a slight stumble at 3500-5000rpm under WOT.

maybe sometimes i just think too much or not enough....

Thanks,
Dolph

 

does it still ping with the vacuum advance disconnected?
you may have the vacuum line hooked to a ported vacuum source instead of straight manifold vacuum.

If you're postive that it's spark knock that you've got,
it might be caused from lean A/F ratio or crusty plugs or plugs that are too hot.

Or could be a blown head gasket as that can cause ping sometimes with any advance setting.

 

It's hooked up to a ported vacuum source, lean A/F maybe, the plugs are new and should be cold enough.

OK, I was under the impression you are supposed to hook up your vacuum advance to a ported vacuum source and not straight manifold vacuum.

should I hook it up ported, yay or nay?

 

I'd say if you like spark knock, hook it to ported.

The other way around it is to limit your centrifugal advance.

FYI, engines run much better with around 15* initial advance plus vacuum advance hooked to manifold vacuum.

15* initial
20* centrifugal
15* vacuum

that's a good place to start.... cept not if you use a ported vacuum source.

 

Why is ported going to make a difference? Ported and manifold vacuum will be the same off-idle.

 

Ported vacuum comes from above the throttle blade and supplies VERY weak vacuum at idle, but it increases as the throttle is openned on up to WOT.

Manifold vacuum is VERY strong at idle and should decrease to almost zero as you open throttle to WOT.

There is a big difference.
Ported vacuum increases with loading
Manifold vacuum decreases with loading.

You want more timing advance at light loads
less timing advance at heavy loads.

more timing advance at heavy loads = spark knock.

 

Ported vacuum is the same as manifold vacuum, except that it comes from a {drum roll please} port just above the throttle blades. So, it is non-existent at idle since at idle the port is above the blades; but once the throttle is opened a little, like at cruise, the throttle blades move above the port, and then the port gets the same vacuum as everything else below the throttle blades.

So, ported vacuum is non-existent at idle, strong at cruise, and weak at WOT.

The only vacuum that increases with engine air demands is venturi vacuum. That's created by the flow of air through the venturi, and has nothing to do with manifold vacuum. It is the type of vacuum that creates fuel flow, and also opens the secondaries in vacuum-secondary carbs. It is not the same as "ported" vacuum of the type that goes to the distributor.

The correct place to hook the distributor is to a ported source. Not to venturi vacuum (which is impossible on practically any carb), not to constant manifold vacuum (because then you will have vacuum advance at idle, which you don't want).

The ideal timing curve varies from one engine to another, and from one car to another with otherwise identical engiens, because of gearing and weight and so forth. But it is usually something like 10-12° "base" or static, 24° of centrifugal starting at 122 RPM or so and all in by 2400 RPM or therabouts, and 12° or so of vacuum advance, that drops out when the vacuum goes below about 12", and is only available when the throttle blades are more than about 15° off-idle.

 

the venturi IS just above the throttle blades.

 

The "port" is in the side of the throttle body, about .060" above the throttle blades when they're in their idle position. It usually a little slot about 3/16" wide and less than 1/16" tall. It is low enough that any siginficant opening of the throttles moves the blades above it.

The venturi is a great big round hole about an inch above the throttle blades, and the throttle blades never move far enough to subject it to manifold vacuum.

 

okay, and what you're missing here is that @ WOT there is no difference if your at the top of the throttle bore or at the bottom. The same amount of air is moving by the port, and at the same velocity, so the same vacuum is generated.
There is more air velocity in the throttle bore than there is in the intake manifold.... therefor more ported vacuum than manifold vacuum. Air velocity by the port = vacuum.

Once again, this is why ported vacuum will increase spark advance under load.... and this can cause spark knock.

 

No.

It is not an issue having to do with air moving, which is the way the venturi works. Vacuum advance has nothing to do with venturi vacuum, or air motion, or air velocity. It has to do with manifold vacuum, which would be high at low load regardless of engine speed; except that it is arranged such that it is "disabled" at idle, and active at cruising speeds.

The "port" above the throttles is effectively above the throttle blades (at atmospheric) when at idle, and below the throttle blades (at manifold vacuum) when the throttle blades openn far enough to pass it. This corresponds to the set of circumstances when you want vacuum advance, which is cruise mode: low load, throttles off idle.

 

What you're still missing though, 305sbc, is that neither manifold or ported vacuum are dependent on the velocity of air, that's venturi vacuum. Both are caused by the pistons sucking air out of the intake while the throttle blades restrict the replenishment of air from the atmosphere.

Try the ultimate test, put a vacuum gauge on the ported and manifold vacuum ports. The ported vacuum port will read zero at idle, but as soon as the throttle is opened slightly, the reading will snap to the same level as the manifold vacuum reading.

 

Air moving through a venturi doesn't cause low pressure (vacuum) SO YOU SAY... so I guess a carburetor does not meter in more fuel as engine speed & throttle angle increases???
ooops I guess we will have to rethink how a carburetor works, throw away all our carburetors, and rewrite all the books since carburetors depend on this to function

I'm giving up after this. If you're convinced that you're right about everything then have at it and be happy. I will do the same.


yep, and at WOT the ported reading still shows vacuum,
the manifold vacuum is almost zero.

 

I'll take a pic of the port in a carb throttle body where the "ported" vacuum for vacuum advance is derived. As always, a pic is worth a thousand words.

Believe me when I tell you, the port for vacuum advance, has nothing whtsoever to do with venturi vaccum and fuel delivery.... it's manifold vacuum that's only available when the throttles are off-idle.

 

No, ported vacuum shows zero at WOT. Try it sometime.

 

Easy guys, I didn't mean to start anything.
I'm sure you guys are just misunderstanding each other

 

RB is right. Period.

Hook up a pair of vacuum gagues to both a full manifold vacuum and a ported vacuum source, as I have, and you'll know that ported vacuum is within 1/2" of vacuum at any throttle position over about 5-10% throttle. At WOT they both read so close to zero it's not worth talking about any more.

 

If you really want to know what your centrifugal advance is then I recommend you spend $5 on a timing tape from Summit that wraps around the outside of the balancer and gives you a full set of degrees to allow you to measure total advance.

Almost all of the V8 HEI's I have tested have centrifugal advance between 18-21*, differences being largely attributable to production tolerances and such.

However, there are exceptions to this that I have found in recent years- most notably some truck motor distributors that sometimes have significantly less. GM liked to play around with stuff like this on an application-by-application basis over the years so there are no guarantees.