I decided to run manifold under the impression that it runs out at higher throttle is this true ?When I hooked it up rpms went up 300 ,and it ran super smooth,but
will it hurt my engine ?How does ported work ?

manifold vacuum is the right setup. No doubt about it! Manifold vacuum sends the vacuum signal as the engine gives it. It's the right vacuum, because it is exactly what the engine is producing with no funny stuff. As I understand, ported vacuum is there for emissions purposes, the vacuum signal is cut at low rpm, making the engine more emissions friendly, because it backs up the timing some? I think? eh? not an expert. But I DO know that full (manifold) vacuum is the right signal for our purposes.

Where does manifold vacuum cut off ? How do
I know if it has to much timing? It's not pinging .

When does manifold cut off ?How do I know if it has too much timing? It's not pinging.

This is the short version. Manifold vacuum advance does not increase the ignition advance at WOT. Since it only adds advance at part throttle it is likely you would hear pinging if the extra advance was going to hurt your engine. It will improve part throttle performace and is almost always good in a street car.

So, this would be the advance responsible for part throttle/cruise conditions ,right?
Would this also affect fuel economy?

I just changed the mild 350 in my 66 to manifold vacuum after years of using ported. I can't tell any difference. The only thing noticable was a rise in idle speed, which I expected. Other than that, it runs exactly the same. Actually, I'm not surprised since ported vacuum is the same as manifold vacuum once the butterflies are cracked. I suppose one advantage of using manifold vacuum is the engine should run a little cooler while idling due to having more advance.

I don't see that it would make any difference at other throttle settings. Perhaps someone could explain what happens at part throttle and full throttle that is different than ported vacuum. Perhaps my engine is just to mild for it to make any difference. I don't mean to be argumentive, but this is my experence.

Engine specs are at the link in my sig. Follow that to the 66 El Camino link. If your interested.

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Pat Kelley
66 El Camino, daily driver
67 El Camino, STRIP/street
http://cochise.uia.net/pkelley/

That clarifies a few difference beteween the two , but I'm still curious as to how much advance it adds,where , and when it gets out.

Oh yeah ,does it affect gas mileage ?

Put a timing light on it to be sure. Disconnect and plug the vacuum advance line. Check your timing at idle and at in 1000 RPM increments till about 3500RPM. This will tell you what you mechanical is doing. Hook up the vacuum advance and slowly do the same. I say slowly so that the engine will be under no load and have maximum vacuum signal. Check the timing again. You should see a noticeable advance with it hooked up. The difference will be what the vacuum adds. Like the other guys said, when you are "on the throttle", you vacuum goes to 0, so if won't come into play at all. Only around town at partial throttle, no load conditions.

Jim R

As far as gas mileage goes....I have no idea. I've always been too afraid to actually check what the gas mileage is on my Chevelle. I know its below 10 MPG though.

JR

xllr8, Manifold is the correct choice for a performance engine, if you have a stock replacement canister, there is a stamping on it that will indicate the amount of advance. Some are as high as 18 and as low as 10. You can also limit it with bushings.

I am using a Crane adjustable Vac. Advance, its cheap and easy.

BTW, I had a simmilar problem as you running hot, the adjustable adv. cured the problem, it did take many days worth of trial and error. I don't even know how much mine actually advances it, it dosn't matter once you get it right. My car idles better, runs cooler, and I did notice an incerase in mileage.

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67 Malibu Conv (always under construction)
mirabile95@earthlink.net

It's great to hear about the gas mileage although my overheating isn't due to timing;it happend way before I started fiddling with that.

There are some numbers on it 481 20 ;how do I read them ?

The "20" means it's a 20 degree canister. Any AC Delco dealer can supply you with a 10 degree of you can use an adjustable.

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Pat Kelley
66 El Camino, daily driver
67 El Camino, STRIP/street
http://cochise.uia.net/pkelley/

Now, lets really confuse the issue.. I have heard that most performance SBC's should be limited to a total of 36 deg. advance. I guess the 36 deg. only applies to initial and mechanical added together? If you think about it, running at 2000 RPM at light load, the vacuum advance would be kicking in with the mechanical and the initial and could easily push you over 36 deg. advance. But, on the other hand, at wide open throttle the vacuum would not be contributing and you would be at whatever your initial and mechanical add up to. Does this sound right? Should an engine ever see more than 36 deg? If you are over 36deg. only at part/light throttle will it hurt the engine? This is the one area of performance cars that I have never fully understood. Will you kind people here in my support/mental health group give me some guidance in this area? Thanks everyone!

My thoughts exactly.If I went by ear I'd run it ,but my mind tells me 22 intial (with vacuum ),and who knows how much at part throttle.

Go69, you're correct. The vacuum is not there during WOT but is at part throttle. At part throttle, the vacuum advance is in addition to mechanical and initial. In my case with 36 deg mech & intital, with vacuum I have 46 deg of advance (I use a 10 deg canister). At part throttle, cylinder pressures are relatively low and flame travel is relatively slow. Adding vacuum advance starts the fire a little earlier to compensate for this. However, running a lot of initial advance with mechanical advance coming in early can push the spark lead too far with a stock 20 deg canister. Hence the need to reduce the vacuum advance to around 10 degrees.

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Pat Kelley
66 El Camino, daily driver
67 El Camino, STRIP/street
http://cochise.uia.net/pkelley/

So how do I know if it's to much ?It runs awesome .

In my case, I could hear pinging under light throttle situations. If your engine or exhaust is noisy you might not be able to hear it. Just a wild guess here but if you are running more than around 12-14 degrees initial you probably should decrease the vacuum advance.

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Pat Kelley
66 El Camino, daily driver
67 El Camino, STRIP/street
http://cochise.uia.net/pkelley/

I'm runing 12 degrees ,and my exhuast is actually kinda quite;my engine makes more noise .I did kinda of think 20 was too much ,
but manifold is the correct connection then right ?

If your initial, and centrifuigal timing numbers are working for you, then don't alter them. Just get a Crane Adjustible unit and follow the directions. But use manifold Vac. Not ported.

You will see great results.

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67 Malibu Conv (always under construction)
mirabile95@earthlink.net

Thanks alot for these replies they have been
really helpful.I was actually running 14 initial with an unknown total (stock hei) in my truck ,same engine with 700r4 vs super t10 in my chevelle, but now I'll be able to keep it safe on the top end and pick it up on the lower.

Does this sound right?

I have a Mr. Gasket adjustable vacuum advance canister. Which way advances the timing and which way retards it? I was told that turning the allen wrench fitting inside the canister clockwise, advances it and counterclockwise, retards it. Is that right?

Clockwise=advance

Counterclockwise=retard

Good rule of thumb, advance 1 turn at a time, drive light,hard,city,hway between turns when you get a ping, then back off 1/2 at a time until ping stops.

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67 Malibu Conv (always under construction)
mirabile95@earthlink.net