Vacuum Questions - Calling Schlemmer

I have the following questions about vacuum.
1. Is Ported vacuum better than Manifold Vaccuum and what is the difference as far as how vacuum is measured?
2. Is vacuum in play at higher speeds?
3. Does vacuum increase or decrease at higher speeds?

I have a 74 engine with manifold vacuum running a 65 dizzy with the vac unit and its screw on metal pipe that was intended for port vacuum. Ran a little wacky so now am running with vac disconnected and plugged and car runs great. I did find a working later type vac unit which I plan to install this weekend on my 65 dizzy. Should I expect a change in how she runs on manifold vacuum after this vac unit swap?

Thanks

If you will pardon the link to another enthusiast car site I recommend any carburetor, distributor articles by this fellow, Lars Grimsrud. While he may speak of Q-Jets and AFB Chevy carbs however he is the best source I have found in explaining vacuum, carb, and ignition operation.

I have included a link to his article on vacuum advance and it's sources:
http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/vacuum_explained.pdf



Thanks,

Mike

Just remember whenever you put your foot into it ,,youre vacuum advance is not in the picture.
Its mainly an economy device.

"3. Does vacuum increase or decrease at higher speeds?

Thats the easy part of the question. ported vacuum increases as the throttle opens , based on air velocity going past the port. manifold vacuum decreases as the throttle opens , because of the larger intake hole

I think manifold vacuum advance units have a smaller swing of vacuum than the ported types, study this page: http://www.mgbmga.com/tech/mgb21.htm

I can't speak to your questions, not being an expert, but I can give some information. A while ago, Jeff told me that the distributor is designed for either ported or manifold vacuum (if by ported vacuum you're talking about a vacuum port on the carb). If you use a distributor set up for ported vacuum with manifold vacuum, you'll get too much advance too quickly, since manifold vacuum is greater than ported vacuum.

At least that's what I understood...

The vacuum advance modules are, indeed, port specific. If you use one that is designed for ported vacuum connected to the intake manifold you will get poor performance. The reverse is equally true. RAY

They also won't last very long.

Here's the scoop. Vacuum is NOT rpm related - rather it increases as the engine load decreases. Cruising or coasting its high, foot to the floor and its very low.

Ported and manifold vacuum only vary at idle, where ported vacuum is zero. Its the additional advance that you get from manifold vacuum at idle that can give a hesitation, or poor off-idle performance.

Manifold vacuum advance units work well with ported or manifold vacuum, but the inverse is NOT true. Ported vacuum units have way too much advance to use with manifold vacuum, and they hold on to the advance at low vacuum levels. When you have manifold vacuum, you need an advance unit that only offers a little advance, and as soon as vacuum levels start to decrease, all the additional timing needs to go away very quickly.

Vacuum advance units help performance AND economy, so yes with the correct unit your acceleration should feel less "flat."

Jeff, how do you tell between a manifold and a ported advance unit? I bought a dizzy from a forum member that I am about to send in for a recurve. Not sure which type it is.

Thanks for all the great information. I never had to worry about vacuum in these cars years ago because they mostly had all of their original parts. Now you buy a B and you never know what you'll find under the hood. I will give an update when I get the Manifold Vac unit installed. Thanks again!

Clean up the vac unit, and you'll see numebrs stamped in it, near the vacuum attachment point. A 10-15-5 is for manifold vacuum (used in 73-75). Anything else is basically for ported, and there are hundreds of variations. Some work better than others. The 5-13-10 is the most common, used from '62-70.

Jeff "help" where are you?
I've got vac unit numbers for you to verify above.
You da man!

That's for a TR4 with ported vacuum. You need a 10-15-5 vac unit with manifold vacuum. At kleast it only offers 6 degrees of crank advance, so you can get by with it if you want to give it a shot. Its not the worst vac unit to use in your situation!

Jeff,

I have the ported vacuum unit 5-13-10 on my distributor, but it is hooked up to manifold vacuum. It works flawlessly with the supercharger. I tried the 10-15-5 vac unit with manifold vacuum and had problems. I tried other vacuum units and the best performance with the supercharger is the ported vac unit connected to manifold vacuum. Are there any long term effects I should worry about. Just asking....

Ray

it may be helpful to think of it this way.

When the carb throttle disc is closed, virtually all the vacuum made by the engine is in the manifold and virtually none is in the carb throat (where the port is). When you open the throttle, the vacuum in the manifold goes down and the vacuum in the carb throat goes up.

So, in effect, one is the mirror of the other. Manifold vac goes down when port vac is up and vice verse. together they total about atmospheric

this means if the distributor vac advance unit is expecting to be plugged into port vacuum and is instead pulled into manifold vacuum, at idle that unit is receiving full vacuum instead of the almost no vacuum it should be getting. That means the distributor has full vac advance at idle and this drops as the throttle is opened instead of having almost no vac advance at idle and having the advance increase as the throttle is opened.

Now...with a supercharger you suddenly don't have vacuum in the manifold any more once the throttle is cracked but varying amounts of pressure. This throws the whole thing even further out of kilter.

Mac,

Thanks for your input and seems this is what a supercharger likes, a lot of advance up front. Maybe why it works so well this way.

Ray

Ray, keeping with the spirit of this thread, the supercharger, when delivering pressure must be applying force against the vacuum diaphragm for which it was not designed. I would think that this would reduce its life expectancy. With my supercharger I'm running a full mechanical advance unit, controlled by an MSD timing controller that retards the timing slightly when under load. Not that different from what the vacuum advance was originally designed to due. RAY '67 Supercharged Tourer

Ray, you're comparing apples to oranges. In fact, a better analogy may be comparing apples to a Snickers bar. You've also got the concept of vacuum all mixed up. Ported and manifold vacuum are identical, except at idle where a ported setup has zero vacuum - the port is shut by the throttle blade. That's the only difference. Vacuum is still created the same way, and works the same way, whether its in the carb throaght or intake manifold. The NEED for vacuum is the same when you pressurize the cylinders however. Essentially you're pulling more air through the intake creating different vacuum levels, which changes everything.